Piston Sizing

[OmniSRT4]

The SOHC motors come with variable size pistons marked :

A
B
C
D

Now, when you order the replacement STD pistons, which range do they fall in?

When they say .020 or .040 over. Which piston size is used as the baseline?

[MachineHead]

They are supposed to be .020 over the A size, but it really doesn't matter. You need to have a machine shop bore and hone the cylinder for the right fit anyways. If it is a decent shop they will measure the pistons first to find out the exact hone needed.

[86Shelby]

Machinehead is right any machine shop worth a darn will number the pistons and then hone each bore to the specific piston that will be going in. That's for any bit of competition use. Around here if you're building a lame-o SBC 350, etc. they measure one or two pistons and hone all cyliders to match them. Hone to match is the key there.

Aren't the letters to signify the number of tenths the bore/piston deviates? Not tenths of an inch but tenthousandths, .ooo1"

[OmniSRT4]

Quote:

Originally posted by MachineHead They are supposed to be .020 over the A size, but it really doesn't matter. You need to have a machine shop bore and hone the cylinder for the right fit anyways. If it is a decent shop they will measure the pistons first to find out the exact hone needed.

Oh I know what needs to be done. This was more for my curiosity.

Anyone know which range STD falls into? (A?)

[csxt#10]

Quote:

Originally posted by Akuma Oh I know what needs to be done. This was more for my curiosity. Anyone know which range STD falls into? (A?)

i would like to know also.what would the c and d,s be

[OmniSRT4]

Quote:

Originally posted by csxt#10 i would like to know also.what would the c and d,s be

- A size 87.442 -87.455 mm
- B size 87.455 -87.468 mm
- C size 87.468 -87.481 mm
- D size 87.481 -87.494 mm
- E size 87.494 -87.507 mm

These are the sizes listed for the pistons, I have that info already.

It doesnt tell me where "STD" (aftermarket) pistons fall in that range but I would assume it would be the E sizes.

(after all, you can only make the bores bigger, not smaller, and if it was based on an A size, then someone with an "E" bore would have no way to seal it properly)

[glhsken]

Sean, aftermarket pistons are based on an 87.5mm "stock" bore size. My .020 overs are 3.464" Straight from Wiseco the MFG.

[GLHS0658]

Sean, they are based off of "A" size. Since "A" fits into an 87.5 mm block bored exactly 87.5 mm

All pistons require clearance. The E piston would not fit into a perfectly bored from the factory 87.5mm block because it is 87.507

The aftermarket pistons are based on bore size. You have a .020 over block and they make the piston to that bore size minus any clearance they require. Forged require more clearance than cast
A Forged piston is smaller than a cast piston made for the SAME bore size because it expands more and will "fit" the bore when warmed up to operating temperature

If the finish honing was perfect at the factory and not a productiion line there would be no need for A,B,C,D,E pistons....there would be only STD

[OmniSRT4]

Quote:

Originally posted by GLHS0658 [b]Sean, they are based off of "A" size. Since "A" fits into an 87.5 mm block bored exactly 87.5 mm All pistons require clearance. The E piston would not fit into a perfectly bored from the factory 87.5mm block because it is 87.507 The aftermarket pistons are based on bore size. You have a .020 over block and they make the piston to that bore size minus any clearance they require. Forged require more clearance than cast A Forged piston is smaller than a cast piston made for the SAME bore size because it expands more and will "fit" the bore when warmed up to operating temperature

All useful info. I didnt even notice the second E number exceeding 87.5 nor was I factoring in the pistons rings.

However, if that is true, the blocks with E sized bores wont seal as well as the block with A size bores.

As for overbore siz, it still has to be based on some base number (which turns out to be A, which was the question!). When they send you the pistons it has to be .020 over some given size. (which turns out to be STD, or A?)

Quote:

If the finish honing was perfect at the factory and not a productiion line there would be no need for A,B,C,D,E pistons....there would be only STD

Yeah, they had manufacturing issues back then. :\

[OmniSRT4]

Quote:

Originally posted by glhsken Sean, aftermarket pistons are based on an 87.5mm "stock" bore size. My .020 overs are 3.464" Straight from Wiseco the MFG.

Makes sense!

[John B]

Quote:

Originally Posted by OmniSRT4 - A size 87.442 -87.455 mm - B size 87.455 -87.468 mm - C size 87.468 -87.481 mm - D size 87.481 -87.494 mm - E size 87.494 -87.507 mm

This post is old; can anyone confirm that this information is correct?