Oshkosh M911 8x6

Truck with Lowboy
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M911 with Hayrack
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M911 Lowboy

The Army has always struggled to come up with trucks suitable for moving their heaviest equipment. The M123 was a second generation heavy equipment transporter. The first generation was an M25 made between 1943 and 1945 of which the Army is said to have acquired 1372. The M123 likewise had its origins in the WWII when it featured an 844 Le Roi gas engine. It was reworked in 1966 to be the M123A1C with a 785 CID V8 Cummins diesel. It saw service in Vietnam primarily in Engineering units, often hauling a D-7 dozer around.

By the 1970's, the army found their tanks getting heavier, and their biggest truck based on a 30 year old design. The Military needs were complicated by the fact that they did not need large numbers of these biggest trucks making it difficult to justify the cost of designing a military transport from the ground up. Enter the Oshkosh M911. Oshkosh has been a specialty maker of all wheel drive trucks for a long time. Their name is familiar pushing rotary snow plows, hauling heavy things in the oil fields, and even as crash trucks at airports.

According to my information the Army acquired about 1200 of the M911's beginning in 1976 for the purpose of replacing the M123A1C. The technology of the truck was contemporary to that era.

For those interested in interior appointments, you will find the M911 pleasantly functional but still decently done. It has a reasonable assortment of instruments, including a combination speedometer-tachometer-tachograph. The electrics are all equipped with resettable breakers. The reset buttons are the hose of red buttons on the dash board and are labeled. Access is easy in two steps--first to the running board, then to the top of the battery box and finally inside.

Sharp eyed viewers will note the pedal where you would expect the clutch. As with other vehicles with a fully automatic transmission there is no clutch, but the extra pedal is the hydraulic retarder built into the transmission. Unlike a Jake brake, it is totally silent, but has good holding power. The transfer case is a 2:1 reduction, and for off highway work, low range is the order of the day. The Automatic transmission does not have a deep reduction low in it, and depends on the transfer case to provide the ratios for off highway work.

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Oshkosh M911 8x6
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Nose is serious
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M911 Interior
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OSHKOSH M911 in shop

The front wheel drive and the differential lockup in the divider box are activated together via a single air flip valve on the dashboard. The PTO for the winch drive is likewise an air flip valve. The large handle under the dashboard on the left is the Air lift axle. It is a variable pressure valve with a pressure gauge to report how much lifting pressure you have applied. The winches are controlled from a platform behind the cab. The hydraulic system is massive. It is a dual circuit pump plumbed in 1" hose and tubing. The fittings are a blend of JIC and O-ring connections. The two winch speeds are obtained by diverting the oil from both pump circuits to one winch. The hydraulic tank is behind the cab and holds probably 50 gallons of hydraulic oil. Given the size of the plumbing and the age, I would presume it is of the 'low pressure' type (under 2,000 PSI instead of the newer now in vogue 4-5,000 PSI hydraulic systems that we see today.

The specifications for the M911 are as follows:

Weight 39,951 lbs
Gross Vehicle Weight 88,000 lbs. rated
Gross Combined Weight 190,000 lbs.
Length 31 feet
Width (rear) 9-1/2 feet
Height 10 feet
Wheelbase 235"
Engine Detroit 8V-92TA at 450HP
Main Transmission Allison CLBT-750 Auto
Auxiliary Transmission Fuller At 1202 2 speed
Rear Axle Eaton DP650 @ 6.21 Ratio
Booster Axle 20,000 lb.
Front 20,000 lb driving
Tires Michelin 14x24 XGC Radial
Maximum speed 44 MPH
Max. speed at 3% grade 14 MPH @ 190,000 lbs.
Fuel mileage at max load 0.6 MPG @ max speed
Gradibility at max load 20%
Fording 36"
Winches 2 ea. Braden 45,000lb
Winch Drive Dual speed hydraulic

Beginning around 1990 the military began replacing the M911 with 4th generation H. E. T. (Heavy Equipment Transport) called the M1070. It has a lower profile single tire, and all 4 axles are driving axles. It also features a 6 man cab so the tank crew can ride in the the truck when it is hauling a tank. The advent of the M1070 set the stage for the Army to begin disposal of the M911's which is why we were able to end up with one. Because it is 9.5 feet wide it like the M123A1c is obviously an off the road vehicle. The major modification that we had to make to put this vehicle to use was to change the fifth wheel. Like the M123A1 the M911 is equipped with a jumbo fifth wheel designed to take an off road king pin size that is larger than the standard highway fifth wheel kingpin. We use a standard sized off road fifth wheel. They similar to the highway fifth wheel but have a joint in them for side to side twist. Such a fifth wheel will encourage the trailer to wander on the highway, but is a necessity for twisty offhighway applications. They come with blocks so you can disable the left right flip-flop for highway use.

