Ring Gap Tolerances vs Service Limits

I'm giving my R850GS a good overhaul and have the heads and bores off (for repainting) and am looking at the ring gaps.

Reading my Haynes manual there are 2 specs for each ring, Min, max and a service limit.
For example:
Top ring: 0.10mm - 0.30mm, service limit 0.80mm.
2nd ring: 0.30mm - 0.50mm, service limit 1.00mm.
Oil scraper: 0.30mm - 0.60mm, service limit 1.20mm.

What is the purpose of the service limit value? Surly if the gap is larger than the max value then it should be replaced, why bother with an even higher value for the service limit.

On one cylinder I have a top ring gap of 0.23mm, a 2nd ring of 0.55mm and an oil scraper gap of 0.48mm - so the 2nd ring is out of spec, but it's well within the service limit. Considering the other two are well in the middle of the tolerance, does it warrant a replacement set of rings?

Padowan said:

Considering the other two are well in the middle of the tolerance, does it warrant a replacement set of rings?

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If you're planning on keeping it a while, and it's already apart, why not replace them anyway? I'd say it's worth it for peace of mind. You won't have that nagging thought in the back of your mind that one of the rings is out of spec and will need changing at a later date.

Yeah, i know that's the voice of reason, i can just see the overhaul becoming a slippery slope if I change everything that's the slightest bit worn.

buellzebub said:

If you're planning on keeping it a while, and it's already apart, why not replace them anyway? I'd say it's worth it for peace of mind. You won't have that nagging thought in the back of your mind that one of the rings is out of spec and will need changing at a later date.

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I think I'd go belt & braces and replace for new anyway if I'd got the engine stripped that far. They 'might' be OK as they are, but certainly will be OK if new.

How Many Miles on it?

Did it burn oil before you stripped it ??

Are the bores heavily marked?

Piston heads/Skirts, Heavily marked?

IN the greater scheme of things If

On one cylinder
I have a top ring gap of 0.23mm,
a 2nd ring of 0.55mm
and an oil scraper gap of 0.48mm -
so the 2nd ring is out of spec, but it's well within the service limit. Considering the other two are well in the middle of the tolerance, does it warrant a replacement set of rings

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If you want to be fussy fit them but If the answers to most of the questions are no or not really Let them be!

I think the primary distances are to account for differences in manufacturing tolerances between pistons and rings

service limits is what you should work to!

Not high mileage - just over 30,000 miles.
Not burning significant amounts of oil, just the "usual" bit of smoke on cold startup if left.
Bores look good, still plenty of witness marks from the honing.
Pistons also look good - bit of carbon buildup on the top which has been cleaned off, skirts show no evidence of any slop.

The main reason for the stripdown was for:
1. to get the heads and bores stripped and painted as the original paint is all flaking off
2. replace the exhaust studs (I need an engineering shop to remove 2 sheared studs from one head
3. investigate the cause of lower compression in the RH cylinder (which I think is valve seating related based on a flood test so I will be lapping those when they go back in)

So the checking of the other tolerances was just cursory while the engine was apart, but like others have said, while I'm in there it makes sense - it's just how much sense it makes for the benefit.

I'm surprised that only one ring would have worn that much more than the others - it's so close to in-spec that a new one might be so close as to be nigh-on identical!

A recent study found that rings gaps of 60thou (old school ) and even more actually made very little difference to the way an engine runs. The only thing they found that made a real difference was to have the ring gap on the second ring smaller than the top one.

Padowan said:

.............

I'm surprised that only one ring would have worn that much more than the others - it's so close to in-spec that a new one might be so close as to be nigh-on identical!

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As you don't know what the gap was new it's impossible to say if it's worn more than any of the other rings.

i.e. It could have started out at the max gap while one of the other rings started off at the min gap. As has been said, the difference between min and max gaps are tolerances for new rings.

All your rings are well within service limits so no need to replace

Andres

Every ring is made to a tolerance which is the min max gap. As the ring wears the gap gets bigger and when it gets to the service limit it is worn out.

Did you check the gaps at the bottom of the bore?

Padowan said:

Not high mileage - just over 30,000 miles.
Not burning significant amounts of oil,

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That alone should answer the question

Padowan said:

I'm surprised that only one ring would have worn that much more than the others - it's so close to in-spec that a new one might be so close as to be nigh-on identical!

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And I reckon that answer would support it

Basically if you are going to race the bike, it may be worth considering

But in general use and the relatively low mileage I would think not necessary