ET: Engine (Twin) Piston Ring Gap

greg brillus

Well Known and Active Forum User
VOC Member
15 thou and no less on the two compression rings, check all of them as you can find differences with them from time to time.
 

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
David,

Just as we need to have a bore clearance greater than recommended by modern piston makers to take account that our motors do NOT have a constant temperature, controlled component expansion operation that exists in liquid cooled motors I think the same applies to piston ring gaps, so I initially used the CP data for Nitros/turbo/supercharged and even then managed to break a top ring!

With CP pistons (forged, low expansion) I now increase the ring gaps by a further 10%, so on a CP piston, in a Comet, I use Top Ring Gap of 0.020", second ring gap 0.025" and oil ring gap of 0.018" min.

No issues with blow by, and no broken rings any more.

M
 

davidd

Well Known and Active Forum User
VOC Member
Martyn,

That is good info. I have used tighter gaps with aluminum barrels and Nicasil, with no bad results, but I don't have a large amount of data, so I cannot say anything definitively.

David
 

CoreyL

Well Known and Active Forum User
VOC Member
Martyn,

That is good info. I have used tighter gaps with aluminum barrels and Nicasil, with no bad results, but I don't have a large amount of data, so I cannot say anything definitively.

David
What gaps have you been running with the aluminum/nikasil liner?
 

davidd

Well Known and Active Forum User
VOC Member
I last ran .0045 x 3.3856 = 0.015" for the top ring and plus .004 for the second ring, which was 0.019". I did not have problems with these numbers, which are street numbers. For racing they should be a little wider at 0.017" minimum and 0.022" respectively. I would err on the side of running larger end gaps rather than smaller. I have had a top ring shatter at a race. A smaller gap does not get you a lot of bang for the buck.

Make sure when measuring the gap that the ring is square in the bore. I push it in the bore with an old piston so I know it is square. Ring gappers are inexpensive and easy to use.

David
 
Last edited:

CoreyL

Well Known and Active Forum User
VOC Member
I just had an aluminium liner made and nikasil coated by Millennium Technologies in Wisconsin (excellent work, BTW) and their recommendation was 6.5 thou per inch of bore. I'm using a forged CP Carrillo piston 8:1 and used 20 thou on the top ring and 24 on the 2nd ring. I didn't file the oil control rails but they were at 15 thou out of the box. Running 3.5 thou piston clearance with the Al liner.
 
Top