Good morning,
I have a 1995 softatial fxstc with about 30,000 miles I bought used about 6 months ago. Riding it home on thanksgiving and it starts acting like it wants to die at idle (figure the carb needs some cleaning try that no go) long story short I check compression have 60 psi in the front and 125 psi in rear do a leak down test and have air coming out of the breathers blowing by the rings. So I tear it down and both compression rings are broken in half on the front cylinder and on the rear 1 ring is broken in 3 pieces and one is still intact. The pistons and cylinders are oddly in decent useable shape with wear you'd expect to see with that mileage. Its has a crane H 306-2 cam with adjustable push rods, I replaced the crumby drag pipes it came with, with a set of pythons recently and it has some sort of aftermarket air cleaner.

Any ideas on what would cause just the piston rings to be fail like that??

 

Improper gapping and running too hot/lean are the 2 most common causes. It they are factory rings I would suspect the later then.

 

It's usually because there was too little ring end gap, but the damage usually occurs early on not after 30k miles.

Detonation from poor fuel or too much ignition timing advance (or the combination of the two) or engine overheating can contribute to that. I'd be curious about the condition of the ring lands when everything is apart. I'd also be curious about the condition of the exhaust valves because short drag pipes and relatively long overlap with the camshaft can have an effect on that.

If you could post a few pictures of the pistons, rings and cylinders there may be a few clues. Also look at your spark plugs for signs of detonation which would typically leave specs on the porcelain part of the plugs.

Paul

 

I wouldn't worry about why they're broken. I'd install new ones correctly and hone the cylinder and call it a day.

 

Anyone with a half of a brain is going to want to figure out why they broke so it doesn't happen again, or even cause other/worse damage.

 

Which you can assess after you replace them. If the carb needs to be jetted or cleaned or both, now is the time. But you can't answer that by looking at the broken rings.

 

all the rings appear to have a slight ridge on the exterior (not sure if that's how they originally came or is a result of damage?)

The exhaust valves both have some pitting The intake vavles both look almost new

The piston lands are in good shape I installed new rings and measured the side clearance top is around .004-.005 and second ring is around .003-.004

this is how the cylinders look after a few mins with a hone both are in useable condition

Sorry probably would have been better to take photos before I cleaned everything up. The heads and piston tops had an abnormally hard and difficult to remove carbon build up especially the rear. And the base gaskets were also very difficult to remove they had pretty much become one with the cylinders. (not sure if this might be a result of overheating?)

I appreciate everyone's responses! My current plan is to hone and re-ring but I'd like to know why they failed to avoid to problem in the future

 

If you ball honed the cylinders, I'd mic them to be sure they are round and not tapered and piston skirt clearance. But you will need torque plates to do that.

The piston does not look like an OEM piston. Are there any numbers or markings that indicate a brand or anything else? Any chance the engine has had work done previously?

The exhaust valves are worse than I would expect from a 30k miles engine. How were the spark plugs?

Paul

 

Since this top end refresh is due to a problem involving piston rings, I would have a reputable shop check/match the pistons, rings, and cylinders for a proper fit..... You may find an issue not visible to the naked eye... like an out of round piston mentioned by pgreer.

I'd want to make sure all three were perfect and matched.... I recently did it with a top end refresh on one of my bikes. I sent off the pistons, heads, & cylinders to a skilled Harley shop. I got back pistons, rings, heads, and cylinders. They were all labelled for assembly to front and rear. He gave it a concentric 4-angle valve job, and checked the springs were to spec. He even cc'd the chambers and did adjustments to have them match. He did very good work. It was really nice not to have to deal with that myself...

Of course, if you have the tools and skills to do it yourself... you've probably already done this....

I suggest a shop, so if you don't have the tools or skills like me, it will be done perfect. It would really suck to have to tear it down again soon.....

Good luck with your repair...

 

I, too, prefer engine spec repairs. Tolarances are everything.
I have lived in areas where the DIY approach was necessary.
Given the option, go with the shop set up for it..