Piston Wall Clearance Question For Glenn
 

Glenn, we are going to start the build on Eric's 331 soon. However, I want to get your opinion on piston to wall clearance. He's using Probe SRS forged pistons and they recommend a clearance of .0030 - .0070 for a 4.000-4.199 bore. Eric had the machine shop hone it for a .0030 clearance.

Now, he is going to be running a blower without an intercooler. Do you think that we should have more clearance due to the extra heat from the blower or will the high inlet temp not affect the temp and there for the expansion of the piston? I know he plans on building the car so that he can run track days with it so it will see extended higher RPM runs which will create higher cylinder temps.

What are your thoughts? Do you think .0030 will be fine? :huh:

When I built the 302 in the Cobra I didn't even check the clearance when I installed the pistons and it didn't have any noticable wear when I replaced the head gaskets a while ago.

I'm no expert (as you all know), but I believe when the Cobra guys rebuild they go .0030over. The original Ford spec is .0010 over and that has caused nothing but problems. When people run any boost in overdrive gears they're playing russian roulette with their motor...

Will the pistons be oil cooled? That should help with expansion issues wouldn't it?

Bob

The 03/04 Cobras have a smaller bore at only 3.55. According to Probe's recommendations with a bore of 3.000-3.624 using their SRS and PRS forged pistons the clearance should be .0020-.0060. If you use their FPS series then it is .0010-.0040.

The difference between the two is the SRS is made of 2618-t61 low silicon forged aluminum and the FPS is made of 4032 low expansion high silicon heat treated aluminum. The high silicon allows a tighter piston to wall clearance because it doesn't expand as much. I'm assuming this is probably similar to what the stock forged piston Ford used is and when guys go with aftermarket forged pistons they increase the clearance.

I know the clearence will be good for normal driving and the occasional drag strip pass. I'm worried though about road racing and the midnight run to top out 5th gear. Not that Eric would do such a thing.

This is a stock 302 block so it doesn't have oil squirters under the pistons.

I think he decided to have the machine shop take the clearance up to .0050 but we are going to call Probe on Monday and verify that would be good for Eric's combo.

I called Probe today and they said that with Eric's build (10psi, non-intercooled) that we should set the clearance to .0045-.005.