Author Topic: Bottom ring broke into 5 pieces, thoughts? (Read 5677 times)

mx250r91 · « **on:** March 16, 2014, 01:43:15 PM »

I tore down the 85 yesterday for a refresh before this season and was surprised to find the bottom piston ring broken into 5 pieces, fully in the correct place. The engine ran fine and compression was ok at 225 psi. The top end has about 50 hours on it since being rebuilt by the previous owner. I wonder what could have caused this? Bad bore job, poor chamfering on the bottom of the ports? I can remember last year before a race hearing a mysterious 'rattle' from the engine. Turns out it was probably this ring rattling in place, may have already been broken then. The sleeve has some wearing above and below the ports where the ring was broken so it will probably take a few overbores to clean up. I'm looking to see if anyone has seen this before or has an explanation of cause so it doesn't happen again. Thanks

havinnoj · « **Reply #1 on:** March 16, 2014, 03:50:14 PM »

Consider poor lubrication from being too lean, oversized rings for the piston, poor ring to cylinder tolerance, or just worn out. How does the piston and plug look?

I doubt this the ring locating pins are your problem but take a look - are they both intact? My guess is that they are since when the rings rotate, it's usually the tips that get damaged and yours look pretty good.

jcs003 · « **Reply #2 on:** March 16, 2014, 04:43:24 PM »

possibly, poor break in procedure.

john

Burns363R · « **Reply #3 on:** March 16, 2014, 05:42:34 PM »

Maybe the ring gap wasnt set right? To tight? Thermal expansion could explain this.

C-Leigh Racing · « **Reply #4 on:** March 16, 2014, 06:52:05 PM »

Show us a good clear close up pic of those port windows in the cylinder.
Neil

mx250r91 · « **Reply #5 on:** March 16, 2014, 07:35:58 PM »

Quote from: C-Leigh Racing;29132

Show us a good clear close up pic of those port windows in the cylinder.
Neil

Here is a picture of the exhaust port where a break was.

Lubrication was good, Klotz R50 @ 32:1 crank journals covered in oil. Top of the piston was perfect, no carbon build up or signs of detonation. It is a cast piston, unknown brand, 66.75mm. The rings were both in their locating pins which were in tact, no ring rotation occurred. Could be poor break in procedure, I think when I bought it there were ~10 hours on the engine after rebuild and it was run on ~pump gas~ gasp! which I changed over to 100 octane race gas when I got it.

Possibly it could be in the assembly, the transmission had a random issue not going into 3rd gear. I've never had an issue shifting an R transmission after rebuilding it which was another reason for tear down, not leaving me with a lot of confidence in whomever put this engine together.

Jerry Hall · « **Reply #6 on:** March 17, 2014, 01:38:11 AM »

I think that your bottom ring is cast iron. I have not seen cast iron rings in a two stroke since the 1970s. Most good piston companies use chrome plated steel rings in their two strokes. Steel rings can be bent or twisted a lot before they will break. Take a piece of the broken ring and bend it and see if it breaks or stays bent. Bend the top ring and see if it breaks or bends like a piece of steel.

Some of the piston kits are now from China. Their quality control is very poor and often make parts from melted down scrap metal of unknown composition.

C-Leigh Racing · « **Reply #7 on:** March 17, 2014, 11:24:52 AM »

I was thinking about the champher on the port windows, maybe not been done & that ring snagged the port, but why only that one ring & not both.
In the pic it looks like may be some champher, but very little. Some porters though will not put a lot of champher on the window edge for fear of changing the port timing.

I guess the best thing, is to check that piston kit out like Jerry mentioned & see where it came from.
Neil

rablack21 · « **Reply #8 on:** March 17, 2014, 12:20:48 PM »

If your rings are not cast iron, I an thinking they might not have been of good quality or to spec. Judging from the breaks, it appears like the ring was or became very brittle. I'm guessing it was like that when installed and not because of overheating during use.

jcs003 · « **Reply #9 on:** March 17, 2014, 04:44:18 PM »

it is possible the ring is the wrong size for that piston. being too big of a ring will surely cause issues.

john

mx250r91 · « **Reply #10 on:** March 17, 2014, 06:18:38 PM »

Quote from: Jerry Hall;29167

Take a piece of the broken ring and bend it and see if it breaks or stays bent. Bend the top ring and see if it breaks or bends like a piece of steel.

I tried to bend a piece of the broken bottom ring and it snapped off instantly and cleanly. Then I bent the top ring in the same fashion and it bent about 90 degrees before snapping off.

It's hard to see in the photo but the color of the metal at the breaks is different between the two rings. The bottom ring is a lighter, slightly more pourous, whiter color metal and the top ring is darker.

I took the piston off and underneath is marked 'ART' which I believe to be a good OEM quality Japanese piston. The rings are just marked 'T' and '2T' and 75 on each. They are the correct size for the piston. I measured the ring endgap of the top ring before I broke it and it was 0.020"

Could this just be a case of a poor quality ring material that slipped by?

C-Leigh Racing · « **Reply #11 on:** March 17, 2014, 08:48:23 PM »

After seeing that pic of the rings, they must be cast iron, because I've never seen a 2 stroke ring break like that. Now I have on Briggs & Stratton iron rings, except for the three piece third oil ring.

When you have that cylinder bored & new piston fitted, be sure they champher those port windows proper.
Neil

kb250r · « **Reply #12 on:** March 17, 2014, 11:32:34 PM »

Aww man...now we are getting who-dooded on our piston rings.!!!...Aww man watch out...flea bay is taking over the world!!!!:securitycamera:

Tbone07 · « **Reply #13 on:** March 18, 2014, 11:47:36 AM »

Wow very interesting. Looks like Jerry was right....as usual lol

Jerry Hall · « **Reply #14 on:** March 19, 2014, 02:08:43 AM »

Quote from: mx250r91;29227

I tried to bend a piece of the broken bottom ring and it snapped off instantly and cleanly. Then I bent the top ring in the same fashion and it bent about 90 degrees before snapping off.

It's hard to see in the photo but the color of the metal at the breaks is different between the two rings. The bottom ring is a lighter, slightly more pourous, whiter color metal and the top ring is darker.

I took the piston off and underneath is marked 'ART' which I believe to be a good OEM quality Japanese piston. The rings are just marked 'T' and '2T' and 75 on each. They are the correct size for the piston. I measured the ring endgap of the top ring before I broke it and it was 0.020"

Could this just be a case of a poor quality ring material that slipped by?

Art makes the majority of the OEM pistons for the big 4 Japanese motorcycle companies. I am surprised that it had one steel ring and one cast iron ring. A lot of the Chinese auto quads and scooters in the 50cc to 90cc engines use cast iron rings and they are always breaking when guys make the ports too wide and or do not know how to put the correct shaped chamfer on the ports to keep them from eating rings.