M1078 cracked block

[markgjsquires]

Was under my m1078 today getting ready to reinstall my oil pan when cleaning up the surface to ensure no gasket material was left and noticed two. Identically placed cracks on both sides of the engine in matching locations. I have never had any mechanical trouble with the truck the engine always ran great when I got it running. Just was fighting some electrical demons. Just wanted to share my unlucky find so others can look for a potential issue as well.

 

[Awesomeness]

Was under my m1078 today getting ready to reinstall my oil pan when cleaning up the surface to ensure no gasket material was left and noticed two. Identically placed cracks on both sides of the engine in matching locations. I have never had any mechanical trouble with the truck the engine always ran great when I got it running. Just was fighting some electrical demons. Just wanted to share my unlucky find so others can look for a potential issue as well.

Most likely your driveshafts are not balanced. Read the document here in my signature, for a ton of information about it, including links to old posts. I had the same thing happen to me, and had to replace the engine.

 

[Towbarman]

Was under my m1078 today getting ready to reinstall my oil pan when cleaning up the surface to ensure no gasket material was left and noticed two. Identically placed cracks on both sides of the engine in matching locations. I have never had any mechanical trouble with the truck the engine always ran great when I got it running. Just was fighting some electrical demons. Just wanted to share my unlucky find so others can look for a potential issue as well.

I had run into a situation like that some years ago with the M1078 LMTV engines. There was a crack at the back of the engine oil pan and Transmission bell housing. Company said at that time it was a casting issue. Haven't seen one since. Recently I had 2 engines come in on work orders for oil leaking around the oil filter housing. During the inspection I notice a crack on the housing under the turbocharger. Both engines in the same place.

 

[Ronmar]

Most likely your driveshafts are not balanced. Read the document here in my signature, for a ton of information about it, including links to old posts. I had the same thing happen to me, and had to replace the engine.

That and operating them well outside the max RPM vs operating angle spec...

 

[coachgeo]

Was under my m1078 today getting ready to reinstall my oil pan when cleaning up the surface to ensure no gasket material was left and noticed two. Identically placed cracks on both sides of the engine in matching locations. I have never had any mechanical trouble with the truck the engine always ran great when I got it running. Just was fighting some electrical demons. Just wanted to share my unlucky find so others can look for a potential issue as well.

Balance drive shafts as others say...... then pin it (old or modern version of lock and stich) -run it -put checking it out at intervals recommended..... some have run pinned blocks for years and years.

 

[Skyhawk13205]

That and operating them well outside the max RPM vs operating angle spec...

I have noticed that I do not see very many FMTV/6x6 reporting the same issues of, torque converter bolts, c6 thrust bearing, cracked blocks, driveshaft seperatation, and other catastrophic failure that are present on the lmtv/4x4. It may be causation/correlation theory but a possibility that most of the problems we see are due to a driveline vibration specific to the 4x4. A good example of how vibes destroy machinery is from the Junkers vibration test. When I got my truck from auction the drivelines were within the TM spec of lash in the slip yoke but there was a noticeable vibration at 50+ until I got the drivelines rebuilt.

 

[markgjsquires]

Thanks everyone for all of the insight. By the sounds of it there are some good fixes that can be done with some time and patience that don’t include a new/replacement engine which is what I was afraid of. I’ll get the driveshafts removed and sent to a local driveline facility and have them balanced/checked and see how bad they were. (Hopefully) they come back to say they were off so that I can assume that’s the issue making the block crack then will tackle the block repairs. I’ll keeping posting to the thread to let everyone know of my findings and what/how I did to repair the block.

 

[Ronmar]

I have noticed that I do not see very many FMTV/6x6 reporting the same issues of, torque converter bolts, c6 thrust bearing, cracked blocks, driveshaft seperatation, and other catastrophic failure that are present on the lmtv/4x4. It may be causation/correlation theory but a possibility that most of the problems we see are due to a driveline vibration specific to the 4x4. A good example of how vibes destroy machinery is from the Junkers vibration test. When I got my truck from auction the drivelines were within the TM spec of lash in the slip yoke but there was a noticeable vibration at 50+ until I got the drivelines rebuilt.

As you increase the driveshaft angle you increase the longitudinal vibration. This has nothing to do with the balance of the driveshaft, and a perfectly balanced driveshaft will experience it. It is a physical phenomenon brought on by changing the plane of a driveshaft operation(angle) using only the 4 bearing points of the u-joint. The greater the angle, the greater the longitudinal displacement. The greater the mass of the driveshaft, the greater the force from the displacement.

