is there any tool that would help me measure piston to wall clearance on my motor ?recently rebuilded and i have a set of srp 9.1 cr pistons 81.5 mml and I'am not sure how to measure clearance ? my gess is tha a feeler gauge would help me ..... ?

 

You measure the cylinder and the piston separately. Then do the math to get the piston to wall clearance. You can't measure it after the pistons are installed into the block.

 

You measure the outer diameter (OD) of the piston skirt, approx. 1/2" up from the bottom of the skirt. Then you zero out your dial bore gauge to the measurement on the micrometer and then you stick it in the bore to get the difference in the OD of the piston and the ID of the cylinder bore.

You use that technique to measure clearances for a lot of things in the motor.

Measurements can also be done at temperature by heating them up in an oven to check the clearances under simulated operating temperatures.

 

So, if you have your block bored to 81.5mm at a machine shop, and buy 81.5mm pistons ... you still have to check this?

And if its off? Then what? I mean if its too close I guess you could hone the cyl out more but if its over then what ???

 

You ALWAYS check your clearances if you haven't measured them before...even if somebody has already given them to you. Assume nothing.

If the clearance is too tight, then you can have it honed out to size.

If it's too loose, well....it depends on how loose. If we're talking close to .0100", then that's clearly too much. Half of that might be ok, depending on the application and composition of the piston.

Along those lines, does anybody know if a larger bore piston would expand more or less than a similarly-designed piston of a smaller bore? If so, then how much of a difference in bore size does it take for us to need to take that issue into consideration? Anybody done any testing on that issue?

 

most pistons come with the piston to wall clearence built in... so if you have an 84mm piston its USUALLY 83.9xx depending on the clearence usually about .0035

this doesnt mean you dont have to measure it but you usually give the pistons to the machine shop when they hone the motor to have it checked

 

Jeff Evan's Engine rebuild guide is absolutely a very nice tech read:

http://forums.evans-tuning.com...d8b43

 

so what about those srp pistons that said they have about 15% of silicone that when they are hot they expand ?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by EGSR &raquo;</TD></TR><TR><TD CLASS="quote">is there any tool that would help me measure piston to wall clearance on my motor ?recently rebuilded and i have a set of srp 9.1 cr pistons 81.5 mml and I'am not sure how to measure clearance ? my gess is tha a feeler gauge would help me ..... ?</TD></TR></TABLE>

I used the 9.9 CR SRP pistons in my rebuild recently. I checked to see if my bores were still round by measuring them in 6 different positions. They came back within spec of the Helms. I got feeler gauges and stuck them in at the top of the cylinder and put in the piston with rod. I used the 0.003" and 0.004" feelers. The pistons would barely grab the 0.003 and wouldn't move using the 0.004, so by this I knew the piston-to-wall clearance was about 0.0035". I gapped the rings and put everything back together. They car runs great and dynos @ 248whp 215wtq on the Greddy 18G B16A using about 12.5 psi. Try and find the extra long feeler gauges. They really help.