Engine Piston-to-cylinder Bore Clearance

stang89bidges

Advanced Member

Mar 14, 2014

727

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74

Fort Worth Texas

Apr 20, 2015

#1

• Apr 20, 2015

• #1

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...

 

Last edited: Apr 20, 2015

liljoe07

5 Year Member

Feb 18, 2009

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Cartersville,Ga

Apr 20, 2015

#2

• Apr 20, 2015

• #2

#6 is .005

I would have #1 rechecked to make sure it wasn't a typo or misread.

 

stang89bidges

Advanced Member

Mar 14, 2014

727

144

74

Fort Worth Texas

Apr 20, 2015

#3

• Apr 20, 2015

• #3

Actually I came up with .0023 on number 6 when I did the math again. How did you come up with .005? Am I doing the math right?

4.002-3.9997=.0023

 

liljoe07

5 Year Member

Feb 18, 2009

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Apr 20, 2015

#4

• Apr 20, 2015

• #4

I think read it wrong and I missed a 9! LoL! .0023 is correct

 

mikestang63

SN Certified Technician

Aug 27, 2012

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Apr 20, 2015

#5

• Apr 20, 2015

• #5

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.

 

stang89bidges

Advanced Member

Mar 14, 2014

727

144

74

Fort Worth Texas

Apr 20, 2015

#6

• Apr 20, 2015

• #6

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.

 

Last edited: Apr 20, 2015

stang89bidges

Advanced Member

Mar 14, 2014

727

144

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Fort Worth Texas

Apr 20, 2015

#7

• Apr 20, 2015

• #7

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?

 

Last edited: Apr 21, 2015

BMW Rider

Mustang Master

Oct 9, 2014

140

277

83

Calgary, AB, Canada

Apr 21, 2015

#8

• Apr 21, 2015

• #8

If the pistons were from that block and going into the same bores and it ran 77K miles already, don't sweat the numbers. The clearances would not have gotten tighter than before it was torn down.

 

Reactions: stang89bidges and A5literMan

A5literMan

At least it is lumpy...

5 Year Member

Jul 30, 2011

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Apr 21, 2015

#9

• Apr 21, 2015

• #9

Agreed. I'd just throw it back together and run it.

 

stang89bidges

Advanced Member

Mar 14, 2014

727

144

74

Fort Worth Texas

Apr 21, 2015

#10

• Apr 21, 2015

• #10

That's what I was thinking. Being a little loose didn't bother me either because it's supercharged and I was told running extra clearance was recommended. I am going to stop fretting and just re-use them I guess.

 

mikestang63

SN Certified Technician

Aug 27, 2012

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Apr 21, 2015

#11

• Apr 21, 2015

• #11

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.

 

Reactions: A5literMan and stang89bidges