1951 M37 with engine knock

[daved]

Help gents, I am looking for some lessons learned on similar problem as I am beginning to run out of ideas.

I purchased a M37, beautiful truck, but it has a distinctive, one knock per crank cycle knock. Previous owner says he has had ith this way for 26 years and drove it over 1800 miles in parades. Compression from #1 back is: 92, 88, 78, 78, 87, 90. Oil Pressure is 35-45 PSI. No Smoke, Starts right up, electric fuel pump and mechanical pump removed. Runs pretty good, tops out at 35 MPH, no misses or backfires. Engine is still installed as I desire to rid the knock without pulling it, hopefully!!! I have checked, adjusted and reset the Intake and Exhaust tappets at 9 and 11 thousandth, replaced the connecting rod bearings which had a little excess spacing and appeared overtightened, were .020 undersized matching depot plate on side of engine. I listened to the engine with metal rod, hard to locate knock. I guess my next move is to pull the flat head and perhaps intake/exhaust manifold to view and check for:

1) Cylinder sleeve wear for possible broken wrist pin, damaged piston or broken ring
2) Damaged or bent valve or valve seat, not exactly sure how to do this?
3) Look for broken valve spring, not sure how to do this either?
4) faulty head gasket for compression disparity between cylinders
5) worn cam lobe, no idea how to check without engine removal and disassembly?

Appreciate any thoughts or guidance on the steps mentioned above or other ideas. I would prefer to fix this issue with engine in truck. Thanks again, v/r dave

 

[daved]

Help?

 

[TGP (IL)]

Sounds like it needs to come apart.
Your compression readings are low.

Hard to say about a knock we can't hear.
Most likely a broken piston skirt or wrist pin
Walking out.

I'm not going to say it's a common problem
But have seen it on more than one engine.
Tom

 

[dmetalmiki]

Try taking off one spark plug lead at a time with the engine running. And listen for a change in engine noise.

 

[teletech]

You have a few issues and it's time to at least yank the head.
The compression is too varied. You need a valve job or rings.
The top speed should be 50+MPH
The knock... if your oil pressure is good, almost certainly a wrist-pin or what it ties to (piston/rod). that can be checked when you pull the head.

 

[NDT]

Crank journal out of round is a possibility and can be tricky to find. Use Plastigage.

 

[T. Highway]

Dave,

Cold tappet clearance should be .010" on intake and .013" on exhaust with properly lapped or renewed tappet screws. If not reworked they should be both set .001" tighter.

To me it sounds like you need to pull the head and take a look at the cylinder walls. If nothing really jumps out as being wrong you will need to drop the pan. Both are easy jobs.
The compression is soo low I'm kind of surprised it ran at all. This truck should run up to 47 miles per hour before the governor kicks in and slows you down. Is the engine a stock T245 or a transplant of something else?

Bert

 

[Gunnermac]

I have an M 43 that had motor troubles, apparently those motors have a number of issues. We fixed it with a small block Chevy motor. Made an enormous difference. Not a hard swap if you don't mind it not being original.

 

[GUNNY 155]

I would pull it and tear it down. It may run but you could be facing a catastrophic failure. A knock is never a good thing. These engines were also poorly balanced so consider having that done when you have it apart. Check the TM's for valve clearance. T Highway is correct that yours are to tight but seems I remember that there was a modification order that may have opened them up a little more. Some how .012 and .014 cold comes to mind but since I am an old guy check the manuals.