M4A2 Sherman clutch shim question
 

We’ve got the two 6-71 Detroit’s out of the Sherman for resealing due to oil leaks.
Both clutches have shims between the housing and flywheel. Hopefully the sketch shows the location and the other photo shows a shim.

One clutch had one shim per bolt, the other clutch had two. The shims are 0.032” thick.

Cleaning up the heat checking involved removing a total of 0.020” from the flywheel and pressure plates, ie 0.012” from one and 0.008” from the other.

The only manual I can find that mentions these shims is for the M36, which uses the hull of the late M4 A2 HVSS. All it says is when friction disk gets down to 0.370” thick, remove the shims. Our friction disks are 0.455” and 0.425” thick so well above this minimum spec.
Trouble is I lost track of which clutch is which when we got then surfaced. Anyone have any info of how the OEM determined these shim thickness?

Thanks,
Malcolm

Attachment 122059

Attachment 122060

Malcolm,

My read is that both engines should have had 2 x 1/32” shims per bolt (for a total thickness of 1/16”)

All references seem to agree with two shims when total thickness of clutch driven plate(disk) is greater than .370”, and no shims when thickness is less than .370”.

Information from post-war civilian manuals

There is some discrepancy regarding minimum thickness of clutch driven plate (disk) which would indicate that it should be replaced.

In my prior post I included images from TM 9-1750G (Power plant maintenance) which indicate the plate should be replaced if it is less than .303”

TM 9-745 (M36B2 vehicle maintenance) agrees with the .303” thickness.

TM 9-731B (M4A2 vehicle maintenance) shows .340” as the minimum thickness.

Fantastic info, Colin! Thank you very much.

You’re right, all the info you gave me clearly states that, with the thickness of frictions I have, there should be two 1/32” shims per bolt. That’s what I’ll install. Mind you, I’ll have to make 12 new shims, which will be a pain. Wonder how much laser or water jet cutting costs??

And the Detroit manual gave some interesting info. The shims aren’t to control the diaphragm spring pressure on the pressure plate, as I had assumed, but to keep the release bearing in its working range. I assume that allows the bearing to release enough to allow the spring to apply full pressure and to pull spring back enough to let friction plate spin freely.

Thanks again. I love MLU.
Malcolm

When I google the shim number listed in Colin's part manual, it does come up as a shim for detroit diesel. Might be worth a try asking the local detroit dealer. The number seems to be expanded a little to D81260-755429 SHIM.

Thanks, Rob. Will do.

It looks to me that the thrust bearing pulls the center of the spring (diaphragm) (away from the flywheel) to release the clutch? The reverse of "normal" If so, then you are still getting full spring pressure, but without the shims, there possibly won't be enough travel at the thrust bearing to properly release.

The rule about Detroits is if they are not leaking, they are out of oil. This applies to Harley Davidsons as well, and most of the older Chev engines.

The old timers always said, "the only thing keeping the oil in was the paint".

Are there match marks on the parts? Supposed to be, but my clutch didnt have any either.

I don’t know, Lynn. When I tighten the bolts clamping the clutch housing above to the flywheel I can feel the torque increasing from the diaphragm spring loading up. If I had 1/16” of shims between housing and flywheel, there would be less compression on the spring when bolts tightened down.
Malcolm

The engine compartment was filthy, it was embarrassing, and it was miserable to work on.
It was time. :)

You mean match marks to identify which parts come from which engine? Didn’t know that so I haven’t been looking for them.
Malcolm

The match marks are usually for keeping everything balanced, but if marked differently could give a clue to origin.

Malcolm, so based on what you have said, then the purpose of the shims might be to prevent over bending the spring and risking breakage ( as well as the travel)

I have to confess, guys, since we removed a total of 0.020” total from both the flywheel and pressure plates to clean up the heat checking, I decided to install both clutches with just the one 0.032” shim per bolt. My justification is that my experience working on GM engines and transmissions is that the design engineers were way too anal about setting allowable specs and wear tolerances. I defy anyone to notice any difference between a 6-71 running with spec 0.016” valve clearance (4 valve head) and say 0.018” clearance.
Also the GM spec of 1.460” for the injector timing, with no tolerance specified, has at least one guy on YouTube making the effort to get it that exact spec, I seriously doubt if +/-0.010” makes an detectable difference.
If we find out I’m wrong on the shims when we get the engines back in the Sherman, I will of course not be posting it here.
Malcolm

The clutch release bearings have been installed. And also the transfer case inner housing with the release forks, and the outer housing.
Much to my relief, there was some free play on both clutch release forks, indicating that the setup with just one 1/32” shim under each clutch cover bolt was allowing the release bearings to unload.
The sketch in post #1 above above shows that if bolting up the transfer case had left no free play on the clutch forks, we would have had to pull the t/c off and add more shims to move the diaphragm spring inner bore towards the flywheel.
Malcolm

Good effort Malcolm. Nice to see you catch a break. :salute: