topend rebuild

[zack221]

Does anyone know where i could find a step by step guide on how to rebuild a topend on a 2 stroke.I am wanting to rebuild my cr125 just so i know i have a fresh topend but i am wanting to read up on it a lot before i tear it down.It will be my first time doing this and i dont know what to expect.I ordered a service manual but i have read that they leave some things out in the service manual.I have searched on here and on tt for a 2 stroke topend rebuild but i cant find anything.

[stevo3176]

Get a clymer manual they are alot of help.

[zack221]

thats the manual i bought but on tt they said it leaves out some things i havent got it yet so maybe it will tell me how to do it

[Fear-Nuttin]

Google it ! There's many articles on "2 stroke rebulds". You can also look at your local bookstore lke Barns & Nobles. But I'm also sure some of these guys in here can probably publish something BETTER than any book available.

[zack221]

ok thanks i cant believe i didnt think of google

[F-16Guy]

For a basic top end rebuild, a Clymer manual should be fine. Start with a spotlessly clean engine and surrounding area, and keep things clean and organized during the build. Also, make sure you totally disassemble the powervalve and associated linkage and clean all of the carbon and sludge off of it. Every part should be cleaned down to the bare metal. Also, pay attention to ring position when you get to that point; follow the instructions that came with the piston. If they aren't clear, follow what the Clymer says. I think most tell you to avoid having the ring gap facing the exhaust port bridge so the ring doesn't catch and break off. Depending on the year, it should have a coated cylinder, so unless there's damage, you shouldn't even need to hone it; just get it clean. Bottom line: Keep it clean and organized, and follow the manual or preferably the directions that came with the part if they're more specific.

[zack221]

ok thanks i found a great review on how to rebuild a 2 stroke so im going to follow it and the manual so i should be ok hopefully lol this is my first time rebuilding a motor.I figured i would have to get it honed (Do i not have to get it honed if i do it right now while its running and not wait for it to blow?)

[zack221]

Well i believe i can handle it my neighbor bought a rm125 today for $20 the guy took the topend off and left it out in the rain so it rusted.But i cant believe how little the pistons are on these things i was looking at my neighbors rm125 there is nothing to it at all im alot more confident i can do it now.It makes you wonder how a piston that little can make that much power.

[F-16Guy]

Originally posted by zack221
ok thanks i found a great review on how to rebuild a 2 stroke so im going to follow it and the manual so i should be ok hopefully lol this is my first time rebuilding a motor.I figured i would have to get it honed (Do i not have to get it honed if i do it right now while its running and not wait for it to blow?)
The coating on a cylinder with Nikasil or equivalent is so hard that a normal hone wouldn't even scratch it. You would need a diamond coated hone. Unless there is some significant scratches, you shouldn't need to mess with it. Just clean all of the carbon build-up out of the bore and ports (as much as you can get to in the ports), get it squeeky clean, and oil the bore with 2-cycle oil before assembly.

[zack221]

ok thanks how long will a topend last the guy that had it before me bought it and rebuilt it and he only rode it on trails it probably has 20 hours on it.

[A A R O N]

Originally posted by zack221
ok thanks how long will a topend last the guy that had it before me bought it and rebuilt it and he only rode it on trails it probably has 20 hours on it.


It depends how hard you ride it and what ratio you run the gas/oil mix at and how cool the engine stays (type of coolant). I would just buy a compression tester and keep that handy and use the specs in the clymer manual so you know exactly when the piston needs changed to prevent cylinder wall damage.

BTW, I to was amazed at how such a little piece of aluminum can haul such a heavy chunk of steel/person around so fast the first time I saw a 125 piston. Truly amazing.

[Fear-Nuttin]

If you have the piston out, and the walls look good, should you still buy new rings or use the old ones?

[A A R O N]

Originally posted by Fear-Nuttin
If you have the piston out, and the walls look good, should you still buy new rings or use the old ones?


Put new rings in.