1911 hupmobile 20 piston clearance

[Jeff Spear]

Hello, I am looking for advice on piston to bore clearance for a 1911 hupmobile. The pistons I'm using are a cam ground cast aluminium .030 oversize and made for a dodge 218 flat head same bore 3.250" and compression height as the hup. The piston box suggests a min clearance of .0015  no max is listed. The pistons have a clearance with the bore at .030 oversize of  of .00175. 

This is where I currently have them set but I am  concerned that this my not be enough  after reading some horror stories from model t guys. Do those of you with pre war engine experience think I should increase the clearance if so how much the T guys run .004-5 but their bore is a little bigger I thought .003 might be ok for me but it seems like a lot.

I'd like to leave it right where its at but if that results in scuffed or locked up engine that would be  a bad day. I understand that with the partial water jacket, thermo siphon cooling, side valves, and not much timing that different specs may apply but how do you know what to do? 

 

Thank you

Jeff

[Mark Gregush]

I have seen a number of post on the Model T Ford Club of America's forum also about going with the .003 clearance, aluminum pistons in cast iron block. In many cases, the expansion rate between the block and pistons has a good chance of locking up or scoring at that clearance. When working on the Model T or A's, we have been going with the .004-.005 clearance to be on the safe side.

Question might be, is that .0015 all around the piston giving a total of .003 side to side or just .0015? I think the old rule of thumb is .001" of clearance per inch of diameter. Different grades of aluminum and cast iron have different expansion rates, then also cooling comes into play. No, I don't totally understand it all, so just want to stay on the safe side

Edited February 15, 2022 by Mark Gregush (see edit history)