Compression hgt ??

dave269

Member
what part of the piston do you measure to figure out the comp hgt ? I purchased a used block w/ a wiseco piston with no numbers on it and when I install it w/ a stk stroke crank & 4.475 rod I end up with .030 pop-up. theres about .080 worth of material left on the top of the piston, would it be safe to remove .020 off the piston & leave .060 ? seems like that might be too thin, So that is why I need to know how to measure the comp hgt because a different piston may be the solution
 
Quick way to measure the C/H is to take a pair of calipers and measure from the top of the piston to the top of the wrist pin boss. Take that measurement and add half of the wrist pin diameter.

Ex. If the measurement you got from measuring the top of piston to the top of the wrist pin boss is .320" and you have a .490" wrist pin (divide the wrist pin diameter by 2 (.490 / 2 = .245)). Add the two together (.320 + .245) and you get the C/H of .565" (Which is standard for the 4.500" rod with stock stroke 2.437" crank.) You can also use the 4.475" rod with this piston but you will have less pop-up as the 4.475" rod would typically use a .590 C/H piston (NLA).
.060" seems a bit thin to me -- I'd suggest a different piston (or possibly a thicker head gasket) if you are trying to reduce compression in the engine.

Hope that helps.

--
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cuts
www.CarlsonMotorsports.com
Celebrating 25 years of service to the karting industry
765-339-4407
bcarlson@CarlsonMotorsports.com
 
Thank you Brian, this piston must have started as a .640 C/H and was milled down to a .588 C/H which is giving me .030 pop-up, The wiseco 1990 has a C/H of .565 which should give me about .007 pop-up. how much piston to head clearance should I aim for ?
 
The .565 piston will be lighter! I would just leave that .007 pop up or deck it flush ! Might even cut more off that .565 piston and then deck the block even more! .030 piston to head clearance is the safe norm. I have ran .020 clearance many times with a light upper rotating assembly "extra mods to lighting the piston even more" . Keep a good check on valve to head clearance also . The big cams limited some of choices of block decking ,piston lighting work and head gasket choices /squish !
 
I just received the .565 piston from jc spec. Yes this piston is a lot lighter & nicer !!! I think this block has been decked pretty heavy already . this .007 pop-up that I'm getting is with a 4.475 rod
 
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