C/R combination

RCJ

Member
Is there a combination that will put you on the correct c/r. Such as 19cc head, .030 in the hole piston, .012 gasket. There seems to be a lot of variance between blocks and heads
 

flattop1

Dawg 89
I belive its .290. . total head depth , head gasket and in the hole .
Add em all up it should be between .290 and .302 to meet the cc check .
More is more cc less is probably illegal .
 
I could be wrong but think he's wondering how guys are reaching it. Big chamber small head gasket? small chamber big head gaskets?
 

DynoDon

Moderator
.295 to .305 most generally is going to be legal. I have had a .308 legal at 26.7 cc. Roll the engine to exact tdc. Measure the outside edge of the piston at 12/3/6/9 o’clock. Divide by 4. Add that number to the head gasket. Then measure the depth of the head from the gasket surface to between the valves. Add that to your numbers and you get a final that should be between 295/305. For instance: piston is 7.25. Gasket is 009. Head is 287. Total is 303.25. Cc is 26.7
 
.295 to .305 most generally is going to be legal. I have had a .308 legal at 26.7 cc. Roll the engine to exact tdc. Measure the outside edge of the piston at 12/3/6/9 o’clock. Divide by 4. Add that number to the head gasket. Then measure the depth of the head from the gasket surface to between the valves. Add that to your numbers and you get a final that should be between 295/305. For instance: piston is 7.25. Gasket is 009. Head is 287. Total is 303.25. Cc is 26.7
So you are only running .017 piston to head clearance? That 7.25 is confusing me a little. i'm assuming that is .00725 down in the hole?
 

alvin l nunley

Premium User
Is there a combination that will put you on the correct c/r. Such as 19cc head, .030 in the hole piston, .012 gasket. There seems to be a lot of variance between blocks and heads
There's a page in nine sheets, my Excel program, that will Calculate it for you. You need to know the bore and stroke.
 

1fasttiller

Member
If stock class racing, you're chasing the wrong thing... Compression ratio is irrelevant. You have to meet the MIN CC check to be legal. Many different builders achieve this in many different ways... you can cut a ton off the head and use a thick HG or you can mill the block and have .002 (just stay under) piston pop up etc. Many different ways to skin the cat and all show up different on the dyno. Some do a little of both. Static compression really isn't a big factor in power anyway with these things. The cam and head flow and carb is where you should be looking.
 
If stock class racing, you're chasing the wrong thing... Compression ratio is irrelevant. You have to meet the MIN CC check to be legal. Many different builders achieve this in many different ways... you can cut a ton off the head and use a thick HG or you can mill the block and have .002 (just stay under) piston pop up etc. Many different ways to skin the cat and all show up different on the dyno. Some do a little of both. Static compression really isn't a big factor in power anyway with these things. The cam and head flow and carb is where you should be looking.
Not sure if that was directed at me or the OP. I'm just trying to see how tight i can get away with. Typically on used motors i'm ending up with the piston is pretty far up the hole and the common gaskets wouldn't give enough Piston to head clearance. But if they are running them that close without issue then i can probably get away with a .009 or .014
 

1fasttiller

Member
Keep in mind per rulebook you cannot have ANY piston popup. I usually build my motors around .003-.005 in the hole to allow for carbon and clearance.

I also use the OEM Honda graphite head gaskets ;)
 

DynoDon

Moderator
you already posted that when this was originally brought up. My question was directed to Dyno Don. Thanks though.
The 7.25 was the average of measuring the piston in the hole at tech. 12/6/3/9 o’clock. Add them together and then divide by 4. Your numbers will be different. Where you get different total numbers between .295/.306 is the measurement I didn’t share. That is how far the valves are sunk in the head. That plays a role in the true cc legality. That is also why the number system is not always accurate. I have done so many and documented the valves to where I can get it close nearly all the time.
 
Last edited:

RCJ

Member
When you add in the hole and gasket numbers together what is the smallest number you would run before the piston starts contacting the head?
 

OVALTECH1

Premium User
I would consider .015 with a good crank and billet rod. However with the flexy clone cranks and a stock rod I would try to keep .025 to .030 for piston to head with stock stuff.
 
Top