Consolidated Diesel 10 Tonner M123A1C

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Off Highway Log Truck Conversion
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20 yard Belly dump

Officially this truck is known as the M123A1C Truck Tractor. It was built for the Army as a prime mover. This particular model was assembled by Consolidated Diesel, but it is built out of Mack running gear with a very rare Cummins V8-300 engine. This configuration was apparently only sold to the military and is rated at 300 HP at 3000 RPM. The truck was first designed for a LeRoi Gas engine and this Cummins Diesel fits just barely under the Hood. The cab on this truck is the same standard military cab used by the army from Korean War times on, but for the most part the rest of the vehicle is quite different from the more common 2 1/2 Ton and 5 Ton Army trucks. The difference is simply size. The particular unit shown here was built in 1963, and was assigned spent its military life at Fort Knox, Ky.

New: Read all about the successor to the M123A1c called the Oshkosh M911

It features 14x24 tires, and is around 9 feet wide, though it can be reduced to 8 feet by removal of the outer duals. The Tandems are massive Mack units with a 10.1 ratio providing a maximum road speed of just over 40 MPH. It has a winch behind the cab and the Army used it to haul tanks and presumably other heavy freight. In its standard configuration it is equipped with steering brakes implying that you can selectively apply the brakes to the left or right rear wheels to assist in controlling spin outs and to help turn under difficult conditions. Although not equipped with a 'Jake Brake', the army had retrofit these units with the next best thing and that is a Williams Air Brake. This is essentially a valve that blocks the exhaust pipe. This causes the engine to slow down rapidly, and provides excellent 'engine hold back' on down grades, and also responds quickly enough to pull the engine down making shifting on the fly much easier. Like the "Jake Brakes" the Williams air brake has a throttle cut out on it so it is only active when the throttle is fully in the idle position. This means that you can turn it on and drive and shift right through it, as it will only become active when you 'let up on it'. The Williams brake modulates automatically because it works harder at higher engine RPM's. this is because it is designed to pass some exhaust gas through the exhaust pipe flow restricter, so the engine will idle normally with the brake active, but as the engine RPMs and therefore exhaust volume picks up so does the back pressure increasing the retardation effect. It more benign than the Jake brake and nasty habit of the Jake brake of locking the wheels at low speed/ off highway conditions is not evident.. The truck a classic Mack transmission combination for a 6x6-- a 5 speed transmission with a 2 speed drop box all in a single unit weighing about 1500 lbs. The gears are all straight cut making upshifting without the Williams Air Brake nearly impossible most of the time, because the truck stops quicker than the engine slows down so you can never get the speeds coordinated to 'get it in'.

This vehicle is particularly popular as an off highway tractor as compared to a highway tractor because it will go almost anywhere. With great ground clearance and substantial weight of its own, it will pull a 20 yard belly dump right over a wind row from a previous spread even uphill. The truck is heavy enough that the empty trailer can be turned around at will without getting stuck on 'wet grass' as a highway truck does with such a trailer. The Army is phasing these trucks out, because their new blitzkrieg battle plans call for a highly mobile military force that can go everywhere at 35 MPH. Since this behemoth will barely go 35 MPH empty on the level, it simply has no place in the Army. The entire vehicle is simply amazing, for its strength and durability. It has a double steel frame and drive axles that are simply large. As to why the Military calls this critter a '10 tonner', I don't have a clue. The Duece and a half got its name because it was rated to haul 2.5 tons off highway and 5 tons on the highway. None of the capacity ratings on this vehicle correspond to '10 tons'.

In trying to guess what its capacity rating might be in a civilian market the situation is likewise not all that clear. If you judge it by the carrying capacity of the 10 14x24 20 ply tires, you don't worry much about overloading it. While the rear ends are not a planetary drive, the brute size of them and the gearing speak of something with a much larger capacity than anything you would normally see on the highway. My guess is that in a civilian rating system, the tandems would have a 65,000 lb or more rating although the military rating is 50,000 lbs. They are classic Mack camel back tandems, but just much larger. The oversize wheels are required to fit over the axle hubs, and unlike the trucks that have a bolt down hub on the end of the axle, these have a splined plate behind a dust cover that transfers the driving torque.