Standard gearing produces ~3300 RPM @ highway speed. At that RPM our max angle should be no more than ~5.5 deg. We run closer to 9 deg… At 9 deg, our max RPM should be under ~1750 RPM…

on the 4X we run a 61” rear DS. That is the maximum specified 1 piece shaft length for the type 16 DS, so we have the largest mass you are going to see in this type driveshaft. The front shaft has about the same angle, but is shorter(less mass).

the 6X has a raised pinion due to the power divider in the front axle right? Much shallower driveshaft angle and reduced longitudinal vibrations… The later A1’s with the raised rear pinion also are better configured…

This is one of the biggest benefits of eco hubs, it brings highway DS RPM down to ~ 1650 RPM And inside the max RPM at our angles… also cuts engine RPM in half at cruise…

From the dana/spicer website…

 

[markgjsquires]

Interesting point. All of the mathematics and theories involved in driveshaft angles that I’d have never thought about. So regardless of driveshaft balance the angles the early M1078’s driveshafts run at is more so the culprit to the vibration issues. Granted I seldom ever held the truck to the floor when I would go on a long drive but I always found 45 mph was the right speed for me at least where I felt minimal vibrations and didn’t get too many middle fingers when passed. I will still plan on taking the driveshafts for balancing just for the sake of why not if I’m this far into it. This is the first and only M1078 I’ll own. So might as well just go all in. We have a fleet of M35a2s on the farm. Just something about military trucks I can’t get enough of…

 

[GeneralDisorder]

Put ECO hubs on it and you'll be much safer in terms of driveshaft vibration with the older low pinion rear differential.

To be fair you have an early A0 truck and the early M35's weren't any better (considerably worse actually) in terms of reliability and teething issues. You have the A2 variant of the M35 and the A3 variant was even better. You are comparing the first generation of the product to the second-to-last generation of another era of product. Not really fair. The A1R and A1P2 are considerably better and have won awards from the military for being the most reliable things they have ever owned. I wouldn't be so fast to rule out owning other FMTV's - just know the faults of the early trucks and avoid them or upgrade them before they turn into the engine falling off the bell housing.

 

[markgjsquires]

Fair enough general. And I wasn’t trying to compare my M1078 to M35a2’s. Just merely the deuce and a half was something I fell in love with as a kid and having never owned anything other than those I got the chance to purchase my lmtv for the right price and have been learning about them as I go. Of course frustration takes over when unexpected bumps come up but like any of them there’s always a fix. I had an old CUCV that used to pull my hair out with at times but still a fun truck to have owned. Being I can turn my own wrench helps just lack much extra time. Thanks for the hint on the ECO hubs. Had never come across those but if my block repairs go successful I’d say those are my next upgrade
.

 

[coachgeo]

Fair enough general. And I wasn’t trying to compare my M1078 to M35a2’s. Just merely the deuce and a half was something I fell in love with as a kid and having never owned anything other than those I got the chance to purchase my lmtv for the right price and have been learning about them as I go. Of course frustration takes over when unexpected bumps come up but like any of them there’s always a fix. I had an old CUCV that used to pull my hair out with at times but still a fun truck to have owned. Being I can turn my own wrench helps just lack much extra time. Thanks for the hint on the ECO hubs. Had never come across those but if my block repairs go successful I’d say those are my next upgrade
.

block repair will do nothing but add life to short life. Not sure how much.... bit of a crap shoot. Id be tossing money in savings account for replacement engine and keep eye on the block to watch cracks after pinning. IMHO plan on an engine that is easier to move closer to 300-330hb with out super degrading of engine life... and non computer to reduce hasssle with matching to Allison. Likely Cummings.

 

[DeMilitarized]

As you increase the driveshaft angle you increase the longitudinal vibration. This has nothing to do with the balance of the driveshaft, and a perfectly balanced driveshaft will experience it. It is a physical phenomenon brought on by changing the plane of a driveshaft operation(angle) using only the 4 bearing points of the u-joint. The greater the angle, the greater the longitudinal displacement. The greater the mass of the driveshaft, the greater the force from the displacement.

