Setting the Piston Ring End Gap

Hi everyone,

hoping for some advice from those who have built engines before, as I'm not 100% sure whether the gap I set for the rings on my CP pistons is ok.

Now the formula for determining the top ring gap listed on the spec sheet that came with the CP pistons is "Bore (in inches) x 0.0045" for street/high performance and "Bore (in inches) x 0.0050" for road racing/drag. it states these measurements are "the minimum gap".

This means in my case, 3.209 x 0.0045 = 0.0144 using the street/high performance formula, and 3.209 x 0.0050 = 0.0160 using the road racing/drag formula.

I thought I'd aim for 0.0160 to be safe, as the spec sheet states that the formulas equal the MINIMUM gap. Although I ended up with 0.0170 in the end.

Turbo applications seem to run 0.0190-0.0200 so it's not that big, I'm hoping this gap is ok. I do recall in one of my conversations with Adrian (TODA) a while back, he mentioned it's better to go wider than tight when it comes to gapping rings, however there's all sorts of information on the internet claiming different things, so if anyone here has actually built engines before, what is your advice here? is this clearance ok? should I buy new rings and go tighter?

You shouldn't go tighter, you will be fine with a larger gap. It will still close up when the engine is at temp and under load. With a smaller gap however if it closes up and the metal has nowhere to go you risk damaging the rings and/or the ringlands on the pistons and probably in either case the bores.

Clearance will be fine. If its out by 5 thou ill be concern.

What engine is this for and what is your expected maximum RPM?

Thanks for the reply guys, it's a B18C2 that will be seeing probably around 8500-8700RPM.

Although looking back at my work, I think I'll need to do it again with a new set of rings as my technique was not perfect.. it's my first time filing rings so I'm not surprised.

I understand I am supposed to file INWARD (handle rotates counter-clockwise), and only file one side - allowing me to use the other side as a reference to ensure that the gap is square. These things I was not aware of.

Also, on this topic, I've coated the cylinder walls with engine oil and am using a piston to push the rings down square, and am drawing them out using my hands to keep them level as they come out, but no matter what it always seems to make light scratches on the walls. I am deburring the rings with 800 grit sandpaper each time as well.

the scratches are very light, by this I mean I cannot catch my fingernail on them, they are only visible. So my question is, will this hurt anything?

I'm confident in my capabilities but this is all new territory to me and it's pretty stressful without any help..

When filing the rings try keeping the ring as square as possible. Dont push the ring on the disc, let the sanding disc do the work. WD40 works well lubricating the walls when measuing piston rings. Download the K20a manual and clock the piston rings/oil rings to that :)

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Originally Posted by fatboyz39

When filing the rings try keeping the ring as square as possible. Dont push the ring on the disc, let the sanding disc do the work. WD40 works well lubricating the walls when measuing piston rings. Download the K20a manual and clock the piston rings/oil rings to that :)

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Thanks for the advice, I'm using 15-40 oil to lubricate the walls at the moment.

If you don't mind me asking, what's the advantage with using the K20a OEM gap and clocked position on the B18C CP pistons??

No gap the piston as to per manufactures specs. Clocking use k20a.

Doesn't matter what weight oil you use to lubricate the wall, just get cheap oil.

Quote:

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Originally Posted by fatboyz39

No gap the piston as to per manufactures specs. Clocking use k20a.

Doesn't matter what weight oil you use to lubricate the wall, just get cheap oil.

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Cool thanks for the info :)

The issue I have with the CP specs is that it gives you the formula for the MINIMUM gap, but nothing about what's optimal. I'm thinking 0.016 to 0.017 is probably a good size, provided that what I've been told about the ring gap being wider not affecting blow by is true. So when I get my new set I think this will be a safe gap to run with.

"Optimal" gap doesn't really exist. The smaller the gap the slightly better the compression gets, along with squish depending on chamber shapes and compression heights etc. When on the minimum gap, the absolute concentricity of the bore matters: any taper, combined with thermal expansion will result in ring bind and piston failure. .016" will be absolutely fine and will err on the safe side in your case. Blow by does not occur until the gaps are much larger - well over .030". But if you start out there, there's no room for ring wear, which is most intense in break-in. Good luck and hope this helps.