1970 Suzuki T500: shattered piston skirt!!!

[ericsattic]

I was riding my 1970 T500 and my left cylinder went dead on me, no noises or anything it just died, luckily I was close to home and managed to limp back home on one cylinder. I immediately pulled the plug and noticed the electrode had just about melted together because it had a small piece of metal stuck in between the electrodes. No hole in the piston, no scorching, burns, or other visible damage to top of piston, cylinder walls as smooth as glass, etc... But I also found a couple of tiny pieces of metal inside the exhaust port... I pulled the jug and low and behold the skirt on the piston facing the exhaust port has been completely shattered!!! The top of the piston looks fine and there is no damage to the rings, etc... Plenty of two-stroke oil seems to be getting to the cylinder, crank, etc... It seems "most" of the pieces of the shattered skirt ended up in the exhaust pipe; I found them there after I removed the pipe and baffle, then lots of tiny pieces fell out. I believe the piston is a genuine Suzuki piston; top of the piston has an "L" on it with an arrow, which is correctly facing the exhaust port. The piston is also correct for the 1970 T500 because the skirts are solid (pistons for the 1968 Cobra are different and have a hole in the skirt). I have done a little research and unfortunately it seems that shattered piston skirts like this are not all that uncommon on the T500. I am just getting the thread started here so I have some questions:

*what are some causes for this type of piston failure - too lean fuel mixture due to carb settings (jets are stock); pre-ignition due to incorrect timing; crappy piston design, etc...?

* just prior to this the plug had completely fouled on a prior ride. A week or so later I changed the plu with a new one, which was in use on this fateful excursion.

* Plugs in the bike were a B7HS, though many people use them I now realize those are not truly correct, they may be too hot, I purchased some B77HC plugs and will use those in the future on this 70 T500 and already put them in my 68 Cobra.

*with regard to fuel mixture I have a habit of putting the 91 octane lead-free gas in my bikes (no more) and in the fall I had loaded this bike's fuel up and ran it shortly with what I now think is too much stabil fuel stabilizer for fuels with ethanol, along with a strong dose of sea foam for winter storage! Even if its not the problem, I now use the lower-octane fuel AND the correct mixture of fuel stabilizer.

*it seems "most" of the piston debris ended up in my exhuast pipe; but what about potential debris i the crankcase? This is an "early" model T500 and has crankcase drain plug holes; could I pour say diesel fuel into the crankcase and attempt to wash out smaller pieces? Does the case have to be split?

*Should I replace the piston with a genuine Suzuki piston, or go with say a wiseco? If I go with something else, it's obvious I should replace BOTH pistons.

* I managed to remove the exterior piston pin "circlip" that holds the piston in place with the piston pin on the connecting rod. However, the interior circlip is very hard to get to. Is removing the other jug the only way to get more space to get to the other circlip, or does someone have any special tricks they can share?


Obviously more to follow, thanks to all in advance for reading and offering some potential advice, Eric

[titan performance]

I have never suffered this problem on the 500, despite riding a great deal of miles on these bikes over the years. I did however come close, but luckily happened to notice a crack in the skirt one time, which would have undoubtedly ended in the same result, probably sooner rather than later.
I put it down to wear and tear and fatigue......they are a pretty big pistons slapping around in there, so they're not going to last forever. I don't think, (I maybe wrong), that it's fuel related, as this usually ends up with a melted piston crown in my experience.
In my younger more wreckless days, I melted several pistons, and would simply nail in another and get back on it, with no regard to removing any debris. I know it's wrong, but I really never suffered any problems as a result. I also shattered a piston in a CR125 crosser, and simply washed out the crank case and put in a new piston, also without any problems. I'm not suggesting you follow my bad example, as a lump of piston finding it's way somewhere you don't want it, could be disasterous.

[desmocat]

I have destroyed several T500 piston skirts in my TR500 race bike. The stock Suzuki T500/GT750 pistons are prone to skirt cracking at due to over-reving or continued operation at high rpm's. I finally went to Wiseco's.

[ericsattic]

I have destroyed several T500 piston skirts in my TR500 race bike. The stock Suzuki T500/GT750 pistons are prone to skirt cracking at due to over-reving or continued operation at high rpm's. I finally went to Wiseco's.

Thank you for this excellent information. My bike is not a racer, it's just a rider, and I do not ride it that hard. However, I would like to replace my pistons with Wisecos. Any tips on where I might find some, especially in the 71mm size as I will have to bore out my standard cylinders?

[ericsattic]

Greetings to all and thanks for the responses and PMs about this. Have not been on the board for a while and I promise not to give you all the lame excuses why. ANYWAY, back to this T500 for what it's worth. I pulled the heads, cylinders, and pistons off and I finally learned why the two stroke addicts are always talking about crankshafts. I believe the reason my piston skirt shattered is because of a combination of excessive piston / cylinder clearance AND a shot crankshaft. It seems to me the bearings on the big end of the connecting rod / crank pin bearings are excessively worn. So, I am going to have the crank rebuilt, cylinders bored, buy new pistons, rings, pins, etc... Probably just a six-pack's worth of repairs for most of you, but for me it's a 1/2 keg job (just wish some of you lived closer to help me drink it - and pitch in on the work)!

I put together a bunch of photos in a picassa web album that can be accessed in the link below. Click on the link, click on the small thumbnail photo in the upper left corner of the page to enlarge the photos and get the slide show started, then just click the grey arrows on either side of the photos to toggle through it all. Read the narrative too, some hidden topics for another post. Looking forward to some comments. PS - from now on when I purchase a "running" bike I am going to pull the heads and cylinders as soon as I can to inspect the pistons and connecting rods for wear before I do too much riding!

Regards, Eric

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