Bore to piston tolerances, crankshaft etc. - Team Camaro Tech
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post #1 of 13 (permalink) Old Aug 7th, 11, 05:39 PM Thread Starter
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Bore to piston tolerances, crankshaft etc.

Hello fellow members,
I know there are many of you that are engine builders.
I would like to know the tolerances held on a rebuild engine.
The shop is boring .080 over on my big block and I'm going to inspect the work done. I need to know how much oversize the bore diameter should be from the pistion.
Also the what are the crankshaft specs.
Thanks in advance
-Bill
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post #2 of 13 (permalink) Old Aug 7th, 11, 06:44 PM
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Re: Bore to piston tolerances, crankshaft etc.

.080 OVER! Whoa! Are they sonic testing this block to see if it can handle that much overbore?

You should be asking the builder what tolerances he is building too. Clearances and tolerances should be the same as factory. That's what you do when you rebuild an engine. Bore to piston clearance depends on the type of pistons being used, cast, forged, hypers. David Vizards book "How to Rebuild Chevy Engines" will be worth the money for the things you will learn.
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post #3 of 13 (permalink) Old Aug 7th, 11, 06:46 PM
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Re: Bore to piston tolerances, crankshaft etc.

Bill,

Your piston to cylinder wall tolerance will be determined by what type of piston you use. Typically a forged piston has a bit more clearance than does a cast piston, or a hypereutectic. The thing to do is contact the piston manufacturer directly and do what they recommend, ask them also about which ring pack they like and the ring gap.

As for the crank bearing clearances, on the mains I go around .0025" and up to .0035" on the thrust bearing. Rods are about .002".

With the bearings I would also call the manufacturer and get their recomendations, and oil preferences too.

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post #4 of 13 (permalink) Old Aug 7th, 11, 07:17 PM Thread Starter
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Re: Bore to piston tolerances, crankshaft etc.

Quote:
Originally Posted by The Brilliant Idiot View Post
.080 OVER! Whoa! Are they sonic testing this block to see if it can handle that much overbore?

You should be asking the builder what tolerances he is building too. Clearances and tolerances should be the same as factory. That's what you do when you rebuild an engine. Bore to piston clearance depends on the type of pistons being used, cast, forged, hypers. David Vizards book "How to Rebuild Chevy Engines" will be worth the money for the things you will learn.
I should have said I'm going with a 402 build kit on my 396 big block
-Bill
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post #5 of 13 (permalink) Old Aug 7th, 11, 07:48 PM
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Re: Bore to piston tolerances, crankshaft etc.

No difference. Chevy called the 402 a 396 for years.

You better have that block sonic tested before overboring it .080. They also need to use a torque plate when boring and honing. Insist on it, IF the block can be overbored .080.
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post #6 of 13 (permalink) Old Aug 7th, 11, 08:39 PM
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Re: Bore to piston tolerances, crankshaft etc.

I should have said I'm going with a 402 build kit on my 396 big block
-Bill

A 402 is only a 396 bored .030" oversize. Where do you come up with an .080" bored 396 to make it a 402? A 396 bored .080" over will more than likely run hot because of thin cylinder walls. Only the very early 65-66 396 blocks were thick to bore this much with no problems.
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post #7 of 13 (permalink) Old Aug 7th, 11, 09:15 PM Thread Starter
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Re: Bore to piston tolerances, crankshaft etc.

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Originally Posted by 1962vette View Post
I should have said I'm going with a 402 build kit on my 396 big block
-Bill

A 402 is only a 396 bored .030" oversize. Where do you come up with an .080" bored 396 to make it a 402? A 396 bored .080" over will more than likely run hot because of thin cylinder walls. Only the very early 65-66 396 blocks were thick to bore this much with no problems.

Well, I am not an engine builder and I'm just trying to learn what are the safe limits.
I do not want to make a big mistake not knowing this critical information.
I'm assuming a 396 should not be bored over .060...
Is this correct and if it was bored more than .060 over it needs to be sonic tested.
I read the cylinders walls need to be greater or equal to .200
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post #8 of 13 (permalink) Old Aug 8th, 11, 12:18 AM
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Re: Bore to piston tolerances, crankshaft etc.

chet and michael are right on this
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post #9 of 13 (permalink) Old Aug 8th, 11, 05:32 AM
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Re: Bore to piston tolerances, crankshaft etc.