Are they Better

The interesting question that we have been trying to evaluate is whether the M911 is actually better than the previous generation M123A1C. The answer is in what you are doing. When it comes to comfort and convenience there is no competition, the Oshkosh wins hands down. It has a Bostrom seat, a fully enclosed cab, heater, defroster, automatic transmission, half again more power. What More could you want? The down side is that even with the set back front axle the M911 is a barge compared to the M123A1C. The wheel base is nearly 5 feet longer, and it doesn't steer as short. The M911 has a conventional enclosed joint driving axle that looks like one from and older 4 wheel drive pickup on steroids whereas the Old M123A1C has a very complex double bevel gear drive with a rotating king pin which allows the tires to steer as short as there is tire clearance and for it to have additional ground clearance under the front Pot.

Lack of spares and age guarantee that the M123A1C's won't be around much longer so the choice is perhaps academic, but when it comes to threading a semi trailer down a crooked road, or going where there is mostly no road at all, the M123A1c is the clear choice, but alas, I think I have some road straightening to do.

Interesting features

There are some things to like about the Oshkosh. It has a pretty decent cab, a rubber floor mat, and a Bostrom seat. The power steering is strong enough, and entry is pretty easy, as you can climb to the running board, then on the battery box and finally get in. A jam switch in the door activates the horn if you open the door with the truck in gear. The wheels are jumbo sized budd wheels and not compatible with the M123a1c wheels which are an abortion wheel that is jumbo sized--but which uses standard lug bolts. For the Oshkosh you will use an 1 3/4" socket for the outer nuts and a 15/16" square for the inner Budd nuts.

The throttle is all air controlled, and consequently the engine will only idle until it gets up air pressure---sort of a guaranteed warmup.


It isn't rocket science to figure out why these vehicles have as standard the Michelin XGC tire. These tires are labeled for 'crane carrier service' and off road use. The tread is a non-directional knobby style. The tires are high speed tires and will operate at speeds of up to 50 MPH or so which contrasts to the 35 MPH speed which is the max speed of the heavy treaded bias ply tires.The tires are ideal for hauling tanks down the interstate, but the treads cut up pretty severely when subjected to heavy loads on steep grades. The tires are also kind of spendy----about a $1,000 each. For our purposes the traditional Bias ply rock service tire costs half as much and works better. The 14 x 24" rock service tube type tire is hard to find these days and your corner tire dealer is unlikely to have it in stock. Denman used to make the tire but has apparently dropped it. The last couple batches of tires that we have gotten in have been YKS branded from China. We haven't worn any of the YKS tires out yet so we don't know for sure how well they hold up, but preliminary results seem satisfactory.

Inside an Oshkosh

You don't have an old piece of equipment around for very long until you get to scatter it around the shop. One of our Oshkoshes was no exception. Transmission oil started disappearing, and engine oil began appearing. This is not a good thing, particularly when it was a lot of it.

The first thing I had to do was to learn more than I wanted to know about old Allison Transmissions.

Understanding Allison Transmissions

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Belly dump
M911 with belly dump
M911 with Belly dump
With Rock trailer
Truck with Lowboy

In the 1990's Allison introduced their 'World' transmission series which is electronically shifted. This isn't what you find in a late 70's Oshkosh. Allison made and older family of auto transmissions which appeared at 500, 600 and 700 series. The first digit of the number has to do with the size. The second digit should tell you how many forward gears it has. Within the 700 series which I am going to talk about, a leading letter of an "H" tells you it was intended for Highway use, and a "C" suggests off highway applications.

