How bad must you melt a piston before it has to be replaced?

Heh, I bet that title made your hair stand on end.

However here's the dilemma.

This past Tuesday I was out riding ALONE through the Michigan woods on my WR500, ringing it out on some deep sandy sweepers somewhere north of Higgins Lake, having a great time. I had been out there for about an hour or so, feeling good, averaging 40 MPH or so with peaks up to about 52 MPH.

I was getting some occasional pinging, but when I backed off, it went away for a while. At some point, I hit some good, drifty areas, and I was spraying sand, when the pinging came back quickly, and a second or so later It felt like I had just applied the rear brake. I chopped the throttle and pulled the clutch, and immediately the engine quit: ...as in LOCKED UP. The fuel filter, which usually is halfway full of fuel, was DRY, as if the tank quit delivering fuel. I have that stupid Blue Lightning tank sealer in my tank, and it's begun to peel. I suspect a chunk of it might have temporarily blocked off the petcock :bang: I probably ought to replace the tank now to get rid of that stuff.

Coasting to a stop in the middle of the hot and very empty woods, with no cell phone coverage, about five miles from anything, in the MIDDLE of the week, following a big national race, the chances of me seeing someone were slim. (I never did). I drank my camelback, sat in the shade, and pushed the bike toward town in 300 yard hops before sitting down to cool off. After half an hour, the piston released, but the engine felt like it had NO compression. I figured that was it, I had holed the piston. Another half an hour later, a kickover produced a very weak compression. So I kicked it over about four times and to my surprise, it started up! I put my gear on, and restarted it, and babied it the 27 miles back to camp. By then, it was running pretty much like normal, and it idled fine, and started fine. Later that day, the compression felt like it has returned to normal :coocoo:

I'm home now, and I've pulled the pipe and the head. What I see is a small score on the front (exhaust side) of the piston skirt, and the very edge of the piston facing the exhaust port is slightly melted off. It looks like I could just pull the cylinder, and smooth off that edge of the piston. It isnt melted down to the ring land, just a slight amount of the edge was white and bumpy. The cylinder surface on a casual inspection looks pretty good. A bit black and oily, I havent inspected it closely yet. No holes in the piston; Before installation, I had used Tech Line ceramic thermal barrier coating on the piston crown and the combustion chamber dome. I suspect that it might have made the difference between a partial meltdown and a holed piston.

So my question is, if I just melted the one edge on the piston, could I simply sand it smooth again, live with the slight reduction in compression ratio, and safely reuse it, rather than shell out $160 for a new piston?

Clean up starts today; and I'm going to remove the cylinder for a thorough going-over. I have about 60 hours on the motor with thus far no issues; Jetting has been spot-on up till now; maybe even slightly rich. I've been one of the lucky few who never fouls out a plug with their air hammer.

It'll be interesting to see how much scoring there is on the bore. Hopefully nothing that can't be honed out. Since the YZ490/WR500 is a cast iron liner, there's no Nickasil to worry about. I probably will order a new piston ; certainly new rings. Maybe I'll take some pictures once everything's cleaned up.

I finally got the piston out today. It was galled up on both sides nearest the wrist pin hole, and there's light scoring on the cylinder wall. The piston got hot enough that the wrist pin discolored with the slightest sign of brinnelling on the needle bearing race. The underside of the dome has baked-on oil, and it's brown all the way up to the wristpin and the back side of the exhaust skirt. It must've gotten really, really hot! Everything else looks great. There's lots of extra oil still in the bottom of the crank case.The lower ring is stuck in the groove and won't move anymore. As for the top, only the one edge that faces the exhaust port, about 1 cm, had visible signs of melting. All in all, minor damage but yeah, the piston's toast. It would've just gotten worse with time and wrecked something else eventually. There was three summers, and 60-some hours of run time on the top-end anyway. I'm not sure if the cylinder will clean up with honing or not. I'll probably put it in a box and send it off to Eric Gorr, since he did the porting for me last time, and let him determine what it needs, and let him pick up the appropriate replacement piston.

