Piston ring end gap

[muskrat]

G'day Fellas.
I've been looking for a decent set of rings for the Cafe. Thought I had a set of GPM's but was gazumped. As she is 72mm I'm looking for T120 Trihard +40 rings. SRM have German made Goetze, another brand is Grant made in USA. While looking at them they had a description below.

The new industry standard for British motorcycles is to gap the rings at .020 / New independent lab tests and studies (in the USA and England) show that any gap between .024 to .010 is efficient, and does not show more oil loss than normal gaping at .010 to .014 or does not show any less engine performance, or any extra loss of compression.
I understand that this information might be difficult to accept or understand because the way we have been doing things for the last 80 years but all the rings that are coming out of the USA (Hastings) are using this standard and the gap spec is .020 / JCC and Hepolite are also using this standard
If you are a purest or are racing the bike and want every mechanical advantage possible, we can have custom rings made with a gap of .010 and three piece oil rings, A piston sample is needed for this process.
In most maintenance manuals it says if the gap exceeds .014 then renewal is necessary.  This reason is more because if the gap has moved that much you know the knife edge of the rings is worn and or rounded, and at that point there will be so much corrosion from engine gases that the rings need to be renewed.
I still gap at 10, does anyone here gap at 20?
Cheers

[limeyrob]

No the last set I did was around 10.  20 would be quite visibly large but if that's what the tests say I'd try it.  Do we know why they are advocating the bigger gap?

[muskrat]

G'day Rob.
I don't know why either. Maybe new piston material has less expansion and can run tighter in the bore. 
So if the rings are made with a 20 gap that's it. The old days you would have to gap them yourself at whatever you wanted.
Cheers

[Swarfcut]

 Don't know what they mean by knife edge, are they saying the top edge of the ring is worn away? Sounds the engineering equivalent of "green wash". Original AE compression rings came as plain, tapered or even barrel profile. More like a bit of frightener style of copy.

 As for gapping, mine have always been within accepted dimension straight from the packet. So never had to risk breaking one....

 Swarfy.

[CheeserBeezer]

In Phil Irving's book, 'Tuning for Speed', a gap of 0.010" per inch of bore is recommended. It is acknowledged by the author that this is a generous gap and is intended for engines which are highly stressed or heated up quickly. This translates to 0.027" which most of us would consider to be excessive but I've come across engines which have been running perfectly well with much bigger gaps than that. I think this is an area where many of us might be overthinking the issue and perhaps getting unnecessarily hung up on what is best. The required ring gap depends on the linear expansion coefficient of the ring material, and only the manufacturer can tell you what that is. So, I'd always follow the manufacturer's recommendation. The worst thing that might happen is the ring gap be set too small and the rings 'bind' when the engine gets hot. Phil Irving's book is highly regarded and I would use his figure (0.010" per inch of diameter) as a maximum. If the manufacturer is quoting 0.020" as the recommended gap I would go with that. Generally, for an A10 in normal use I wouldn't go below 0.015", for fear of binding the rings when hot. This could cause serious damage. I suggest that the difference in performance between rings gapped at 0.015" and rings at 0.020" would be undetectable.

[groily]

https://hastingspistonrings.com/tech_tip/checking-compression-ring-gaps/
I've always gone 3-4 thou per inch of bore as a rule of thumb when new, but never worried if the gaps were a bit more. Which is what Hastings themselves say here, with their minimum of .0035".
But, as CB just said, considerably larger gaps won't have any obvious bad effects on most engines though. There are plenty of big singles chuffing around with 30 + thou gaps and no worries.
One man's 'worn out' is another's 'nicely run-in', as ever.
The degree of visible blow-by past the rings is what normally dictates next steps for me.

[limeyrob]

When I've stripped failed engines the piston damage is often at a ring gap where combustion has got though the gap and damaged the land then the ring starts to move up and down and widens the gap - well that's what I think happened, who really knows?  So I've worried that a larger gap would let the flame though, but perhaps 14 vs 25 thou is irrelevant in this context? 
Which reminds me, when I stripped the A10 (recently very poor rebuild) the compression rings were at 180 and the oil ring at 90, I would normally space them out equally at 120 with the oil gap to the rear.  It got me thinking, were they right on this?

[Triton Thrasher]

As this question appears to have a Triumph connection, I’ll post this with a clear conscience. 

People who have a workshop manual but not the updating Service Bulletin are sometimes perplexed by the gap on new rings.



On a related point, you can get better ring sealing by making the 2nd ring gap 25% larger than the top ring gap.

[Swarfcut]

 Rob, In my experience piston damage where the top land is eroded away is a result of leadfoots continuing to drive with the obvious signs of over advanced ignition. Think Fort Transits with 2 Litre Pinto Engine, contact breaker dizzy, Main Dealer spanner monkey getting it wrong.  Had a few through the shop like that, always on number 3 cylinder.

 Reckon you have to be a pretty slick molecule to work your way past any size ring gap maze, but it does make sense to position them at 120, at least on assembly.

 Swarfy.

[Colsbeeza]

I have always set mine at 0.012" or 12 thou by the old rule 0.004" per inch diameter. When fitting new rings I have found they are never the same. The last set I fitted had most at 0.010-0.015" and one at 0.005", so just ground the end of that one. I presume that rebores are difficult to get perfectly within tight tolerances so gap must allow for that. Given the steady pace I ride at, I am not too concerned. 0.020" may be appropriate for modern higher performance motors.
Col

[chaterlea25]

Hi All,
Apparently it does not matter where you put the end gaps on the rings as they rotate relative to the piston as the engine runs, since reading about this on the net I'm not so particular any more.

As to end gaps I aim for 3-4 thou per inch of bore

John

[muskrat]

G'day John.
You are correct about them moving around the piston. I still put the gaps 120 apart. My thinking is when new and not bed in having them at 120 there's less blow by right from the start.
Cheers