Engine Piston-to-cylinder Bore Clearance

stang89bidges

Advanced Member
Mar 14, 2014
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Fort Worth Texas
I just got my engine back from the machine shop. I am very concerned about these measurements they took on my cylinder bores and piston diamaters.

Tell me what you make of these numbers:


Cylinder#1
Bore=4.0223
Piston=3.9987
Clearance=.0236

Cylinder#2
Bore=4.002
Piston=3.9995
Clearance=.0025

Cylinder#3
Bore=4.0022
Piston=4.00
Clearance=.0022

Cylinder#4
Bore=4.0026
Piston=3.9988
Clearance=.0038

Cylinder#5
Bore=4.0021
Piston=3.9989
Clearance=.0032

Cylinder#6
Bore=4.002
Piston=3.9997
Clearance=.0023

Cylinder#7
Bore=4.0027
Piston=3.999
Clearance=.0037

Cylinder#8
Bore=4.0023
Piston=3.9978
Clearance=.0049

Have you guys ever seen numbers like these? Did I do my math right? If I did then I fear the worst...

My book says Piston-to-cylinder bore clearance = .0034 to .0042

Those numbers up there are just crazy, some of them are .02 off!

This can't be normal.... Can it?

The engine was super clean when I tore it down, not driven hard, only 77,000 original miles on it...
 
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where in the cylinder were the measurements taken? Normally they take them at the top, middle and bottom to measure any taper of the bore.. #1 is spec'ing out as if it was bored ..020 over. Sure you didn't slip a digit?

Are you using cast or forged pistons? Forged pistons tend to be looser when the engine is cold and snug up on a hot engine vs. cast pistons.
 
OEM forged pistons. I didn't slip a digit, it's what the machine shop wrote on the pistons for me.

EDIT: And when I say OEM I mean I am reusing the stock original forged pistons that I pulled out of the engine to re-use.
 
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Your right though, if the machine shop slipped the digit and the bore was actually 4.0023 then the clearance comes out at .0036

So let's say that is the case and the clearances are as follows:

.0036
.0025
.0022
.0038
.0032
.0023
.0037
.0049

Allowance is between .0034 and .0042

This gives me only 3 that are in spec and one that is .0012 off. What would your advise be? These pistons were only cleaned up. The bores nothing done. They still have nice crosshatch. Would you stick them back in and use them? Are these numbers not that bad? Or would you take everything back in to shop and have them checked again?
 
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You'll be fine. If you are planning on running a supercharger, you should adjust the gaps accordingly.o your top ring. Normally it's about .004 for each 1" of bore so on your motor you'd be at .016. With a supercharger, you can open the gap up to .020 as the added pressure and heat will make it expand more.

It's been a few years since I've assembled a supercharged motor and I'm sure ring and piston technology have changed with new materials. I'd recommend you check with the machine shop or your suppier of parts for their recommendations.

Once you get the short block assembled, it should spin freely with no binding. If you encounter any, STOP and disassemble it.
 
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