The vehicle is listed with a light weight of just over 30,000 lbs, and is military rated for a payload of 30,000 lbs off highway and 35,000 on highway for a GVW of 60,0000 lbs and 65,000 off and on highway respectively.. In terms of a practical capacity, (I.e. what will it manage), the limiting factor, as with most military vehicles, is the engine horsepower. By this standard, if you keep the gross combination weight to the 100,000 lb range, it will handle grades to 18% in second gear (low range) at around 4-5 MPH, and will handle moderate grades under off highway conditions p to the 10 MPH range which is as fast as you want to go anyway. The official brass plate on the dash board is consistent with this. The Military rating is 80,000 "Towed load" on the off highway, and 120,000 lbs 'on highway' which seems reasonable in experience. With an 80,000 lb GCW one would hardly give a thought about driving anywhere that most of the wheels would stay on the ground. With weights beyond that, you really prefer something that looks like a road. If you double the gross weight to 200,000 lbs., predictably it goes up the hills half as fast, which means that you are in the big hole on every serious hill and on about a 20% adverse or a little steeper, it simply says, I don't wanna . . . . and beyond that you try to stay out of swamps and on pretty solid roadways..

Trucking' at 2.5 MPH is sort of boring, if you have a few miles of it to go, but the vehicle still responds fairly well, and you really don't get a sensation that it is structurally overtaxed. I've twisted out a few rear ends, drive lines and axles in my time in highway trucks applied to off road applications. When you start operating one over the limits of its structural integrity you can usually feel it. A front wheel may come up, and you can sense the drive train winding up and it is usually not much later that something lets loose if you don't let up first. This 10 tonner provides no sensation of that right up to the point of engine flame out, indicating to me that the vehicle has been successfully designed essentially so that it will run out of power before it will hurt itself.

All in all, with a 100,000 lbs. (which is about what it weighs with a 20 yard belly dump full of rock) one feels pretty smug about going 6 wheelin' where ever, but beyond that, one tends to get a bit choosy about where he goes, at least to the extent of trying to stay on a road..

For more pictures and views of various off highway trailers see the Belly Dump page, the Hayrack page, and the Lowboy page.

Tires, Wheels, and Brakes

The 10 hole Budd wheels for the 14x24 tires have a larger off highway bolt circle, but use the standard truck lug bolts. This is an oddity, because in civilian applications typically an oversized lug bolt is used with these wheels. The wheels are of a standard split rim variety, and you fix flats just like on any truck with split rims. You simply lay the tire down, grab the grubbing hoe and beat the tire back from the snap ring. Then you pry the snap ring off and flip the tire over on a block of wood and beat on the back side until it falls apart. The only real difference is that you need 2 men to flip the tire over or one really big guy. Unfortunately, new tires of that size run in the $500-$600 range, while Army surplus ones can often be found for around $200 each mounted on wheels. The downside of the surplus tires in that they tend to be old and the rubber is soft, resulting in rather bad tire life. Tread life for the rotten rubber military tires seems to be measurable in hundreds of miles not thousands.

M123A1C converted to off highway log truck

The braking system is somewhat unique. The vehicle, of course, has full air brakes with S-cams and even has front wheel brakes. The rear brake cans are a bit odd, but a standard BW can be bolted in place of the issue brake cans. One of the first modifications we made or betting one of these vehicles was to add spring lock brakes to one axle. Somehow the flip lever on the floorboard which operates a drive shaft parking brake on the back of the transmission did not impress us as an adequate backup braking system for a vehicle which in combination my weigh over 200,000 lbs. As big as this truck is, it uses the standard cab found on Deuce and a half's and 5 tonners which isn't all that big inside, which makes for some problems. Truck tractors classically have a Johnson bar hand brake handle on the right side of the steering wheel for activating the trailer brakes only, but this vehicle has a second 2-way Johnson bar on the left. It is sort of a knee basher when you get in the truck, but believe it or not this control is actually a steering brake control. That's right, this truck has steering brakes. Pull the lever one way and it sets the right rear brakes and the other way and it sets the left brakes.