In the Oshkosh M911 you will find a CLBT 750 transmission. The "C" tells you it is off highway, the 'L' tells you the converter has a lock out in it so that about half way through 2nd gear a friction inside the converter is set eliminating converter slippage. Transmissions without the "L" in the model don't have this feature. The "B" in the model tells you that the transmission has a retarder which in the case of these transmissions is a 5 inch segment bolted between the bell housing and the main transmission body. The retarder consists of essentially a paddle wheel in a chamber which is normally empty, but when you hit the retard mode, it is pumped full of oil, and the paddle wheel has to sheer oil against a stationary set of opposing blade. Mechanical Energy is converted to hot oil. Being an input shaft retarder, the effectiveness of it is a function of what gear you are in. If you follow a rule of going down hills in a gear that you could go up them in, the retarder will serve you well and keep your foot off the service brake as it produces somewhere around engine horsepower. The "T" tells you its a transmission so that doesn't add much to your knowledge. However, the fact that the model is a 750 and not a 740, tells you that the back end of the transmission has an extra set of planetaries bolted on it which provide an additional deep low gear.

Still additional features that were available for this transmission that are not used by Oshkosh include a drop box on the back which allows direct support of front wheel drive, and an extra long bell housing on the front that allows the mounting of an engine powered PTO on top the bell housing.

This truck has a drop box and a PTO for the hydraulic powered winches but they are not on the transmission. Oshkosh put a mechanically shifted 2 speed behind the automatic which has the PTO, and then the drop box is still another box behind that.

As implemented in the M911, the CLBT 750 Allison Transmission is direct mounted to the back of a Detroit Diesel 8V92T. Transmission cooling is accomplished via a water to transmission oil heat exchanger mounted below the radiator which is plumbed to the water pump with a 2 3/4" combination of hoses and pipes. You will dig your 2" open end wrench out of your tool box to disconnect the Oil lines to this exchanger.

Since This transmission is not equipped for an engine driven PTO, the converter bell housing is 'dry' implying that there is not suppose to be any oil in it, and this was an important clue. You see, since the transmission cooler is an external critter there is no cross connection between the engine and the transmission by which transmission oil can get into the engine except via the rear main bearing seal in the engine and through the converter bell housing. One can think 'bad engine seal' but when you realize that converter bell housing is suppose to be a dry housing you get the message that the problem is more serious than that.

The usual diagnosis is a cracked or leaking converter. In any event it had to come apart. The question then became how and what. On a survey, it didn't take me very long to figure out the disassembly sequence. Removing the transmission and leaving the engine is not an option. For a host of reasons the transmission cannot go back as it has a close coupled 2 speed (on a very short drive line) right behind it, and it can't go down because the front axle,drive line, and even a frame member are all under it. I didn't want to take the cab off and try to take it out the top so it was coming out the front.

Fortunately, taking the transmission out with the engine is a breeze. Indeed after doing it I would never attempt to leave the transmission in this vehicle if it was the engine I wanted. In fact you can't. Engine and Transmission share a rear mount saddle, and by the time you have undone it there is nothing but the drive line holding the transmission in the truck. Consequently disassembly is as straight forward as can be. First you remove the hood, fenders, grill guard and radiator. Then you deal with all the wires and hoses going to the engine, (and the transmission). After undoing the mounts you swing the engine and transmission together right out the front. It turns out that the lifting hooks on the back of the engine provide the proper balance. I first removed the assembly with a spreader bar, but that isn't needed. With a 2 point lift on the back of the engine the assembly is slightly front heavy, and you need to lift the front a little to get the oil pan over the front motor mount, so for reinstalling I used my main sling on the lifting eyes on the back of the engine, and ran a small 'come along' to the front lifting eye. I used the 8,000 lb service truck boom for the job (with a snatch block), but in doing so one should position the service truck thoughtfully (and very close), and don't forget to set the outriggers. As I have said elsewhere, an 8,000 lb crane rating means that you can actually handle about 4,000 lbs, if you are careful and keep it close, and that engine and transmission assembly weigh every bit of that.