Quote:
Originally Posted by 1962vette View Post
I should have said I'm going with a 402 build kit on my 396 big block
-Bill

A 402 is only a 396 bored .030" oversize. Where do you come up with an .080" bored 396 to make it a 402? A 396 bored .080" over will more than likely run hot because of thin cylinder walls. Only the very early 65-66 396 blocks were thick to bore this much with no problems.
he factory 1970-1972 396 engine actually displaced 402 cubes. They just called it a 396 for badging purposes. OP, stop what you are doing right now. Get your engine back from that builder. Buy the book I mentioned and educated yourself before shopping for a machine shop or engine builder. No reputable builder will bore a block .080. If it needs that much to be cleaned up, get a different block.
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post #10 of 13 (permalink) Old Aug 8th, 11, 05:53 AM
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Re: Bore to piston tolerances, crankshaft etc.

Best thing to do is have the block sonic tested as you have to many guys assuming it will not go that far or it will over heat!!!! Sonic test it instead of guessing!!!!

If your checking bore clearance check it with the head on and measure form the bottom of the cylinders or just use a torque plate as that what your pistons will see for size once the heads are bolted on!!

Here is a thread on plate honing I did years ago.

Values Of Torque Plate Honing
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post #11 of 13 (permalink) Old Aug 9th, 11, 04:26 PM
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Re: Bore to piston tolerances, crankshaft etc.

Quote:
Originally Posted by convertible68camaro View Post
Hello fellow members,
I know there are many of you that are engine builders.
I would like to know the tolerances held on a rebuild engine.
The shop is boring .080 over on my big block and I'm going to inspect the work done. I need to know how much oversize the bore diameter should be from the pistion.
Also the what are the crankshaft specs.
Thanks in advance
-Bill

From the Chevy Power Catalog for BBC Mark IV:


It lists High Performance clearances as:

Main clearance .002 - .003

Rod clearance .002 - .003

Crank end play .005 - .007

Running a little extra clearance, say .003” instead of .0025”, makes an engine less vulnerable to crankshaft, rod, and block deflections and misalignments that come from the higher loads seen in High Performance engines.

For me personally, I run 5W30 synthetic in my 540ci BBC Street/Strip motor, which I intentionally built with .003 clearance on the rods and mains. And on the dyno it made 781 HP, 710 ft lb without issue. The motor uses a Titan gerotor “high volume” oil pump, and it has a hot idle oil pressure of about 30 psi, and a rock steady max oil pressure of 80 psi, which also shows that there is no sign of aerated oil with this setup.

It’s like Reher-Morrison says in their Pro-Stock Engine building book, running a little extra clearance, is ALWAYS better than running a little less clearance. Words to live by for Gearheads like us.
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post #12 of 13 (permalink) Old Aug 9th, 11, 05:28 PM
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Re: Bore to piston tolerances, crankshaft etc.

Bill,
I would like to make 2 comments.

1. Most people still do not understand this but . . . the actual clearance is designed into the piston. You finish the block to the exact bore size. The piston will be smaller by whatever the clearance is supposed to be. For instance if you have a forged piston that is supposed to run at .005" clearance, the piston will actually measure .005" smaller than the bore size. So your .080" oversize piston will actually measure .075", or .078" etc depending on the clearance.

2. I would not be a bit concerned with boring that block .080" oversize. Unless you are going to race it exclusively or run a blower and nitrous, I dont think you will have any problems. On a STOCK TYPE engine, my rule of thumb is that if they make pistons the size you want, then it is ok to bore it to that size. If you really are a chicken, then get it sonic tested, but I dont think you will have any problems.

Just my opinion,

Bill Koustenis
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1971 Chevelle "Heavy Chevy" original owner

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post #13 of 13 (permalink) Old Aug 13th, 11, 06:48 PM Thread Starter
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Re: Bore to piston tolerances, crankshaft etc.

Thank to all that replied to my post
The block is going to be bored .040 over which is a great relief to me.
-Bill
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