I put solder in the c shape and did both sides of both pistons numrous times and could not get the same reading as taking the head off and doing it the correct way. I was .003 to .005 bigger every single time. It is not accurate imo. If you stick it through the head and your getting a reading closeto the low end of your range you wanna be. You may be off. This was my experiance so take it for what its worth.