GS850 piston ring orientation and markings

[eil]

Hello! I finally decided to bit the bullet and swap the head/carbs on my '80 L model and '79 G model. No mysteries/issues on the L model since I did a top-end rebuild on that a couple of years ago. However, the G has been more of a challenge.

When I got the head off the G, I saw quite a lot of carbon buildup on the head and pistons. I was going to just leave it alone since the bike ran fine and didn't seem to be burning much oil at all. But the more I looked at it, I decided to go ahead and dig in a little deeper while I was there and clean everything up. Reasoning that if something went wrong a few years down the road, I would kick myself in the butt for now being more thorough right now.

So i pulled off the pistons and to my eyes it looks like this engine overheated at some point in the past and a bunch of oil got baked onto the pistons themselves. I've spent a few hours now scraping the crud off the pistons, the rings, the ring grooves, clearing out the oil holes, etc. Now I'm putting everything back how it was and have a couple of questions.

1. Ring markings: The FSM says that both top rings are stamped with the letter "R". Most of mine are stamped with "N". Or Maybe Z. But definitely not R. What does that mean? Are these oversize rings? Aftermarket? (A few are missing the mark altogether, I assume it has just worn off.)

2. Ring orientation: At least one of the top rings (I don't remember which) was upside-down with the "N" facing toward the piston skirt. Not a big deal, as far as I know. Technically should work fine either way. But I found one of the 2nd rings without a mark. How do I know the right orientation in that case? The FSM notes that the 2nd ring has a specific tapered profile but I've tried looking at it every way I can think of and I can't see a taper on _any_ of the 2nd rings with the naked eye. Is there a trick to this?

And not that I _want_ to do this, but what exactly would go wrong if the 2nd ring is accidentally installed backwards?

I could use the L model for comparison except that one's already mostly put back together...

Thanks for any hints.

 

[tom203]

Let's bump this up.....someone here must know..

"FSM notes that the 2nd ring has a specific tapered profile but I've tried looking at it every way I can think of and I can't see a taper on _any_ of the 2nd rings with the naked eye."

have you tried stacking them on each other to see if the taper can be discerned?

 

[Grimly]

I came across this while installing the factory rings on my 850. The ones that were supposed to have a taper on them, didn't. The markings were on the top surfaces of them, so I went with that. I'm fairly sure that the lack of a taper would likely have meant they could have been installed upside down with no detriment.

 

[Steve]

If there are marks, the marks go up.

If there are discernable tapers, the larger side of the gap goes up.

If there are no marks, look even closer for a taper.

What would happen if they are upside down? They would not seal as well, allowing more oil to bypass the rings, possibly even baking it onto the pistons.

.

 

[bobgroger]

If there was a lot of carbon look at the valve stem seals....

 

[bwringer]

A certain amount of carbon is inevitable with any air-cooled engine. Heavy buildup, or "wet" or "fluffy" or otherwise unstable buildup can indicate a problem, but there will always be some.

And I would presume that upgraded valve stem seals are going to be part of any work that involves removing the cylinder head.

 

[eil]

Thanks for the replies, I had given up on any hope of a response... but as usual you guys came through anyway.

I lost the reference but somewhere either on this forum or elsewhere on the internets, I ran across a thread from someone asking the same question. One of the responses shows a snippet from a manual. Not sure which one but definitely not the 850 FSM. Apparently the rings marked "N" don't have a taper and can go either way. What those "N" rings are doing in my '79 850 I'll never know.

By the way, I got the bike put back together Sunday night and it runs absolutely fantastic.

Edit to address the carbon issue: I believe in a previous life this engine was abused... we know it was running lean at one time because of the blue on the exhaust and the carbon on the pistons seems to me like baked-on oil. There's no mechanical damage to the cylinders or pistons that I can see. But it could also have been bad valve seals in the head that the PO replaced. In any case, the head that's on it now has relatively new valve seals.

 

[Grimly]

By the way, I got the bike put back together Sunday night and it runs absolutely fantastic.

Excellent.