A Series clearance question

[revingtosh]

Good evening all.

Just a quick question with regard to piston / bore clearance on an A Series 1380.

Assuming that the bores look healthy (no scoring etc) whats an acceptable gapping distance (in thou) in between the rings and the bore?

Im about to whip the head off mine to check this out as well as valve stem oil seals etc, and so was intending on measuring up as my engine is a bit smokey.

Thanks in advance,

Mike

[Snake the Sniper]

There shouldn't be any gap between the rings and the bore! They should, however, be a gap in the ring itself, ie it's not a complete circle. Damned if I can remember what's recommended though.
Before you pull it apart, get it compression tested or leak down tested. That will tell you what's leaking and by how much.

Edited by Snake the Sniper on Wednesday 3rd December 21:23

[revingtosh]

So what is meant by "gapping the rings" then?

Obviously there must be some gap otherwise they would break wouldnt they?

Am i barking up the wrong tree? Is it the split in the rings that i should be measuring?

[Cooperman]

With an older engine the problem is that the bores tend to wear most right in the middle of the stroke between TDC and BDC. That's because the piston speed is greatest at mid stroke. There may also be a 'step' at the point where the top ring comes up to. If there is it needs re-boring.
You need to measure the bore diameter at the top and half way down using a bore gauge. If the difference is more than about 0.001" (one thou) you should really go for a re-bore and new pistons. Alternatively, if the wear is small you can hone the bores with a decent 3-legged hone and fit new rings.
The rings should have a fitted gap of about 0.003" per 1" of bore diameter. To measure the gap, take the rings off the pistons (very carefully!), slip them in turn into the bore and using an old piston with the rings removed, push the ring down the bore for about 1". Then measure the gap with feeler gauges. If the gap is too small, i.e., less than about 0.008", increase the gap by linishing the end of the ring until it is correct. However, most rings will be the correct gap, so it won't be a problem. Remember, the rings must go onto the pistons in the correct order and the right way up - most important. You really only need to check the ring gap if fitting new pistons or new rings.
If you really want to do the job properly, get re-bore, fit decent pistons, the Hepolite 21253 is excellent for a 1275 engine, check the crank and re-grind if necessary and fit all new main, big-end and thrust bearings plus a new oil pump anmd timing chain. Then, with new valve stem oil seals and, if necessary new valves, you'll have the basis for a really good engine. Also, if you have a standard bore 1275 cc and go out to 1330 cc, that's +0.060" on the bore, you'll increase the power by about 3 to 4 bhp without any other mods and the torque will improve a bit as well.
I hope this helps,

Peter

[Skyedriver]

Just to confirm, Peter's text, the "gap" is the "gap" between the two ends of the ring, not the gap between ring and bore.
Hope I am not treading on toes, sorry

[revingtosh]

You might not have seen my other thread, im looking at going down the nitrous route, but properly.

Any ideas on pistons?