6 replies to this topic

[Timmyquick]

Hi all, first ever mini build! Started engine assembly today. Struggling with piston ring gaps.

Engine is 1275 bored 30 thou over for high compression pistons, which it had originally. New pistons and rings don't say what gap should be.

With rings 1 inch from top off bore I have

Top ring gap filed down too .010
Second ring gap filed down to .008
What should oil rings gap be?
Is that too tight? Haynes says that's minimum any suggestions as I keep reading conflicting info.

Thanks.

[Cooperman]

I have always worked on a minimum gap of 0.003" for each 1" of bore diameter.

years ago it was not uncommon for rings to come up a bit tight and the gap width had to be increased by gentle filing and final smoothing off. However, I can't remember the last time I had a ring come up tight and on a recent +40 thou rebore to 1310 cc the ring gaps on the 21253 pistons came up at around 15 to 18 thou.

[Spider]

Good pick up on the ring gap.

Before you file the rings, I'd suggest checking the piston to bore clearance. You can do that well enough with feeler gauges. You'll need to find the piston manufacturer's data for what this clearance should be and where to check that clearance on the piston. All the piston Manufacturers have their own special mixes of the alloy used in the pistons and so they expand at different rates, hence why you need to refer to the piston manufacturer. Check all bores against all pistons. If you have the right clearance, then check the ring gap 2 more times and try each ring in every bore as there'll be some variation here as you may find some sets of rings will gap better in particular bores. Only then if you can get the minimum gap you need (the manufacturer will have data on this, but you many need to track down their catalogue) though it is usually as Cooperman has suggested, in the order of 0.003" per 1" of bore diameter. Don't fret if they end up on the big side, the losses from it are so small, that they can't be measured. 

[Timmyquick]

Not measured piston to bore clearance, have assumed when machine shop bored the block it be right. They had the pistons when doing the block. The pistons are P21253-30. The packaging on rings say Grant, instructions do not say what clearance just which ring goes where on piston. Tried rings in all bores and could only just get in a .004 feeler gauge in them. So I got the file out for top rings, Looked in Haynes there minium figures seem really tight in comparison to Google search! According to Haynes top ring 0.25mm, second ring can be 0.20mm on 1275 12A.

[Spider]

This is from the Grant Catalogue;-

 

Never best to guess and google can be quite onerous.

 

 

Not measured piston to bore clearance, have assumed when machine shop bored the block it be right.

 

If I can butt in here, I'd suggest this is not a wise train of thought. I actually assume they have done it wrong and then set about proving otherwise. The guys who work in these shops are only human, you don't know what's on their minds and they are also working under commercial pressures, they do make mistakes. Say here. if the bores are too tight (and the initial ring gaps suggest this), then after your engine seizes, all you can expect to claim on them for is to rectify the tight bore, they won't replace your pistons and rings, let alone gaskets, oil etc wasted in the process.

 

I'll just add here that I use a lot of Grant Rings, I do like them for most builds. It's very rare indeed to find a set that's tight and even then, it will only be by maybe as much as 0.001".

[Timmyquick]

Thanks for info, really useful. I will try and measure piston to bore clearance with feeler gauge.

[Cooperman]

Yes, in general ring gaps with the 21253 pistons usually come up at around 15 thou on average.
If yours need the gaps increasing an all rings on all 4 bores, it would suggest that the bores are slightly small.
They could be honed to give the correct piston fit and the rings should then have the correct gap. Remember, if the bores are 3 thou undersize, the ring gap will be reduced by over 9 thou. If you have 4 thou ring gap, honing 1.5 thou on the bore will bring the ring gaps into tolerance. However, the piston clearances must be checked.