The Cookie Cutter

As any trucker knows who has done any heavy trucking under difficult conditions, when the going gets tough, a semi truck tends to not want to steer. The torque dynamics of a hard pull have the rear truck wheels trying to drive out from under the fifth wheel which takes weight off of the front wheels, and additionally, the tandems have to slide sideways to accommodate a turn anyway, and there is a point at which the front wheels even with front wheel drive, simply can't get ahold of enough to turn the truck. Muddy conditions and extreme loads headed uphill are a couple of opportunities to this no turn syndrome to appear. Steering brakes solve this problem, but they must be used judiciously because they will fold you up quicker than you can say 'jack knife' if you give them a chance. A classic way to do this is to apply the steering brakes when going downhill under slick ground conditions while in front wheel drive. The following happens. The wheels on one side will lock and lose traction sliding easily. The rear wheels on the other side will consequently turn twice as fast. The front wheels will be unaffected and will turn at the normal speed and discourage the vehicle speed from increasing. The trailer will push on the back of the truck and help make it all happen. The problem with cutting cookies with a 6x6 is that you shortly meet the semi trailer going the other direction which spoils an otherwise thrilling ride.

The Fifth Wheel

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The standard fifth wheel that comes with these trucks is not usable in the civilian market because it uses an oversize kingpin. (The 5th wheel has been removed in this photo.) We've seen a few heavy off highway trailers equipped with this special oversized king pin, but they are few and far between. Besides adding spring brakes, accordingly, the other modification that had to be made was to replace the non-standard fifth-wheel with a more conventional one. This not to say that we put a standard highway fifth-wheel on. No way. There are Highway fifth wheels and off road fifth wheels. We rounded up one of the latter. The difference is that the off-highway fifth wheels have a left to right tilt in them whereas the highway models don't. In off highway applications the left/right tilt give the vehicle additional flexibility on uneven ground, but on the highway simply gives the trailer a chance to chatter. Because of the big tire size, the fifth wheel height from the ground is unavoidably significantly higher than that of a standard highway truck. This is why the belly dump sort of bobs along behind, but the added height improves the ground clearance is generally a positive feature.

Another standard feature shown in this photo is a heavy winch behind the cab. The Army assumes that the trailers will be laying on the ground, and one of the purposes of the winch is to drag the trailer up on the back of the truck. The winch can also be used to drag loads up on a beaver tailed trailer, or presumably if you get mired down in the muck, you can drop the trailer and drive the tractor ahead and then winch the trailer forward. The winch is designed for serious winching. It is driven with a full torque PTO off the transmission main shaft and has 3/4" rope. An engine throttle control and a forward and reverse air activated controls are provided for shifting the winch. The winch also has a level wind accessory designed to make the cable spool on the which evenly even if you are pulling off to the side. Some of these trucks were equipped with 2 of these winches, but alas the VanNatta model only has 1 winch.

Other Military equipment dealers with web pages include SECO in Augusta, Georgia.

The Transmission

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Off highway Logs

The transmission in this model is a story all by itself. It is a Mack manual transmission. It features in Mack tradition a 5 speed main box and a 2 speed transfer case integrated into a single package. The transfer case is a separate casting bolted to the back of the main box, but the output shaft is a single long shaft reaching through both boxes. The main box is of the constant mesh type in that the gears are always engaged and a small shifting spline slides to lock the proper gear to the output shaft. It is not a synchro-mesh transmission, in that the extra hardware to synchronize the gears is not present. Without an engine brake, the transmission is nearly impossible to shift because the truck slows down more quickly than the engine does, so that upshifts in particular are very difficult.

The point here, however, is to discuss the lubrication and oil leak issues with this transmission. This particular model transmission is notorious for leaking oil out around the input shaft, and thence into the clutch and bellhousing. We have don't some major explorations into the finer points of this issue and have our own conclusions which may or may not be correct but are based on the best information we can gather. Recognition of the leaking transmission syndrome is easy. As soon as you use the truck enough for the oil to heat up, it starts running out of the bottom of the bell housing profusely.

Solving the problem is less easy. First to describe how the lubrication system in this transmission works. It turns out that this transmission actually has 2 lubrication systems that really confuses the issue. There is a gear drive oil pump mounted on the bottom front of the transmission which connects to the lower cluster gear. This pump has external plumbing (copper tubing) which goes back to the transfer case. The oil intake is a screen in the bottom of the transfer case and the output is to the transfer case. It pressure feeds into the output shaft which is rifle drilled from the rear and oils a couple of gears that ride on the output shaft within the transfer case. This is critical because the winch drive PTO is connected to the back of the output shaft and when the transfer case is in neutral so as to operate the PTO only the top shaft in the transfer case turns and there is no splash to splash oil anything. Beyond that there is a small tube running forward which spays some oil from a hollow bolt onto the reverse and first gear in the back of the main box. That is all that the oil pump does, other than on some trucks, the oil on the way is routed through an oil cooler. It seems that the cooler was somewhere along the line a retrofit kit for this transmission, as was a temperature gauge whose sender unit was inserted into the drain plug of the transfer case.

