Suzuki 2 Strokes

I started a tear down of a GT550L engine today. I heard this motor run and it had a heavy knock in the left side/left cylinder when hot. I even took it for a test drive. Everything worked just fine except for the knock. It has the updated starter clutch so the electric start works, the transmission is quiet and tight, the clutch is smooth and she still had great throttle response! The owner thought it had a failed con rod bearing, swapped out this engine for one from a parts bike he had, and wanted to know if I wanted the engine for spare parts! If the price was right, sure I'd take it. He said come and get it before he throws it out!! I don't know if I even hung up the phone, but I was out the door and into my car as soon as he said that!
Got it home and it's complete, just missing the carbs. I pulled off the head, normal carbon build up and didn't see or feel any excess play in the pistons. Pulled the left cylinder off and removed the piston. The con rod has 1.250 mm shake side to side, small end looks good, large end looks good. I can't see that it was a con rod knocking. I cleaned up and measured the piston, it's within specs, but it's CRACKED!! The crack runs about 1/3 the way across the crown and down the right side into the piston pin bore! I'm hoping this is all I'll find wrong with it!
Cliff!

Thats a great deal getting that engine, if you can replace the piston and test it out on a bench you would know for sure if the piston was causing the noise before you go any further.

The rod sound fine just so it has no up and down slop on the pin. Most of the time pistons crack from being worn out and sloppy in the bore. I tore a 380 motor down for a knock and the wrist pin had a groove worn in it I couldn't believe, the wrist pin bearing was shot.

Ive seen a rod that had the top end pin so badly worn it visibly elongated the rods eye. The pin had to be forcibly pressed out since for it to pass out one side of the piston it wanted to re-round the rods eye.

Nice score, Cliff. It always a good thing to have a nice spare engine waiting on the bench.....

Rick, a nice spare engine? Ha! I think this is engine no. 5 that I've picked up!! I'm going to have to split the cases on this one though. The crankshaft seal behind the alternator is leaking injector oil! That's a flanged seal and cannot be changed without splitting the cases!

I was asked what would crack a piston. With this engine, my guess would be detonation. After cleaning the carbon off the crowns I can see the dimples and pitting. Anyone have any other thoughts or causes? All three piston crowns are pitted the same amount, had the same carbon build up, the skirts show no signs of overheating or being seized. The only other thing I saw, other than the crack in the left piston, was the left piston crown has a flat spot in the middle about 17mm in diameter with a slight indent right in the middle of it, the center cylinder piston crown is rounded with no flat spot at all, and the right cylinder piston crown has the same diameter flat spot as the left but no indent. This engine is all original with 40,000 plus miles on it!

Cliff, IIRC, the original pistons on the early bikes did NOT have a machined spot face in the centre of the crown....but the later engines and replacement pistons DID have that spot face. Maybe your engine has already had a couple of pistons replaced along the way.

Indent ?? Yep, probably a detonation episode or 2 caused that. The timing for that cylinder may have been a little too far advanced.....or maybe some of the carbon got hot after a long run and caused the detonation. You/we will probably never figure out exactly what happened. Sometimes it can take only one or two good detonations to crack a piston....if they're severe enough. The stock cast pistons are NOT exactly the strongest examples of the piston makers' art.....