'39 Model B connecting rod shims

Pilot199

Member
Just replaced the full set of standard Clevite bearings on the engine & left the crankshaft standard. The main bearing caps were previously shimmed but the connecting rods had none.

Upon reassembly using the manual's torque specs. the engine is too tight & will not turn (even with a pry bar).

So the question is, what shims are typically added to the connecting rod caps?

Any help is appreciated.

Thanks,

Gary
 
Normally the stack is .010" thick. The stack consists of four shims glued together that are .0025" thick. Any good machine shop or an AGCO dealer can get them.
 

Too bad you didn't keep checking that the crankshaft would still turn freely as you snugged the caps down.

It can't do the bearing inserts OR the rods any good to get reefed up tight like that with no bearing clearance.

I would seriously consider having a machine shop check the ''big ends'' of the rods for ''out of round'' before proceeding.
 
Loosen all 4 rod caps two turns and then check rotation. See that all is free on the mains before you start working on the rods... Do one rod at a time, then RECHECK by rotation ....As mentioned, you might need a few thousandths of shims... They may not all or any need .010 .. As the crank journal wears, you remove a shim or two to get back to PROPER clearance..
 
You should be checking the journal diameter and bearing ID to verify a proper shim... then check with plasti- gauge to verify clearance... and you know the 4 rods do not all go in facing the same direction, right ?
 
Thanks Steve, yea the crank turned fine until we torqued the connecting rod caps. As we backed them off slightly, one at a time, the crank turned much easier. It turned fine with only the main caps torqued down.

We're going to get shims & plasti-gauge tomorrow & try it again.

Thanks for all the input gentlemen. I'll let y'all know what happens.

Gary
 
To set bearings that you need to shim is a bit of a process. First you finger tight the bearing cap with no shim. Measure the gap between the cap and the block or rod.
Inst all shims .001 to .002 less than this gap to give you crush or a tight fit between the bearing shells and the rod or block. Now with plasti gauge in snug up the
bearing cap good and check the clearance. If more than the spec you remove shims and file the bearing shell to reduce the clearance and maintain the proper crush. Always
keep the crank tight on the side of the bearing away from the plasti gauge for a good reading. Check your manual for proper clearance but I think mains would be .002 to
.003 and rods slightly less, .0015 to .0025. But check a manual.
 
If your installing new Standard bearings on a in pec used crankshaft and want a .002 clearance you will ost likely need to file the bearing ends.A used crankshaft will have a smaller diameter than when new. You will want about .002 bearing crush where the two ends meet. With the crankshaft being smaller you would get more than the .002 crush. The rod and caps will not move so the ends of the bearing haves will push in on the crankshaft acting as a brake when torqued to spec. Shims were in the packs of 5 per shim so .002 of the shim pack to be removed to take up the wear by the owner. I took pictures to show how I do this years ago and explained it in posts. I didn't add them under the pictures in my album.
Look Here
 
It is hard to get less than .002 on a crankshaft that is not fresh ground. That would be because without being fresh ground it will be slightly out of round at best. I always tried for the .015 on a fresh grind but couldn't get it often. It would take reconditioned rods without shims, I think anyway.
 
I respectfully think you know this stuff and probably good have lots of experence but did not understand what you would file? If the bearing were already tight then backing off and working with shims to get fit is going to work. If talking about filing the edges of the bearing would not that cause it to be tighter yet?
 
(quoted from post at 15:57:04 08/14/22) I respectfully think you know this stuff and probably good have lots of experence but did not understand what you would file? If the bearing were already tight then backing off and working with shims to get fit is going to work. If talking about filing the edges of the bearing would not that cause it to be tighter yet?
*** When both connecting rod shims are new (.010" thick) and installed and the rod bolts are tight, the connecting rod big end hole is round. New bearing shells installed in that round hole are crushed to a specific squeeze tension. If you start removing shims (.002" at a time) then the big end hole in the rod ain't round anymore, so you are crushing the bearing shells tighter than they should be. So, the pros file some off one side of each bearing shell to get the shell crush back into an acceptable range. How one knows when enough filed is enough is a black art. I've never worried about it because when I have a crankshaft reground and install new undersized bearing shells with new shims, there isn't anything else to do but assemble it !!!!! Everything is correct !!
 
With the bearing shell on a flat surface, surface plate, table saw even, band saw table, etc., round side up, slide it under a rigidly mounted dial indicator. It is
easy to see how much you are removing when filing the ends. File evenly so the shell doesn't rock when sitting on the flat surface.
 
Without shims the where the rod bearing shells fit it is .010 shorter than it is wide. New it would take a .010 shim pack to make it round with the rod cap tight. When you install the bearing shells with the caps just snug you would have .002 more clearance between the crankshaft pin and the bearings than you want. When the cap is torqued to spec you would then have the clearance you want. This is called (crush) If the crankshaft pin was was wore down .002 this would cause .004 crush. The rod and cap are heavy enough that they will not expand which causes the extra .002 of bearing shell to be forced out to the crankshaft pin keeping the crankshaft from turning. Filing .001 off each bearing shell will give you the proper .002 crush without forcing the bearing shell from out to the crankshaft pin.
 
Thanks so much for the explanation from you and Doc Allis. I remember seeing this once in the Service Manual when working on a WD that had run low on oil and spun a rod bearing and beat the Rod out of Round and ruined the journal on the crank shaft. With a Regrind and undersize bearing the Rod always got tight no matter how it was shimmed. It took a good used rod to get it tightened up right. I Mistakenly thought you were talking about Filing the Cap which some old mechanics did once in a while to Tighten up a wore out engine. Cleddy
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top