The oil compartment of the main transmission and the transfer case are separate. Each has its own level plug and they are at significantly different elevations. The second lubrication circuit and the one that causes the grief is in the input shaft. There is a splash catcher high on the right side of the main box. This catch basin has a cast drain hole that runs forward and down and around to the front main bearing. As is common with manual transmissions the transmission input shaft is held in position by a large ball bearing in the front of the transmission case which in turn is held in position by a collar that bolts on the front of the transmission case. In turn the collar has a tube in it that sticks forward around the input shaft and the clutch throwout bearing assembly slides forward and backward on this. Of necessity there must be some sort of an oil seal around the input shaft to contain the oil that runs from the catch basin down into the front bearing. This oil which runs down in front of the front bearing also serves a secondary purpose. The input shaft is rifle bored and oil is inserted into the center of the input shaft in this area and pushed backwards to oil the needle bearings between the input shaft and the output shaft and is further pushed into the front of the output shaft to oil the second, third and fourth gears which ride on the output shaft. A small bore riding piston pushes the oil into the input shaft circuit. The sealing to the front is accomplished with 2 sets of spiral threads machined into the input shaft. The primary 'screw' is about half an inch long, and is supposed to screw the oil to the rear as the input shaft turns. It screws within a presumably close fitting surface on the inside of the collar that bolts over the front of the transmission. In front of the primary screw on the shaft there is a grove in the collar which contains a drain hole cast through the bottom of the collar to allow 'leak by' to drain off and run back into the transmission. In front of this drain hole is a longer set of screw threads which turn inside the tube on which the throwout bearing rides which are supposed to catch any oil that tries to escape forward and push it back toward the drain hole. What is clear is that this technology has not always worked as intended. After having one truck turn into a leaker, we outsourced a transmission replacement and got an 'official government rebuilt' transmission installed and found that it leaked just as badly afterwards as before. Not being happy with that result we assembled a collection of spares, junkers, takeouts and the like until we have a total of 7 transmissions for one of these trucks in out shop as once, and then we spent 2 days just looking at them. What we found was that none of the seven transmissions were the same. While they all had the same part numbers and the like, each one had been modified in some fashion during the life of the transmission presumably to address the leakage syndrome. Two of the seven transmissions had had restrictions driven into the oil feeder port so as to reduce the flow of oil getting into the area in front of the front bearing, and several others had various retro machining done to the drain hole in the collar so as to provide for enhanced drain off capacity. The rebuilt transmission which we had installed by the outsource shop contained a radical but dysfunctional modification. When we pulled it down to see why it leaked (It had a 1990 government rebuild tag on it), we found yet a different 'mod'. The primary screw seal had been grooved to take a synthetic seal and the secondary screw and was turned off the shaft. All of the same might have worked, but for the fact that the transmission had been reassembled with a badly worn collar which had destroyed the seal to the point that it didn't seat on anything. Apparently 'government rebuild' does not mean or include the installation of a new collar on the front of the transmission, and there are a large number of badly worn collars circulating around in the salvage parts channel, some of which have been modified to increase the drain hole size and some of which haven't. It seems that one of these collars can be easily destroyed by having the bell housing bolts come loose, permitting the transmission to become misaligned from the engine, and once this occurs there is not much holding the oil back from running out.

A combination that doesn't leak.

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After a lot of reflection and review of what didn't work as far preventing leakage around the input shaft of the transmission we finally made a command decision of what 'our mod' would be. The 'standard' drain hole for returning excess oil to the transmission is about half an inch long and no wider than a hack saw blade. Others we looked at had be attacked with a router and some were 3/8 inches wide and pretty much the whole system had been turned out on a lathe to make this possible. We thought about all this and went for an intermediate drain hole. We put a thin grinding wheel on a small die grinder and widened the hacksaw blade thick grove to about an either of an inch. Additionally we undertook to constrict the flow of oil into the front area by driving a short piece of 1/2 inch OD copper tubing into the rifle port which carries the oil from the catch basin to the front shaft area. Two of the transmissions we examined had had this done, but one of them was much more heavily restricted with only about a 3/16 inch passage open. That transmission was also out of service due to a front bearing failure which convinced us that was too much restriction.