[edit]

Looks like Eric is out of the office for all of September. Anyone else have a suggestion as to where I should send my piston and cylinder?

2-Strokes 4-ever said:

A good friend of mine does most of the motor work for Paul Strickland at Strictly Hodaka. He says the best boring/replating work he has seen is coming from Powerseal. Not at all to take away from Forward Motion, too bad he's not available right now.

Click to expand...

Interesting idea: Nickasil the bore. I wonder if Nickasil, coupled with Ceramic thermal barrier coating the piston dome, (and of course BULLETPROOF fuel delivery) would help. I was running it pretty hard in some really hot conditions and it was pinging. I dunno.... Perhaps I should go back to buying the Cam2 at $8 a gallon and running that when in the woods. Been thinking that maybe I should fix it & sell it to a collector, & buy something newer with 200 or 300cc less engine... However it's been a solid bike thus far & never stalls, even when I'm in trouble or in the wrong gear. Starts every time on the second kick, unless I forget, and leave the petcock on when parked. It's also kind of cool, motoring about town on a street-registered 500 two-stroke. I once even entered it and rode it in a Harley Davidson charity ride and it out-thumped the thunder.

I've downloaded Powerseal's work order form and will probably give them a go.

Thanks!
-Dan

Thanks FNG,

Actually, I had Eric Gorr recut the cylinder head for use with 93 octant fuel already and it certainly made it more useful of a machine. I went round and round on the jetting, and cleaned up the bottom end and midrange with swapping out from a Q8 to a Q4 the needle jet and putting in a leaner low-speed jet. I've moved the needle up & down, put on different expansion chambers & mufflers, fiddled with the main jet (and settled on the stock 440 main), and finished up with a swap to FMF power reeds to get the engine running smooth & starting reliably. Eric Gorr even ported the cylinder for trail use. I've had Garry from EFM Autoclutch custom-CNC an autoclutch for this engine, and I've milled, extended, & TIG welded the magnesium clutch cover in order to give it easy-to-use, dial-a-wheelie control, so that if I stab the brake, I don't accidently stall the motor. Now it'll torque through the tight woods in second gear, and still wail down a long sandy straight at 84 MPH. After running it in stop & go street traffic, doing 50 miles of highway, keeping it together for a cold, rainy 5-hour long trailfest, I thought it was getting pretty bulletproof. Then what I think was a chunk of that failed tank liner plugged up the fuel line during an hour's worth of sandy fire trail blasting and viola! It leaned out, got hot, pinged, and seized. I don't know the extent of whether it was the 93 octane (probably not), or the lack of fuel delivery that locked it up. I do know that at least twice that day I had to back off because the engine had been pinging. Fuel related or fuel delivery related, I am not sure. I always thought that as the day got hotter and more humid, the fatter an engine runs. This bike rips on cold weather days... and the plug in the head is blackish whereas it's normally a dark tan - so I think it had been running richer than normal before the failure. YZ490/WR500's are supposed to be particularly susceptible to temperature, barometric changes, and load requirements so maybe that was part of it. I'm just trying to think of ways to make it more resistant to those kinds of problems.

Well the cylinder is off at Powerseal and they are going to Nickasil the cylinder. I had to cut expenses a little so I chose to NOT ceramic coat the piston this time. Gad, I wish I still had some of that Techline Coating around... The stuff I put on the combustion chamber is still perfect and intact. The carbon nearly washed off the chamber dome with just a little spray of Simple Green.

Anyway, I Hope to have it back together in time for the annual Michigan Trailfest, though I may not be "in shape", having logged only a total of one afternoon of trail time this whole year... It's getting to where I am not sure why I'm still trying to do this stuff. I should just act my age, get fat like the rest of my neighbors and get old like a good, dependable husband should.