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Topic: Piston to wall clearance (Read 719 times) previous topic - next topic

Piston to wall clearance

I'm going to call Federal Mogul tomorrow, but does .0018 clearance seem a little tight for forged pistons? Since these were originally put into a block 6 months ago, I no longer have the paperwork that came with them. I know they say they're made to expand less or whatever, but that seems like hypereutectic clearances.

I'd have to look up their model number, but they're some of their SpeedPro Forged pistons with I think +2.7cc...similar to stock forged but .030 over.

Also, some machine shop guy at the new place is telling me to get 4.035 file to fit rings - I call bs on this? Why should I need such large rings when they'll just be shaved down to standard 4.030 file-to-fit size?
1988 Thunderbird Sport

Piston to wall clearance

Reply #1 –
Yeah, that sounds a bit tight to me. I think 0.003 - 0.004 is about as tight as you wanna get with forged pistons - even the tight-fitting ones.

I'm not sure if it's unusual to get slightly larger file-to-fit rings - it's just insurance to insure that you don't start with gaps that are too large. In my case with the supercharger and previous issues with butting rings, I went pretty big on the gaps so there was quite a bit of grinding - doh! Anyway, the builder is probably just a "better safe than sorry" type ...
11.96 @ 118 MPH old 306 KB; 428W coming soon.

Piston to wall clearance

Reply #2 –
I agree that the clearance sounds a bit tight, but could be right with the newest tech in pistons. I have done a few hypereutectic piston builds long ago and they are tighter than I would have thought. Most of the stuff I built used .007" to .010" of clearance. Too tight will be a problem. Too loose just cuts the life of the motor.

The guy is right about buying .035" over rings if you want to set your own end-gap. Or you can just stick .030" rings in and hope they are right.

There  are as many opinions on cylinder wall clearance, finish, cross-hatch, end gap, etc as there are builders.

Piston to wall clearance

Reply #3 –
The motor is supposed to have light boost applied so the ring gaps will need to be slightly larger and the machinist will be gapping the rings (most of his labor prices are pretty low with exception of decking the block) and I just want this thing done. I'll be calling federal mogul in a few and will post back what they say in terms of all this.
1988 Thunderbird Sport

Piston to wall clearance

Reply #4 –
If under boost, I would want a little more cylinder wall clearance too.
Check with F-M and see what they say.