With these modifications we put the unit back into service and have found that the leakage has been eliminated. We also applied 'Lock Tite' to all the bell housing bolts in an effort to assure that they don't work loose and spoil the repair. While our repairs have not stood the test of time yet in that they could result in a delayed lubrication failure the early observations indicate our work is successful.

Update: June 2000: We have considerable time on two trucks that had the transmissions repaired as described. Both repairs have been successful, and the leakage stopped. If we created a lubrication problem, it hasn't shown up yet even though we have worked the trucks pretty hard. As for tires, we finally gave up on the rotten Vietnam war surplus rubber and bought a new set of kicks for one of the trucks from an outlet for Denman Tire. The 14x24 Rock service tires are plenty spendy and are almost special order, but I like them already. The traction is much better. I don't know what the tire life will be yet, but it is obviously going to be much longer as the rotten surplus tires often came apart in days for a few weeks.

Dateline January 3, 2001:

Transmission repairs continue to be satisfactory. Heavy usage of new domestic rock service tires has been a success. In several hundred hours of heavy usage there have been 3 flats--one tire twice. All have been from tube failures, apparently from very small pieces of gravel mounted with the tires due to sloppy sanitation at the tireshop. An engine apparently gulped some water in the fuel and an injector seized. This caused the push rod to drop free and the cylinder went out of service. A fresh injector and a more careful water watch seem to have managed the problem. Loose bolts seem to be an ongoing issue. The right front stub axle bolts have come loose a couple of times as did the U-bolts holding a front spring. Exhaust brake usage has blown out a couple of manifold gaskets, and the huge power steering unit bolted outside the frame behind the left running board comes loose from its mounting from time to time. Even the vibration locking nuts don't seem to hold it down from the beating it takes. We've lost 2 air compressors on the engine now. Both apparently to the same fault. The rod bearing seized on the compressor crank shaft and broke the connecting rod. We assume this is a lubrication failure. The compressor on this Cummins sits in the back of the 'V' and the oil is fed up through riflings and into the back end of the compressor crankshaft. We are paying more attention to oil change intervals and are no longer idling the engine while loading the truck as a remedial measure.

As noted in a photo above a truck has been converted to an off highway log hauler. A Peerless trailer with a Page suspension makes up the set. The bunks are 10 feet wide and the stakes in the range of 6 feet high. The tires on the trailer are 12.00x24 tube type, typical of the off highway haulers made by Kenworth and Pacific in the 1970's. There are variants of the 12.00x24 that will handle near highway speeds unlike the 14.00x24 which should not be operated at speeds over 30 mph. All of this is not an issue in our application as the trucks don't make it beyond 10-15 mph and usually a lot less than that.

Dateline March 2002:

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The YKS Tire
The Denman Rock service tires installed about 2 years ago are worn out. The treads are worn down to where one would consider recapping, but the casings in general are trash. Of the dozen we bought 2 years ago, 3 have been scrapped, and a couple more have had sections. There are enough nylon threads sticking out through the tread to make it clear that recapping is not an option. These tires have lasted something less than 1000 hours, and have operated at an average speed of about 4 miles per hour. They have been on the truck pulling the 20 yard belly dump, and have had to work on severe adverse grades, as the logging roads are all engineered 'downhill to market', but the rock has to go the other way, so this truck has spent much of its time pulling adverse grades ranging to 15% and a little more here an there. This sort of grade with all the rock that will fit in a 20 yard belly dump---usually over 20 yards because I load it until it runs over the sides--gets you in second gear, low range, and in some places a real grunt and groaner in second gear. Although these tires present an amazingly large footprint, there is still significant slippage.

Our second effort at buying tires has been a set of YKS off road tires made in China. The tread looks a little harder than that of the Denman tire, however, only time will tell how it wears. I maintained inflation on the Denman tires at 55 PSI. This is well below the maximum for the tire (110 psi), but is consistent with what the Army uses, and meets the factory specs for that tire size considering the weight being hauled. The idea of the lower pressure is to provide a little more flexing, and a bigger footprint with less slippage. Unless someone convinces me otherwise, I'm inclined to experiment with a little more pressure in this set of tires---75 PSI comes to mind--to see how that works out. With tire costs in the range of a dollar a mile, I clearly need to change the formula.

