A gent in the Denver Colorado area made some Diesel conversion heads for TT, Magnum and ASP engines about 12 years ago. He passed them out like candy. He made the contra with a 1/32" gap all the way around it. The O-rings would blow out after about 5 minutes of operation. I have never had a Davis contra out of the head so don't know if he uses O-rings or not. I know his contras are a good, tight, precise fit. Yesterday I inspected the piston fit on the SuperTigre .90 I ran for John, I set my Torque It torque wrench for 10 inch lbs, the compression was soft, I had to go to 20 inch lbs. For a good seal. I did the "snug the bolt" and then turn the bolt an additional 45 degrees, i couldn't turn it 90 degrees. It took 22 inch lbs to move it from there. That's not scientific but it worked.