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pictures are kind blury,..its hard to tell why motors go some times.
if it was mine,....i would have it re chromed,
get two piston sets,....freshin up both sides.
be sure to clean the crank case real good,...
 

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It does not appear to be a burndown since the piston crown is intact on the exhaust side. Looks more like the ring snagged in the exhaust port.

Look closely at the piston and see if the ring gap rotated to the exhaust port. Is the piston ring locating pin still in the piston? Is the engine high mileage? If the locating pin is still there, did it migrate inward in its hole? Any of these factors could have allowed the ring to rotate and cause what you see. Or, the ring could have simply fatigued, broke, and that piece snagged.

Cats of your vintage had a plethora of piston and ring problems. Best bet is to have the cylinder reworked, pull the other side down, change both pistons with Wiesco slugs, which will come with rings, wrist pins, wrist pin bearings and locks. Millenium technologies or US Chrome offer a cylinder exchange program, where you buy one of their reworked ones and return your old one as a core. Cost is about half of what a new one would run you from Cat, and its every bit as good. Run a ball hone through your PTO cylinder a few passes (removes the glaze, adds some cross hatch), wash the cylinder really well in some hot soapy water. Blow dry it dry, wipe a little oil on the cylinder walls. When your new piston sets and reworked cylinder arrive, put it all back together. Premix your first tank of gas, make sure the oil pump is bled. Fill the cooling system and get the front end of the sled up off the ground; this will allow any trapped air in the rear heat exchanger to burp to the front and escape (leave the coolant bottle cap off while this occurs), it may take awhile. Start the sled up. I usually take a long welding rod with a hook on one end, reach down between the throttle bodies and snag the oil pump lever and pull it up to full open. Its going to be smoky in your garage. Do this for a minute or so. You then know the oil pump is purged. Allow the warming coolant to circulate to the rear exchanger and feel for it returning back to the engine warm. You no doubt will see air bubbles returning into your coolant tank. Top off the level as necessary. Very important to get the cooling system bled of all the air. Lots of Cats, yours included, the ass-end sits higher than the engine. Trapped air will sit right there, and the coolant won't push it out. You must get the engine up higher than the rear to get this done.

Should be good to go another several thousand miles! I've done several, mine and a friends.

ps: now is a good time to check the throttle body rubber mounts for cracks. change them if they are. The large rectangular rubber boot that feeds the throttle body pair is prone to literally disintegrating as well.

Good luck. Post if you need help.
 

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Discussion Starter · #6 ·
thanks,I looked and the pin is pushed into the piston like you said.thanks for the advise on bleeding the system,i'll do that.trying to buy a used one off ebay so i can get it running for the kids.i'll change to the weisco's so it wont happen again though!
 

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i just had to do my friends girlfriends dads 98 zr600. mag side...pin went threw the top of the piston, ring spun and boom jug gone, piston gone. the motor had JUST been rebuilt not 300 miles ago
 
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