Once you get the engine and transmission on the ground then you have to separate them. This is no easy task. You need to support the engine so it won't fall over, and shift your crane to the transmission. These transmissions are flex plate connected, so you get to take out a pipe plug on the right side of the engine bell housing (on the front) --- the starter is on the left side find a circle of 12 bolts behind the plug which fasten the flex plate to the transmission. Once you have those, you can take out the bell housing bolts and complete the separation. In my case, I didn't find the expected cracked or leaking torque converter, though the 'dry' housing was full of oil. Instead I found the bolts behind the converter that hold the transmission bell housing to the retarder and to the main transmission housing loose which allowed the leakage. This is not a usual problem with this transmission making me think that the Army mental midgets who put the Army 'transmission rebuilt' tag on the transmission some years back forgot to use a torque wrench where they should have. The other irregularity was with the rear engine oil seal. It is a two part seal. There is a flat ring that slips over the crank shaft for the seal to turn against, and the seal itself. Those seals exist in both 1 lip and 2 lip versions, the latter being for wet bell housings. What I found was a 1 lip seal and no 'journal guard' on the crank shaft. This meant that the seal was sealing against the shaft itself instead of the composite slip on ring which was missing. The seal could hardly reach the shaft all of which explains when the oil went through it so quickly. You could almost pour oil past it.

The Allison transmission folks for a princely sum resealed the transmission and dyno tested it, and I reassembled the truck. With a helper it took about 2 days to put it back together.

CLBT750 vs. HT750

Many folks are more familiar with the HT750DR Allison and may wonder what the difference is. The models that start with "H" are intended for highway applications and the "C" series is a 'construction'/ off highway application. From 10 feet away they look the same. I'm told internally the valve body has some differences but the exterior view doesn't disclose anything of significance that is different. We have a 750DR in a White-Volvo Expediter and they appear the same, except that the HT750DR doesn't have the optional segment between the converter and the transmission that houses the retarder.

Both transmissions have identical specifications for maximum torque (1300 ft. lbs), and offer a variety of converters to match various engines. Presumably if you have a high horsepower, high torque engine, you wouldn't match it with a high torque rise converter unless you liked to hear the sounds of snap and crackle.

What you find in the HT750DR transmission is a deep reduction first gear (7.97:1 to be exact). This compares to a 5.182:1 ratio in the CLBT750. The good and bad here is that the CLBT 750 starts out in first and dynamiclly shifts in and out of first as needed. In the HT750DR, you normally start out in second gear, and first is a granny gear. You only get it when you manually put the shifter in first position, and it stays there until you manually put the shifter somewhere else. This ability to shift in and out of first as needed makes the CLBT750 a better off road transmission where extreme torque loads are the norm---even though it is not as deep a low. Oshkosh deals with this ratio limitation with a 2 speed manual box behind the automatic transmission. Conversely, on a highway application where just occasionally you need a creeper gear, or need to drag yourself out of a tough spot now and then, the granny gear is just fine.

Understand that both of these transmissions are 'history' at Allison these days. These hydraulic mechanical shifted automatics have been replaced with the Allison World class transmission family. The most significant difference in the World transmissions is that they have integrated electronics to manage the shifting. Electric selonoids control valves that open and close in response to electric signals generated by an on board computer. With the electrnic family of diesel engines, this has enabled a futher level of integration in that the engine and the transmission can talk to each other which improves the ability to calculate proper shift points.


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Taper Locks set
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20 pound Sledge
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Locks now Loose
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Taper Lock

Well, we have had a couple of these M911's around for nearly a year now. Reliability has been high. We pulled the lift axle off of one because we don't use it and the frame member that holds it up broke causing some grief. We have moved the fifth wheels forward 4 inches from center over the tandems. All things considered we would go more but tailswing constraints make this difficult. We also stuck 4 inch blocks under the fifthwheel elevating it even some more. This was to give our belly dump some tire clearance, as in uneven ground the rear axle of tires would come up and hit the bottom of the trailer frame and this is a great way to get stuck.

We have also learned to use the lockup when there is even a thought of a spin out. As noted, the lockup engages the front wheels and serves as a differential lock as well---the same control does both. If you spin out on a rock road with out the lock up set, the truck starts jumping and you can't stop quick enough. Even with a heavy load it will jump about 18 inches in the air and since this is only something that happens on a steep up grade with a heavy load and good traction you have the rubber bands in that auto transmission wound up really tight. I don't know why an axle doesn't break of something, but it hasn't yet, but we have learned to be more aggressive about using the lockup when you are on a heavy adverse grade with some loose rock.

Still Later

Just some random comments: The trucks come equipped with Bostrom spring suspension seats. That is a good seat for its day, but air ride seats have been around for 40 years, and as well as these things ride, an air seat would be a big improvement.

