A little stumped on a topend rebuild?

[zrxrider]

I picked up a 2000 yz125 for really cheap. Brought it home and went over it before trying to get it running. Took the carb off, cleaned it, added fresh fuel, new plug and a quick look over. It was missing the kicker & rear spacer but bump started down my hill and to my surprize it fired right up and ran great too!!

I plan on keeping the bike now as its so fun to ride. It looks like the cylinder was never off and I want to rebuild the top end just so I don't have any problems later. Doing a search, I see Yamaha has A/B/C/D pistons. Should I just replace with the same one or go with a Wiseco or just replace the rings if the piston looks good? I also saw I should use a 54mm/240 ball hone to clean the cyl. up......is that correct?

I've rebuild old kz1000 inline streetbikes before so this should be a piece of cake but just want to make sure.

 

[slodad]

Many of the people who seem to know about such things recommend that a ball hone not be used on 2 stroke cylinders because of the chance of damaging the nikasil plating on the edges of the ports. The difference in size between an A piston and a D is small enough that it's likely you will still be within the usage limits for the cylinder regardless which size you get, but going with the larger may extend the time before a new one is needed.

Seriously consider fixing the kickstarter. There are few things less fun than stalling a bike where you can't easily bump start it.

 

[zrxrider]

After getting it running I found a used kicker & wheel spacer for $30 delivered. :yeehaw: Its way better than bump starting it. :laugh: Still need a handfull of little items to make it perfect.

 

[zrxrider]

Ok, took the topend off today. Everything looks really good. Should I be ok with just a hone and new rings? The piston has no marks at all........What about the Wiseco "kit"?

 

[zrxrider]

update: I called Wiseco and they said I wont have a problem using there kit in my stock bore. I'm going to order the kit which comes the gaskets, pistons assy. & bearing.

 

[yam*91*]

weisco is the best way to go in my opinion.. might not have to hone the cylinder if its not beat up...sounds like it ain't...but if you still want to hone it just to be sure i would take it to a professional just to be on the safe side...you mess it up you pay for it...they mess it up they pay for it

 

[zrxrider]

I'm going to get a flex hone and run it down the cyl. just a few times to get some more cross hatches. This book I have, "Motocross & Off-Road performance handbook" says to get a 10% smaller hone then the bore size & use 240 grit.

 

[_JOE_]

Where did you see to get a hone 10% smaller than the bore?

 

[zrxrider]

Its in that book......seems kinda odd, right? Man, its hard to find good solid info. on doing this topend!

 

[_JOE_]

What edition do you have? I have the third edition and it says to use a hone that falls within the bore size range, 240-320 grit aluminum or silicon oxide. It also says that the hone cannot cut into the plating, but will knock down the peaks of the original hone scratches possibly resulting in accelerated piston wear.

I would be nervous that you could do more harm than good by honing. Personally, I would stick to chemicals and a light abrasive to break down any residue on the cylinder walls. Or send it to someone who KNOWS what they're doing and has the proper equipment. Some things are better left to professionals.

 

[zrxrider]

Joe, i'm guessing its the 1st edition as it on covers up to 2000 models. I used to work at a bike shop years ago and i'd rather do the work myself. I've used a flex hones on kz1000 cylinders with no problems but they wern't plated. I'm just trying just to get a solid answer on the proper way to do this.

 

[_JOE_]

Is a kz1000 a 2t? The part that scares me is chipping the plating near a port. A 4t cylinder is much less risky to hone. Not saying it shouldn't be done, but unless it's got some very light scratches it's probly not necessary.

 

[zrxrider]

2t? You mean 2-stroke? A kz1000 is a 70's Kawasaki streetbike.

My cyl. doesn't have any scratches other than most of the oem cross hatch marks & some shiny smooth parts. A guy that sells hones said I should use a 240 or 320 grit flex hone. I still haven't got a piston yet but i'm in no hurry, got all winter to fix it. :(

 

[_JOE_]

My point is that a 4t (4 stroke)cylinder has no ports to snag. If you feel it best to do it, by all means go for it. Last time I checked hone tools were pricey. So is a replate if the plating chips.

 

[Redrodent]

After much consideration and research ( I also have that book) I decided that the process was being way over thought. I went and got some 320 grit sandpaper and followed the original crosshatch, half from the top, then from the bottom. 5 mins and your done. You can see the color change quickly as it cleans. That's all you're trying to accomplish anyway. On a 2t, with such narrow, low tension rings, sealing is less of a problem. A DRZ400 (4t) I recently used this method on, leak tested excellent at 2%.

 

[zrxrider]

Redodent, I just happened to have some 320 sandpaper and tried your method........came out real nice! I think your right about it being "over thought". OK, off to the next step....... :cool:

 

[zrxrider]

I picked up a OEM manual for my bike and it says to use #400-#600 grit sand paper to clean the cylinder. Maybe someone will find this info. useful. :cool:

 

[whenfoxforks-ruled]

I like the green or dark red scotch brite pads. Give it a good bath in hot soapy water, dry it with a soft towel, douche with wd-40, and good to go. Does your manual give the thrust washer clearance for the big end of the rod? How are the main bearings, any up and down play?

 

[zrxrider]

Does your manual give the thrust washer clearance for the big end of the rod? How are the main bearings, any up and down play?

I think it does but I haven't checked. The main bearings feel perfect, no up and down play at all. The motor looked really good, even the oem piston. Crossing my fingers that the piston comes tomorrow in the mail so I can put it back together this weekend. :ride:

 

[whenfoxforks-ruled]

Can you perform a leak down test? Always leary of old crank seals, and 125's especially!