91 5.0 Gt

[WEIRDGUYWITHAKNIFE]

I am working on the above mentioned model, it is also a convertable. I do not have a set of micrometers handy with me and need to determine the size of the pistons. In the service manual it states there were 3 sizes used that year for that model, listed as follows:
5.0L (1)
Red Piston 3.9989-3.9995 (101.572-
101.587)
Blue Piston 4.0001-4.0007 (101.603-
101.618)
Yellow Piston 4.0013-4.0019 (101.633-
101.648)
5.0L HO (1)
Red Piston 3.9972-3.9980 (101.529-
101.549)
Blue Piston 3.9984-3.9992 (101.559-
101.580)
Yellow Piston 3.9996-4.0004 (101.590-
101.610)
5.8L (1)
Red Piston 3.9978-3.9984 (101.544-
101.560)
Blue Piston 3.9990-3.9996 (101.574-
101.590)
Yellow Piston 4.0002-4.0008 (101.605-
101.620)
however there are no identifying marks or colors whatsoever on the pistons. How are they identifiable? Thankyou

 

[rbohm]

the stock pistons are 4" in diameter. the numbers you are seeing is what the pistons measured, or can measure since you are being shown the tolerance allowed in the piston size.

 

[WEIRDGUYWITHAKNIFE]

what is with the colors? Thankyou

 

[WEIRDGUYWITHAKNIFE]

How can theese be piston tolerances when the bore is 4", I could see it working on the red one

 

[rbohm]

when i talk ab out tolerances i mean the piston is supposed to be 4" by the engineering drawings, but an allowance is made, aka a tolerance, to either side of the engineering spec.

 

[flstang65]

Not to mention you are talking thousandths to ten thousandths of an inch. The piston rings and the expansion and contractions of all the metal ect...blah blah blah. Lol. Do yourself a favor and get a micrometer and get the accurate numbers. You will find yourself using it way more than you think once you get it anyways.

 

[horse sence]

What they a saying is ,even though the piston may be 4" you can't have the bore exactly 4" or the piston will not fit in the bore .The tolerance is the amount of over bore that the piston requires to operate .Cast pistons require a different tolerance than a forged piston .
The tolerance on the rings would be the amount of gap between the two ends of the ring .different materials used in the rings require a different gap or tolerance .Same with the crank and bearings .

 

[65ShelbyClone]

If your engine has been run and has any kind of miles on it, then the pitons aren't going to match those numbers anymore anyway whether or not you have a set of 4" micrometers.

Those tiny variations in piston size were used to streamline manufacturing. Instead of having a set of pistons follow the block all the way through machining and assembly, they could instead Machine the block in a separate facility and later mix and match the red/blue/yellow sizes to get the right bore clearance. Those minute oversizes can also be used sometimes in used engines that have too much clearance, but don't need an overbore. It's a popular technique with motorcycles.