This is one truck that you never worry about running out of power. In absolute speed it is not terribly fast, but with well over 100,000 lbs gross, it will go up 15% grades about as fast as you can stay in the cab on a pit rock road. It will not only to up these hills, but with the retarder it will go down the other side with scarcely touching the service brakes. On the new generation auto transmissions Allsion has put the retarder on the back of the transmission. This means it works as well in high gear as in low gear, but if you are wanting to use the retarder in low gears, there is no substitute for having it on the front of the transmission. The hydraulic retarder is essentially a paddle wheel in the oil which converts mechanical energy to heat. What is really interesting is to watch the temperature gauge on the transmission oil. The sender unit is locate on the oil exit, before the oil goes to the oil cooler. When you tromp the retarder hard, you can watch the temperature gauge jump right up there. Indeed you can warm up the transmission oil a lot faster in the retarder mode than any other way. the converter has a lockup mode that locks the converter out when you achieve speed equal to about half way through second gear. Consequently when you start out in a hard pull, the converter will work for a while and you will see the oil temperature gauge wake up, but as soon as you make it to second gear, the converter locks out (that is the "L" in the CLBT transmission), and the oil temperature goes back to sleep until you stand on the retarder. The cooling system and heat exchanger in these trucks is truely massive and I have never come close to overheating the transmission, but you can sure make the transmission temperature guage jump with the retarder. Dateline 11/28/2004 7:23:46 PM

Detroit Diesel 8v92TA

I've been learning about these engines the hard way. It seems that they have a nasty habit of taking water in the crankcase--particularly the early ones. It turns out that the engine design was reworked to address some of the problems relating to taking water, and the engine was released as a 'silver 92' supposedly with all these issues fixed. Of course these Oshkoshes are old and came out with the early engines. You can tell them apart because of not only what the serial number is, but where it is located. The early engines have the serial number behind an oil line on the right rear side of the block below the exhaust manifold. The silver 92's have the serial number on the top front of the block on the right side. I've got good news and bad news. One of the two M911's we have has a silver 92 in it. It seems the taxpayers must have paid for an engine change sometime in the past. The bad news is that the other day the new one turned up full of water. Not just a little, but suddenly so much that the radiator went low and the chickens and geese came on.

I"ve been asking around as to where water comes from in these engines, and the answer is all sorts of places. On the front they have a gear driven water pump that can lose a seal and fill you up. On the back the air compressor can do the same thing. Otherwise, it is either an O ring on the bottom of a sleeve, a cracked sleeve (or one with a hole in it due to iron worms (corrosion), or a failure of one of the numerous o-rings that Detroit uses for a head gasket.

12/25/2004 9:40:07 PM

I'm still waiting for a report on the engine. Dropped the oil pan off and poured the water in the radiator. I didn't have to wait long to see the problem. Water was running down both ends of the engine (inside) in large volumes (enough to put a bucket under). With that observation the engine went to the engine hospital, and I don't have a report yet.

2/27/2005 9:04:58 PM

Well, the engine is back from the shop, $4000 later with an inframe overhaul. What was discovered with that the army depot screwed up a perfectly good engine. One of the heads hadn't been reasonable torqued down and this let the head gaskets assembly fail. Also a couple of cylinders had been badly honed when the Army had taken it apart so it got an inframe and should be ready to go. It wasn't warn out or corroded through, just messed up by bad mechanics work. It sort of fits the pattern as this is the same truck that had a transmission service because the bolts that hold the transmission segments together came loose (or were never torqued). The army has a habit of taking their equipment apart from time to time and inspecting it whether it needs fixed for not. Unfortunately with the quality of the mechanical work they seem to do I suspect they make more problems than they find and fix.

Sunday, March 19, 2006

Eaton DP650 Double Reduction Rear Ends

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Eaton RP650
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Frog and Spider Gears
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Spider Gears
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Parts book

Well it is back to the shop for the rock truck.

I was off getting a load of rock and when I when to pull away from the shovel, it didn't go right. Assuming that the tandems were '3 wheeled', I set the lockup which locks out the interaxle differential and engages the front wheels and went about my business. Later when I got up the hill, I kicked the lockup out and started down a little pitch, and the truck took off free wheeling. Then I knew I was in trouble. I got rid of the load of rock and diagnosed that the rear differential input shaft would turn but the wheels didn't and it made crunching and grinding sounds ocassionally jumping but not doing much.

