Is .003 over good for standard?

35th T-bird SC

Registered User
Machine shop just honed my cylinders and I told them I wanted to stay standard size. They honed and it measured out about .003" over 3.81". They said that should still be good for standard pistons. What do you guys think?
 
I am no machinist or very knowledgeable of what the acceptable wear limits are. But looking at the factory manual (you should get one)

SC Piston to bore clearance 0.0040- 0.0035in. (Doesn’t make sense to me or they got it backwards)



In the specifications section they show three service piston diameters offered.

Red 3.8095 - 3.8101in.
Blue 3.8107 - 3.8113 in.
Yellow 3.8119 - 3.8125 in.


Service Piston selection bore diameters

3.8110 - 3.8122 in. Red
3.8122 - 3.8134 in. Blue
3.8134 - 3.8146 in. Yellow



So if your bore is 3.813 in. it looks like you would use the Blue.


Like I said above. Get the book and verify what you use with the machinist.

I can see why they just bore engines. You could have three types of pistons depending on how they measure out.
 
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Thats the piston diameter though. I'm looking at cylinder wall diameter. I have the same thing in my haynes. What do the codes mean? Mine are slightly different.
Coded red 3.8095-3.8101
coded blue 3.8107-3.8113
coded yellow 3.8119-3.8125
 
The way I read the manual the Bore diameter is the cylinder wall measurement.

So if you bore diameter is 3.813. The replacement piston is the blue code that covers 3.8122 - 3.8134 in. bore.
 
While you won't know until you check with a Ford parts department, historically it has proven difficult to get Service pistons for engines not currently in production. Plus this would also mean that you would have to use Ford parts with the attendant price. Aftermarket pistons will only be available in standard and standardized oversizes like .020, .030, etc.

So if you want to try and use your existing pistons, or the Service oversizes prove to be unavailable or prohibitively expensive, the real question is what's the actual piston to bore clearance, and what's the acceptable clearance? Answering those questions will tell you if you need to go to the next economical oversize.
 
Is this right to do that?
Measure the diameter of the piston at a couple spots with a micrometer and measure the cylinder wall with inside snap gauge and micrometer then subtract the difference.
 
^^^^
That's pretty much it. About the only thing to add is that there are specific locations to measure the piston. A piston is not perfectly round, it generally has a slightly wider measurement on the skirts than at the ring lands. You could picture it as a sort of barrel shape. Just so that you know to look for the widest measurement and not just arbitrarily select a point to measure.

Oh, and a dial bore gauge is a bit easier to deal with, but if all you have is a snap gauge and outside mike for the bore then that'll have to do.

edit: almost forgot, don't forget to measure the bore at top and bottom to find any taper and take that into account when selecting pistons, if necessary.
 
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