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Sun Dec 14, 2014 6:37 pm

I had my engine rebuilt an year ago but think that some things were not done well so i am preparing myself to tear down the engine and measure everything.

The first questions that comes are what is thrust cut and when is it needed to resurface each half of the case?

Thanks

Sun Dec 14, 2014 7:29 pm

You don't machine the case halves , all you do is check and make sure the mating surfaces are true and flat so they will seal , if you machine the mating surfaces you will have to do major machining and its very involved , most builders and machinist will torque the case halves together and check the crank bore for correct size and line bore the case if necessary , the thrust cut is don't on the flywheel end where the thrust bearing goes , if this area of the case is pounded and not correct in size it is machined for the proper thrust bearing , they come in sizes to correct worn out cases .

Sun Dec 14, 2014 7:43 pm

raul arrese wrote: You don't machine the case halves , all you do is check and make sure the mating surfaces are true and flat so they will seal , if you machine the mating surfaces you will have to do major machining and its very involved , most builders and machinist will torque the case halves together and check the crank bore for correct size and line bore the case if necessary , the thrust cut is don't on the flywheel end where the thrust bearing goes , if this area of the case is pounded and not correct in size it is machined for the proper thrust bearing , they come in sizes to correct worn out cases .

Thanks, i think i get it.

Sun Dec 14, 2014 7:52 pm

BeetleUy wrote: what is thrust cut and when is it needed?

Thanks

The front-most main crankshaft bearing in the engine (i.e., bearing #1) is also known as the "thrust" bearing. It is called this because, every time you press the clutch, you are exerting longintudinal forces (forward and aft) on the crankshaft, and something (in this case, the #1 bearing) must be there to accommodate those continually varying thrust forces. There are also other forces playing into this, but suffice to say, the acceptable "play", or movement, for the crank in this axis is about, say, .004 to .006 inches (some will say slightly different numbers).

Anyway, after years of use, this "thrust" bearing is getting pushed back and forth, and eventually, the two shoulders securing this bearing in its saddle begin to deform that part of the case, wearing it out. Once you have your engine apart, you can evaluate that area of your bearing - the interface to the crankcase - and determine if your #1 "thrust" bearing is loose, or wobbling in it's home position. Basically, the two shoulders of the bearing should install snugly into the saddle of the case. If the "standard" sized bearing no longer fits snugly, a machinist will "thrust cut" that portion of your case for you, and then you will purchase new main bearings (or at least the #1 main) with thicker shoulders on it, to accommodate the spaces where the worn material was machined away from your original crankcase saddle.

The .004-.006" of play referenced earlier (known as "end play") is adjusted between your new, properly sized main bearing, and your flywheel, using specially sized shims to allow you to adjust it (usually with a target of .004 to .006 inches).

Sun Dec 14, 2014 9:13 pm

You can also get the thick shoulder bearing and cut the shoulder of bearing to fit case.... Maybe not best solution, but it works...

Dale

Mon Dec 15, 2014 3:49 am

Thanks Guys

Dale M. wrote: You can also get the thick shoulder bearing and cut the shoulder of bearing to fit case.... Maybe not best solution, but it works...

Dale

Is it a thick shoulder thrust bearing?

Mon Dec 15, 2014 5:28 am

Yes- they come ins several over sizes and you can have the case cut to fit the bearing (most common) or you can just clean up the case and fit the bearing to the case- which many prefer as you get more rebuilds from and engine that way due to limited over size selection in bearings.

Mon Dec 15, 2014 5:34 am

raul arrese wrote: You don't machine the case halves , all you do is check and make sure the mating surfaces are true and flat so they will seal , if you machine the mating surfaces you will have to do major machining and its very involved , most builders and machinist will torque the case halves together and check the crank bore for correct size and line bore the case if necessary , the thrust cut is don't on the flywheel end where the thrust bearing goes , if this area of the case is pounded and not correct in size it is machined for the proper thrust bearing , they come in sizes to correct worn out cases .

I think I read that the factory would shave off .001/.002 of the case haves on some of their factory rebuilt motors and even had a code for the process stamped before the engine number.

Mon Dec 15, 2014 6:16 am

If a case is warped or worn to point that the case halfs must be trimmed to make it useable again, I would through it away and find another case.
I agree that machining the thrust bearing instead of the case is the best way. Doing it this way you can take just enought materal off to clean up the damaged surface, then machine the bearing to fit it.

Mon Dec 15, 2014 7:04 am

What exactly is it that makes u think this needs done. Have u or someone familiar with these engines measured the crank play, up/dn, end to end??

Mon Dec 15, 2014 7:47 am

Dale M. wrote: You can also get the thick shoulder bearing and cut the shoulder of bearing to fit case.... Maybe not best solution, but it works...

Dale
That's what I like to do !!