The first order of business was to remove the rear drive axles. They are of the full floating type but not as easily removed as just undoing the nuts. Behind the nuts are 'taper locks'. I have some photos in the margins illustrating them. The problem with axle shafts is that they get turned and want to work on the stud bolts. The taper locks assure there is no slack around the studs so the axle cannot work. This is a good thing until you need to take them apart.

The traditional solution for getting an axle loose is a big hammer. I had two axles to remove. I tried blinking the axle on the end with a driving rod, and actually got one of them loose this way. You see the marks on this one from that attempt but it failed. I therefore increased the level of violence and applied a 20 lb. sledge hammer to the task. About half a dozen strong blows and the axle bounced nicely and the locks were all loose. While they didn't exactly fall off, I was able to screw them off the studs with a small pipe wrench. They don't have threads in them but having been clamped on a threaded stud, they think they do, so turning them makes them come off. The last photo shows what one looks like all by itself.

Removing the axles served two purposes. First it verified that they weren't broken (a possible cause of the rear axle not working, and they have to come out anyway in order to remove the differential. The axles were not broken, but did show cuttings on the splined end confirming what I by new knew, i.e. that there was a serious problem inside the rear differential.

The next task was to remove the differential from the axle housing. This is pretty straightforward and I was glad it the rear one, because the front one has divider box integrated into in and is under the fifth wheel anyway. I removed the trailer ramp from the back of the frame, to gain access on the top. The only thing in the way was the torque bar that keeps the axle from rolling. It was easily unbolted on this model and lifted out of the way. Then I had to disconnect the drive line by taking apart a universal joint which wasn't too difficult. These have bolt in caps, so you need a small chisel to release the locks and pull too bolts and then drive the drive line first one way and then the other to pop the caps out of the yoke. Next you wiggle the center part of the U-Joint out of the yoke and set the driveline off to the side. The Differential is held in place by a bolt circle which I made quick work of with a small impact wrench. IT is common to weld something on the top of differentials so you can hook them with a chain hoist, and this one already had a big washer welded there. I calculated the differential would come out the top which was a good thing because if you have to let it down you have to jack up the truck considerably because it won't slide out from under the truck otherwise. I was correct and it will come out the top, but you do have to jockey it considerably to get the job done.

As shown in the top photo on the left the differential was unremarkable in appearance when removed from the truck. In Eaton terminology the entire drive set is known as a DP-650. The 'D' tells you it is a dual drive, and the '65' tells you it is a 65,000 lb rear end. Of the two pots this is the Rear pot or the 'RP' unit. More interesting it is a 'Double Reduction' rear end. Pinion and bull gear are a 9 on 41 ratio producing a 4.55 reduction. However, inside of that is a planetary reduction that takes the gross reduction down to 6.21. In lighter applications this rear end can be a two speed rear end, with dual ratios of 4.55 nd 6.21. If you will study the parts breakdown you will see that the difference is that the two speed units have a sliding sun gear, which in one position will lock the planetary reduction out. However in the 65,000 lb configuration, the rear end is locked in the double reduction low ratio and there is no shifting fork.

Harking back to the photos on the left there should have been 4 spider gears in there instead of what looks like in the photo as crushed gravel. The all rotated on the frog which you see there broken. The largest piece of a spider gear we found was 2 cogs. The planetary gears were still intact but the carrier housing for the spider gears, though not grenaded was distorted and there is little salvage in this unit. The entire differential carrier assembly is pretty much toast. The yoke and pinion assembly and the bull gear are ok, but that isn't much considering the whole thing.

Dateline March 2009

When you hear a lot of crunching in the rear end and the drive shaft turns but the truck doesn't move.....

Well, it happened again. Same back pot same truck. Same story. At least this time we knew exactly what the problem was. The Rear end in this truck is the original design. It has been upgraded at least twice to try and make it stronger. Finding parts are a challenge but we located a supplier who was able to find a set of 'Series II' upgrade parts for the rear end. A set of 'Series III' parts could not be located, and reportedly they do require some modifications to the carrier for installation. The Series II is a bolt in upgrade, that is suppose to provide a strong set of spider gears. We hope so. Smooshing spider gears is not a good thing. Apparently though, it isn't surprising that the 2nd axle of a set of tandems is the one that fails. Experts say it's the one that works the hardest. Fortunately, it's fairly easy to change with a chain hoist down from the top to lift it up through the frame, but the costs are enormous.

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