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The AMC, FORD, & CHEVY Guy??!!
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Discussion Starter · #1 ·
Ok so my brother and I are rebuilding the top end of a '78 Chevy 350 truck block 4 bolt mains that was attached to a TH400. When we took the worn out factory timing chain set off to replace it with a double roller we noticed what appears to be an excessive amount of crank play. We didn't have a dial indicator with us so we just used a tape and the flexplate that is still bolted to the engine.

We measured the crankshaft endplay to be about 1/16" which is 0.0625." From everything I've read that is way too high. It looks really bad being able to freely move the crank fore and aft. The motor going to be mated to a manual trans and I would assume this needs to be addressed before we spin a bearing?

Off came the oil pan and everything looked good there. No metal filings. Motor ran fine in the truck without any noises.

From what I have been reading the #5 main has the thrust bearing in it. Can this just be replaced and everything should be ok or will this motor need a complete tear down of the bottom end and all the main bearings inspected?

The plan was to replace the 1.72" heads with 1.94" 76cc heads from a '74 truck. We have a dual plane weland intake & 600cfm edelbrock combined with a mild summit racing cam. At some point the low compression factory pistons were replaced with stock GM flat top pistons and it has a factory cast crank with cheap rebuild cast rods.

Thanks guys.
 

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78novaman said:
Ok so my brother and I are rebuilding the top end of a '78 Chevy 350 truck block 4 bolt mains that was attached to a TH400. When we took the worn out factory timing chain set off to replace it with a double roller we noticed what appears to be an excessive amount of crank play. We didn't have a dial indicator with us so we just used a tape and the flexplate that is still bolted to the engine.

We measured the crankshaft endplay to be about 1/16" which is 0.0625." From everything I've read that is way too high. It looks really bad being able to freely move the crank fore and aft. The motor going to be mated to a manual trans and I would assume this needs to be addressed before we spin a bearing?

Off came the oil pan and everything looked good there. No metal filings. Motor ran fine in the truck without any noises.

From what I have been reading the #5 main has the thrust bearing in it. Can this just be replaced and everything should be ok or will this motor need a complete tear down of the bottom end and all the main bearings inspected?

The plan was to replace the 1.72" heads with 1.94" 76cc heads from a '74 truck. We have a dual plane weland intake & 600cfm edelbrock combined with a mild summit racing cam. At some point the low compression factory pistons were replaced with stock GM flat top pistons and it has a factory cast crank with cheap rebuild cast rods.

Thanks guys.
Not a good sign, could be the bearing but usually the thrust face on the crank is out to lunch as well. You need to get a really accurate measurement to be sure.

Bogie
 

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As oldbogie said you need to get an accurate measurement.I copied a piece from a Car Craft mag a few years ago regarding crank endplay.I will quote(condensed) what it said in this Q&A article by Terry McGean;" We touched base with Mike Johnson at JMS Racing Engines in Monrovia,California for his input.He went on to explain that the .005-.007 inch clearance spec is the Chevrolet blueprint spec,intended for freshly built high performance applications.However,the wear spec given by GM-the one used to determine if a part is junk or not-allows for up to 0.020 inch.
Guy
 

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The AMC, FORD, & CHEVY Guy??!!
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299 Posts
Discussion Starter · #5 ·
Thanks for the replies.


I will work on getting a better measurement. I had been wondering what GM's "worn but ok" spec was.


New crank is not what I wanted to hear. This motor only has about 20,000mi on a rebuild. My dad paid good money to have a local shop fix it up about 15 years ago. Motor has been sitting for the last 10 years and was still clean as a whistle inside.


Luckilly we have a whole parts engine from a '74 pickup. Hopefully the crank will be good in that if need be.
 
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