Standard gearing produces ~3300 RPM @ highway speed. At that RPM our max angle should be no more than ~5.5 deg. We run closer to 9 deg… At 9 deg, our max RPM should be under ~1750 RPM…

on the 4X we run a 61” rear DS. That is the maximum specified 1 piece shaft length for the type 16 DS, so we have the largest mass you are going to see in this type driveshaft. The front shaft has about the same angle, but is shorter(less mass).

the 6X has a raised pinion due to the power divider in the front axle right? Much shallower driveshaft angle and reduced longitudinal vibrations… The later A1’s with the raised rear pinion also are better configured…

This is one of the biggest benefits of eco hubs, it brings highway DS RPM down to ~ 1650 RPM And inside the max RPM at our angles… also cuts engine RPM in half at cruise…

From the dana/spicer website…

View attachment 906424

This is just another benefit to the ecohub concept. It reduces driveshaft speed by half brining it closer into spec.

 

[TheRealJohnnyB]

I have noticed that I do not see very many FMTV/6x6 reporting the same issues of, torque converter bolts, c6 thrust bearing, cracked blocks, driveshaft seperatation, and other catastrophic failure that are present on the lmtv/4x4. It may be causation/correlation theory but a possibility that most of the problems we see are due to a driveline vibration specific to the 4x4. A good example of how vibes destroy machinery is from the Junkers vibration test. When I got my truck from auction the drivelines were within the TM spec of lash in the slip yoke but there was a noticeable vibration at 50+ until I got the drivelines rebuilt.

Is it the idea that the driveshaft is at too extreme of an angle on the 4x4's but pretty straight on the 6x6's?

 

[Ronmar]

Is it the idea that the driveshaft is at too extreme of an angle on the 4x4's but pretty straight on the 6x6's?

I am not sure what the rear shaft angle is on the trucks with a raised pinion, but Yes, steeper angle of the lower pinion axles, coupled with the ~3350 driveshaft RPM at 60MPH. At ~9 degrees on the older 4x4 trucks, highway RPM is double what the engineers say a driveshaft should be spun at. they are being spun at the same RPM, and If the raised pinion angle is over 5.5 deg, they are still outside the max rpm vs angle spec, but less angle = less longitudinal vibration/force/stress…

 

[TheRealJohnnyB]

I am not sure what the rear shaft angle is on the trucks with a raised pinion, but Yes, steeper angle of the lower pinion axles, coupled with the ~3350 driveshaft RPM at 60MPH. At ~9 degrees on the older 4x4 trucks, highway RPM is double what the engineers say a driveshaft should be spun at. they are being spun at the same RPM, and If the raised pinion angle is over 5.5 deg, they are still outside the max rpm vs angle spec, but less angle = less longitudinal vibration/force/stress…

Have you seen shims used on the rear axel to help compensate and lower the angle? Makes me reconsider to pick up a 6x6 for the long haul....

 

[Ronmar]

Have you seen shims used on the rear axel to help compensate and lower the angle? Makes me reconsider to pick up a 6x6 for the long haul....

you want the input and output pinions very close to/at the same angle, so the only reason you would use shims is to put the two pinions at the same angle. The only way to reduce driveshaft angle is to raise or lower something…

you could put a high pinion axle in the rear end(raises the rear pinion) like they did on the A1R 4x4. You could lower the engine and transmission a little…

Of course you can install eco hubs and bring the driveshaft RPM back to an acceptable level For the angle we currently operate at.

 

[GeneralDisorder]

From what we can tell the A1R LMTV rear axle is literally the same housing as the MTV intermediate axle. It's got slightly different guts with no power divider and the output where the rear driveshaft would otherwise be located is blanked off. The TM refers to this axle as the "top feed" variant.

If you can find one (extremely difficult), they also require a different driveshaft or having the existing driveshaft shortened. The low pinion driveshaft has an overall length of 150.11 centimeters, while the top feed variant on the LMTV is 141.22 centimeters, and the MTV driveshaft from transfer case to intermediate axle is 135.64 centimeters.

The angle is much less. Probably 1/3 of the angle the low pinion 4x4 axle runs - if I had to guess from working on my truck (recently replaced my u-joints).

 

[Ronmar]

My low pinion DS is currently setting at 61” or 155.25CM, but I have no weight on a bare chassis right now so my suspension is extended a bit high pinion being 1/3 the angle(~3 deg?) and a few inches shorter(less mass) would definitely reduce the longitudinal forces. Under 5.5 degrees would even bring it back inside max RPM…

 

[GeneralDisorder]

Yeah I imagine they measured it with it fully compressed. Those measurements are from the part number specifications.