Std size forged piston question

[tminer]

I know that forged pistons run more bore clearance than our stockers. TRW makes a std size forged piston. Are they slightly smaller in diameter to make up for the expansion, or does one just hone the bore to compensate?

The specifics: my preliminary inspection shows that my bore has .0005" total taper (difference in top and bottom diameters, not taper rate), and is up to .001" out of round. Std bore, still has crosshatch visible, along with some skirt scuffing. Would it even be an option for me to use th Std size TRWs with a hone to the .0045" clearance?

tyvm,
Tom

 

[turbo nasty]

All cyl are the same on out of round as the specs u say ??

Do you have a ridge at all ??

Why did you tear it down in the first place??

 

[tminer]

I only checked the cylinder that had the worst scuffing. Just preliminary measurements. I haven't even cleaned the block yet.

There is some carbon that gives a texture change, but couldn't catch a fingernail and pistons slid out with no "feedback" when the rings hit the carbon.

I'm preparing for some future abuse. B & B, upgraded bearings, deburring, girdle and/or steel caps, etc. I'd like to go with the TRWs, but will go the more expensive route (JE etc, or virgin block) before I do .030 over.

Tom

 

[Justin Terry]

Bump a hone threw it with whatever stone you want and it should be fine with the clearance of a standard bore piston. TRW can tell you what they want for piston clearance. But usually
.004 - .007 is fine.

 

[tminer]

I think that I have pieced it together, +/- .0005.

TRW shows clearance of .0035 90* from pin center. Merkel recommends .0045, doesn't say where but says that it is .001 more that TRW specs. He bores and hones .030 overs to 3.8315 to achieve the .0045 but TRW says that the .0035 is built in, hence the .0005 discrepancy.


Tom

 

[Justin Terry]

Yes the TRW piston has .0035 clearance built into it at the point on the skirt but pistons have a taper built into them so even though it has that much clearance right there it will have a different amount of clearance at the bottom of the skirt due to the fact that less heat gets to the bottom of the skirt that it does at the top so it grows less at the bottom of the skirt. Forged pistons grow and they are cam ground so they arnt completly round. Some parts of the piston grow more under heat than other parts hence the reason they are cam ground.