Engine Failure

Dateline August 2002:

Well, I haven't had a chance to find out if the YKS tires are any good or not. The truck has been a hanger queen most of the summer. First it went through the shop for a burned out exhaust system, and broken transmission mounts, and engine mounts, and a perennial problem of a main air line leaking. Then it came back in the shop for an 'engine missing problem'. For that we gave it a compression test and found all cylinders pretty uniformly putting out just over 200 PSI of compression. A couple of the injectors were found to be clogged and after cleaning them the truck sounded a whole lot better.

Meanwhile a companion truck which has been pulling a hayrack for hauling short logs met a tragic end. First, it was inappropriately parked near a timber falling operation, and took a tree across the cab. This was a strong negative on the cosmetics of the truck but didn't do any structural damage to it. A few weeks later, however, a valve seat dropped out of one of the heads and got hammered into little pieces and driven into the top of # 4 piston. This cause the engine to knock and misfire. All things considered, that truck went to the junk yard.

Consequently, my trusty rock hauling 10 tonner got reassigned to haul logs. It brought in one load, and on the way out to the woods for the second load there was a tremendous commotion under the hood, and things were never quite the same.

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crumpled con rod and broken sleeve
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Distorted flywheel bolts, scored main bearing
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Crankshaft end---One dowel sheared and the other hole distorted.

We towed it home and pulled the engine for failure analysis.

Our findings are as follows:

  1. The immediate cause of failure was a broken connecting rod. The top came off the rod allowing it to depart the wrist pin. The piston was pushed into the head bending a valve and cracking the head, and the loose rod broke the skirt off the piston and the sleeve finally catching on part of the block and doubling itself up.
  2. The engine bearings were an interesting study. Although the engine was tagged for having been overhauled in 1983 by the army, both the main bearings and the rod bearings for dates of 1989.
  3. The rod bearings looked serviceable even on the cylinder that had the broken rod. The crank was clean and at worst the rod inserts showed that a very minor amount of metal had moved.
  4. Of the surviving rods, the wrist pins were loose enough on the rods that you could feel some slack when rocking the rod left to right. The wrist pins are splash oiled.
  5. All three center main bearings were ugly and into the copper. One main bearing was badly scored, though the crank itself was smooth.
  6. The Front and rear mains were better, showing wear, but not into the copper.
  7. The camshaft bearings were in poor condition. There was visible wear on the end bearings of the camshaft and the end bearings were severely worn. All the intermediate bearings had been hot enough to cause heat discoloration of the camshaft, and at least one intermediate bearing likely seized and turned.
  8. The flywheel was nearly ripped off the back of the crankshaft. In this engine an adapter bolts to the crank and the flywheel bolts to the adapter. The Bolt circle to the crank is augmented by two dowels. One dowel was sheered off and broken. The other dowel was bent and the hole in the back of crank egg shaped. All the bolts though tight and wired were bent and contained offsets of about an 1/8th of an inch in them at the sheer point. While the cause of the trauma to the flywheel mount is not completely clear, My guess is that the trauma of the connecting rod jamming against the block and bending itself double probably account for the deformity of the flywheel mount.
  9. The engine had an electric oil pressure gauge which I suspect was not profoundly accurate or reliable.
  10. The cylinder walls looked good and there was almost no ring ridge.

We have run the engine for about 7 years while accumulating perhaps 1500 hours on it. The engines known history included at least two other problems. The coppers around at least 3 injectors had leaked for the last 2 or 3 years allowing water from the water jacket to enter the injector bleed off circuit and ultimately ending up in the fuel tank. There as no evidence that it ever leaked to the crankcase. We managed the problem with radiator sealer and frequent draining of the fuel filters. We confirmed the diagnosis when we had the injectors out and found radiator sealer around the injectors.

The engine had a history of losing air compressors. The Air compressor sits up on top of the engine in the "V" and is gear driven off the camshaft gear. It is oiled via a rifle drilled crankshaft in the compressor which is fed from the main gallery which runs down the top of the engine between the valve lifters and the camshaft. In the early years I had this truck I had a habit of letting it idle for the 10 minutes or so that it would take to load a load of rock. After the second air compressor seized up and broke its connecting rod, I quit idling the truck and lost no more compressors.

