P&C swap gone bad

[flashy]

I've been bitten by the 'dark side' of CAD. I bought a used 6401 Makita that I didn't REALLY need on e-bay for $230 and loved it. Found out here that I could soup it up to a 7901, so I took a perfectly good saw and decided to have some fun. I downloaded some instructions and ordered a kit and went to work. The instructions were ok but not great, and all was going well until it was time to slide the cylinder on the piston. I got the ring inside the cyinder and noticed that I needed to rotate the cylinder about 5 degrees. When I did that, I heard a 'click' and thought 'uh-oh'. Slid piston back out and found that I had modified the ring into a 2 piece! CRAP! Newbie-boy didn't know that piston rings were so delicate.
Now I'll order a new piston ring AND A RING COMPRESSOR KIT and try again. Will I be able to put a ring on the piston without breaking the ring again? Where on the piston does the piston ring gap need to be, or does it matter?
Another question I thought of for future reference----- When you remove a flywheel, do you have to be careful about messing up the timing? Is it manufactured to only fit back on a certain way?

 

[husky362]

look at your old piston .....see the ring grove has a small pin in it ,,,
keeps the ring from turning on the piston the ring has a U shape to it and must match up with the pin

 

[Edge & Engine]

Right, the ring gap MUST be in position at the pins in order to install the cylinder.
The flywheel is keyed, you don't have to worry about messing up the timing, just make sure the key is still in the crankshaft then line up the groove in the flywheel and slide it on.

 

[motomedik]

Be sure also that the piston's arrow is pointing toward the exhaust. The locating pin in the piston may be offset in the ring groove. Be sure that the notch in the ring ends corresponds.

 

[flashy]

thanks, all

Guess my instructions were bad. No mention of ring groove pin. Will a new ring go on easily?

 

[Urbicide]

Don't forget to coat the cylinder, piston, & wrist pin bearing with 2-cycle oil before reassembling.

 

[jra1100]

Good lucks

I just changed the piston and rings on my Jred 920. First time for me to, and it was an adventure. Mine runs strong and will get even better I hope. The search function on AS is a great source for info on this type of work. I also have a Makita 6401 that I bought on ebay. I gave $160 for mine and am very impressed with how well it cuts at 64cc. I have toyed with the idea of going to 79cc's also so let us know how it goes when you get it together. JR

 

[Lakeside53]

Guess my instructions were bad. No mention of ring groove pin. Will a new ring go on easily?

Yes.. but it's very easy to break them... I walk them on with a razor knife as a separator. MAKE SURE IT'S UP THE CORRECT WAY.

 

[andrethegiant70]

On the bright side, the lesson sticks pretty well when you muck it up. Not a terribly expensive mistake, just more of a PITA. Be sure to get those wrist pin clips in well! Now THERE'S one that will cost you some coin!!

 

[Tzed250]

:agree2:

 

[flashy]

ok

On the bright side, the lesson sticks pretty well when you muck it up. Not a terribly expensive mistake, just more of a PITA. Be sure to get those wrist pin clips in well! Now THERE'S one that will cost you some coin!!

Clips went on good. Not easy, but good.