The BME spec for piston to head is .060 , around .025 more than a steel rod engine, when doin engine math if you use the .060 figure you are shorting the compression ratio quite a bit, So what I was asking was what was a good figure to use for engine math that was closer to actual ratio after heating the rod ,stretch or what ever you wan to call it , since Alu grows, swelles stretches or what ever more than steel if you had a real number you can figure the ratio closer, Take Bills engine , he had piston to head interference at .054 so we now know in a running engine with a alu rod the .054 clearance is no longer there , thats why you got the shiny spots so if most engine math is done using .035 or .040 piston to head math a Alu rod engine should figure a lower than .060 number, now my head hurts also