Conclusions of failure analysis

This engine as two oil sumps. A smaller rear one and a very deep front one. It is small but drops more than a foot down. The high spot in the middle provides front axle clearance. It has tandem oil pumps. One scavenges the rear sump and moves the oil forward to the main sump while the other feeds the engine. Given the shape and depth of the oil sump, the engine would have to be virtually out of oil or tipped to the point where the wheels weren't on the ground for the oil pickup to suck air.

Since the oil gallery is almost directly above the mains with rifle drillings going almost straight down, contamination in the oil gallery would fall on the main bearings. The cam shaft is oiled with rifles between the main bearings and the cam bearings---so oil goes straight up to get to the camshaft which is directly above the crankshaft.

The central mains (one in particular) are scored up, but the rods which are fed from the mains are not. The oil gallery is fed from the ends, but neither end main bearing suffers.

My estimate of the situation is that when the engine was last rebuilt, some cuttings were in the oil gallery which dropped into the mains and cut them out fairly a long time ago; that this in turn caused low oil pressure during idle which cause the long standing issues with the engine of adequate lubrication--causing the compressor failures, and also accounting for the cooked camshaft bearings.

None of the lubrication or bearing problems is what killed the engine however. It died because a top came off a connecting rod allowing it to come free of the wrist pin. Those pieces are beaten up so badly it is not really possible to tell what happened, but since the wrist pin is splash oiled, and the pin itself looks ok, I can only surmise that for what ever reason, a Rod simply got tired and broke. My assumption is that the flywheel mounting damage occurred from a sudden stop when the engine came apart.

The only thing know for sure is that the engine is scrap. The block is ruined, the crank is ruined along with the camshaft. One head is cracked and both had leaking coppers. I salvaged the electrics, the PT pump, and the injectors.

The New Engine

There is always hope.

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V8-300 canned in pressurized storage

Faced with this dismal problem, I looked around on EBAY and lo and behold I found it. There it was on Ebay, a V-300 Cummins rebuilt by the Toole Army Depot some 17 years ago and stored in a sealed, pressurized steel container ready to pop into my truck. The container (which had a lid similar to the bottom half seen here) with its cargo weighted 3950 lbs and had to come clear across the US, but I got it. the engine likely would have been in better shapebut for the fact that a few years ago a City down in Florida who got the engine surplus pulled it out of the box and started the engine to verify it was good. Unfortunately, they didn't fully put it back to bed and left fuel in the PT pump so I got an engine with a gummed up stuck down PT pump, but this was a small issue as I happened to have a 'take off' pump from my blown up engine. Also it is not a big deal to get a PT pump scrubbed out and serviced. Seventeen years is a long time for an engine to sit around after an overhaul, but if one is ever going to survive, having it canned with some moisture absorbing bags is the way to do it. Other than the PT pump which was gummed up, I pulled one valve cover and looked and what I saw was as pretty as you please.

One might ask how I knew the PT pump was gummed up. Actually that wasn't hard to diagnose. You grab the throttle shaft on the side of it. It should turn freely. It is just held in the pump with a snap ring, and sealed with one small O ring. It should turn from stop to stop very freely. If it doesn't you have a problem. On a conventional inline engine you can remove the shaft with the pump on the engine and scrub the shaft down, but one the V-series the pump is inside the "V" so I had to remove the pump to get access to pull the shaft. Once I got the pump off, I found that the pump would not rotate freely either, suggesting that it could use a good case of dismantled and scrubbed out. My solution was to swap out the pump for a spare. Only time will tell if there are other problems with the engine, but...... so far so good.

Later: Requiem for this engine. About a month after I got it working, it was all over. The bottom radiator hose came off and I didn't notice it, and once all the water is drained out the temp. gauge doesn't show anything, and consequently I cooked 'er good. A sad and distressing ending. At one point we had 3 of these trucks running, but it was one less after that. When you run 'em hot until they seize up---you have a lot of scrap metal.

- - Updated 01/20/2013
- - Updated 04/26/2008
- - Updated 8/10/2002
- - Updated 11/30/2006
- - Updated 6/11/2006
- - Updated 03/27/2008