GY6 150cc -piston ring replacement

Posts: 21

Hey all,
I have deteremined my main cause for having oil and major carbon build up on the spark plug, and in the intake and exhaust valve chamber. Come to find out, after talking with an experienced mechanic, a compression test will not insure your rings are bad on your piston. I'm propably the only one who didn't know that. Learning quickly. I know for a fact the reason this happened and it was solely due to that orginal crappy air filter assembly the cart came with. I know that excessive amounts of dirt went through that carb. That has been resolved, but the damage has occurred regardless of precautions taken. Regardless, I now have to access the piston and begin this process. I have the intake and exhaust valve, & Cam assembly removed at this time. Now looking directly at the piston, wondering what to do first. Looking to see if anyone has photos, or some insight to help me walk through this. I have a tomberlin Manual, but it does not explain how to do this. I am mechanically inclined, just not engine savy. any help would be greatly appreciated.

Posts: 1889

The cylinder slides right off the four studs. The piston is held in by a roller that is retained by a spring clip on either side.

Posts: 2640

pghruby is correct. The "jug" is only held on right now by your gasket. A couple LIGHT TAPS with a plastic hammer or small rubber mallet will break the seal... dont hit it hard, just tapping it loose. Then slide the jug upwards and off the piston. The piston has a pin through the center of it called the wrist pin. there should be circlips on both sides. They look like rings, with a little impression on the piston to use a hooked tool to remove the clips. unless you have replacement clips, it is best not to pry too hard on these, they bend easily. Remove both, and the pin will most likely need to be pressed out with a clamp or press tool. You can use a carpenters clamp and some sockets, but you cant clamp directly onto the piston or it can damage the piston. Once you get the wrist pin out, the rest is smooth sailing!!! just clean off all the carbon, remove the rings, clean the ring grooves with an old pistong ring (break it in half and scrape gently through the ring grooves)

Clean the piston off really good in soapy hot water once the carbon is cleaned off and the grooves are cleaned out... Then inspect it really good for any damage or cracks.

Once that is all settled, install the oil rings first... then the compression rings, working upward... The last ring should be the thickest rings, and if it has a bevel (should be beveled on the inside diameter) the bevel faces upward. DO NOT spiral the ring onto the piston... use care not to distort the ring, you dont want it looking like a lock washer when installing it into the groove.

Once all the rings are in place, this is how they should face:

Check the piston for the word IN or an arrow pointing in any direction. That is your INTAKE direction. That needs to face the carburator. Once you found the direction the piston is going in, you need to line up each ring in the proper direction...

The oil ring expander (bumpy ring) gap is pointed to the front of the motor, even to the wrist pin (gap should face the flywheel on motor). The lower oil ring gap is supposed to be 45* away from the oil ring gap (45* away fron center) in either direction. The upper oil ring is the same, just in the opposite direction. SO... Lower oil ring can be to the left 45* from center and the upper oil ring can be to the right 45* from the center....

THEN the lower compression ring gap faces 90* to the left of the wrist pin, and the upper compression ring (thickest ring) should face 90* to the right of the wrist pin.

Once those are all lined up, you soak the cylinder "jug" with oil on the inner walls, and then slowly slide it back on the piston, pressing in the rings all around until each one goes into the cylinder... DONT FORCE THE CYLINDER!!!! Let it slide over each ring as you get each ring to fit.

then you simply put everything back the way it came off!!! check your valve clearances for lash and you should be good to go!!

Good luck!

John

Posts: 21

Thanks guys. This type of info really goes a long ways.

I am beginning to think I can conquer anything on this machine. Changed out my jet, rollers and spring clutch. Had some difficulties, but I was successful.

There is no better experience than hands-on.

I am now anxious to conquer this last challenge. Hopefully then I can do some serious riding !

Posts: 566

Nice job John, I could be wrong but double check the center ring I believe the bevel goes down. the oil rings ( the bottom ones) can be tricky the corrugated one goes on first then a thin one on the very bottom then the next thin one on the top ,so that all three parts make up one ring. Take note when disassembling them.

Posts: 4286

ya"ll are good Very Happy

another thing to remember is to put some rags around the opening of the crankcase. dont want the wrist pin clips to fall down in there.

Posts: 2640

I think you are right Tim! I was just checkin one of my old books from the ASE, and it doesnt mention anything about beveled edges on the rings, but the placement of the gaps are all there! lol... They only thing I would really worry about when putting new rings in are those clips!!! what a pain they are on the wrist pin, trying not to bend them!!!

So, in that heap of wrds a couple posts up, edit it to freflect bevel down on that compression ring! sorry bout that!

john