2 stroke top end rebuild

[Twheat18]

I'm getting started on prepping my bike (2001 KX 125) for the ice this winter and as part of the the maintenance I'm putting a fresh wiseco top end in. I've never done a top end before so I've been reading and talking to various people about their recommendations when doing one. It seems to be the general consensus that with any Nikasil plated cylinder wear is going to be extremely minimal on the cylinder wall and therefore just a piston and rings will do the trick, no honing and definitely no sleeving (assuming no scores in the cylinder).

I'm curious to know what your opinions are, if you have done one yourself.

brap brap brap, long live the 2 stroke.

[Rada]

With a Nikasil cylinder there is no honing.

The cylinder is either good or not.

You can check the cylinder to piston clearance to get a feel for wear.
Unless there is obvious damage you should be fine.

When it is time to go together it is all about the clean.
I like to clean everything with HOT soapy water then assemble.

Don't drop a circlip into the bottom end.

Pre-oiling the piston is up for debate. I usually go with a small amount of 2 stroke oil on the piston skirt.

Pretty much a piece of cake.

[Fast_Eddie]

It's pretty easy to do. All the tips I can think of a common sense.

Make sure to hold the power valve linkage securely with something before turning any locknuts or bolts. The mechanical limits on most designs are fragile.

Put a clean rag into the crank opening and water passages before scraping the base gasket off.

Remove the rags from above before putting the cylinder back on.

Make sure the power-valve is cleaned of all goo and that it operates very freely before re-installing the cylinder. (most bikes need the power valve cleaning even more than the fresh rings.)

Lube that cylinder like an old cougar before you put the piston back in.

["Dangerous" Dan K]

I did a top end on an 02 KX125 a while back. The cylinder and head were scored, but not too bad, and the bike has run like a top since I put it back together with the original head/cylinder. The cylinder actually held up REALLY well when compared to the piston/head. I'm sure I have pics around somewhere.

Anyway, easy peasy job, and odds are the cylinders will be OK.

[Paul_E_D]

I've done a few...hundred. (seems like it anyway). Like everyone said, just inspect the cylinder. If that bike has a powervalve, then there will be worn parts to replace there... basically any shaft or bushing is suspect. Clean and adjust everything as well.

Otherwise, just make sure to put the piston in with the ring gap towards the back of the bike. Be sure the snap rings are seated well and with the opening opposite the access hole. A little 2 stroke oil on the ring and a dab of grease on the orings (if it has orings, and a thin smear on both sides of the base gasket is good. Torque carefully and follow instructions re grease on threads or not.

["Dangerous" Dan K]

The 01's and 02's are the same engine, as far as I've ever been able to figure out. That said, there is a powervalve on my 02. That was the worst part of the rebuild, it was so caked with carbon or whatever it took got knows how long scraping at it before it became a moving part again.

[Twheat18]

Awesome, thanks guys. Sounds like confirmation of what I was hearing. That will be the plan. Inspect cylinder for damage. Clean the living piss out everything including power valve. little 2 smoke oil for first run and bolt her back together.

["Dangerous" Dan K]

Just to put it in perspective and give you an idea what to expect... Mine was having detonation issues.

Piston:



Head:



Cylinder:




That cylinder was in really good shape, even with the head and piston being that bad. All I did was piston & rings, even re-used the original head. Been running like a top since, including a beating @ Southwick, which is as hard on a 125 as anywhere I've ever ridden.

Hopefully you're just a couple hours, a piston and a ring away from being ready for the ice!

[Twheat18]

Dan, thanks a lot for those reference pics! Motor will be coming apart this weekend, will be interesting to compare.

["Dangerous" Dan K]

NP. I have more if you have any questions, but there are certainly more knowledgeable people than me on here for this stuff. That was my first top end rebuild, and I got 90% of the assistance for doing it here.

[gregp]

IMO - That doesn't look like detonation damage. It looks like it ingested some hard parts...

[Paul_E_D]

Nope, That's textbook detonation. Random pocking, plus fuzzing towards the outer edge of the squish band. A foreign object leaves a much more regular and geometric pattern, especially toward the outer edge, where it would be deeply imprinting the head and piston. We could likely identify the part from the impressions left behind!

[JettaJayGLS]

Get it back together while the dirt season is still here....not just for ice.

[Twheat18]

Get it back together while the dirt season is still here....not just for ice.

not gonna happen. top end is the 1st thing, then gotta replace my linkage bearings, fork seals, reassemble her. build proper ice tires. plus I enjoy ice riding wayyyy more than dirt. I only rode her 3-4 times this summer.

[Paul_E_D]

if my 125 doesn't sell, this might be the year I get out on the ice!