Piston coatings and clearances

Ive got an all aluminum 400 sbc with about 25 passes on all parts since new. Had 10-71 standard helix and im putting a 14-71 retro hh on and all new injection so im gonna need some tuning time to get a tuneup I can trust. When i pulled the engine down my bore to piston measures between .006 and .007 so im already getting loose but everything else is looking great. I didnt put the engine together originally so im assuming that it was put together loose cause i usually start with about .004 then on next tear down end up about .007 but this is my first blown alcohol engine so maybe it differs from NA. or nitrous injection. Planning to tune to about 30 lbs boost and 32 degrees timing then tear down and inspect again . Hopefully i can do this within 10 or15 passes. So i guess what im wondering is Can you coat a blown alcohol piston skirt to help with piston to cylinder wall tolerance. Im at .007 now and need to cut about .002-.003 to clean up bore. Probably .002 will clean cylinder nicely. Was hoping I could coat piston .003-.004 and use pistons one more time to keep my piston tolerance around .006 to .007. Still loose but maybe would buy me some tuning passes before I buy new pistons and bore sleeves. ANY HELP AND INFO WOULD BE GREAT!!!!!!!!!
Like i said im new and learning my way around blown stuff. Thanx for reading and anything you can tell me whether about tuning, engine building particulars for blown alcohol or just good advice will be more than appriciated... Thanx,,,,, Brian

 

clearance

At operating temp. I bet you are close to where you want to be. The piston will grow more than the liner. Thats my guess, what do you guys think???

 

On a full blown aluminum motor, I run at .010 clearance (.005 per side).

 

So with all aluminum you need more clearance.
Brian

 

clearance

hey 400,
where are you measuring the piston diameter??? you should check it just below the oil ring and 90* from pin. (across thrust)

if it is less than .007 you are living dangerously! on a 4.0 to 4.2 bore up to .010 is fine. on nitro (4.188-4.250) they run .011 to .013. i once ran .021 clearance (bye mistake) on a blown alky chevy bb and it ran great until the walls cracked. chuck