Hey everyone, I just picked up a '94 YZ 125 dirt bike that needs some engine work. I bought it with the cylinder head already removed and was told that it would need a new piston, piston ring, and sleeve. The connecting rod also has some play at the crankshaft and the person selling the bike wasn't sure what it needs down there to fix that, so I'm just going to tackle as much as I can to get this thing back in good running condition.

I tore a bit of the engine down tonight and since the cylinder head had been off and the bike was stored outside there was a bit of water mixed with the engine oil. The connecting rod is a little rusty up top but I'm sure a machine shop could clean it up for me, or I could maybe purchase a new one. But the crankshaft rotates with no problem so I don't think there should be any problems with it, maybe just some new bearings?

I've worked on MANY car engines over the years, and my last project was actually a Yamaha 3.0 V6 in my '92 Taurus SHO. I bought a 3.2L block and had it bored out o 3.3L, and assembled everything myself from the rotating assembly, cylinder heads, all the way to the ignition wires. The whole entire engine. However, I've never worked on a small engine like this, and I'm not sure exactly what parts are inside just yet that may need to replaced. I've never touched a 2-stroke engine, but I know I'm 100% capable of doing this on my own.

One of my biggest questions though is regarding the cylinder bore. I'm not sure how fast this bike is in stock form, but I love power and I'm thinking that I may want to do a slightly larger piston. My weekend toy is a 10-second '05 GTO, and every other car that I've owned has been a fast car.... so I can't let this bike be boring to me.

If I want to do a larger piston, can I simply purchase a thinner sleeve for that larger bore? Or is there more to it than that? I've been doing a little searching the past few hours and haven't come across anywhere that sells hi-performance pistons, like Wiseco pistons, along with the sleeves so I'm not sure what this larger bore will involve.

Are their any good online stores that sell a good variety of different parts for this bike? I need a few things like a new clutch cable, maybe all new plastics (just because), new clutch and front brake levers, gas cap, etc.

Thanks in advance for any help on this!!

-Andrew

Hi Andrew,
first off I think your bike has a nikasil cylinder and I also think you can have a sleeve put in it .
the problem is in order to get the hole big enough to make a noticeable power increase it will take the bore past what the ports will alow. this is the reason you see the real big bore kits that cost so much (thousands of dollars) because they replace everything and actually change the stroke to increase total cc's to above 150 cc's and I think even up to 175cc's .

by increasing just the bore to the max you will only get maybe a few cc's out of it . my friend re-sleeved his out of what he thought was a better financial decision(rather than replace the nikasil cylinder) . he ended up with the largest size piston after it seized a few times(the downside of machine shops and aftermarket sleeve's etc.) ,and gained no noticeable power improvement . It seemed to be rather expensive to me, and in general a real pia to get the parts then get them machined in. In some cases it is cheaper to replace the cylinder.

just to clarify if your cylinder is nikasil I am saying it may be cheaper to replace the cylinder if it is damaged .
if your cylinder is not nikasil you can go up to the largest size , but I would not expect any major power increase. actually i dont recommend going above the needed size incase (in fact) it will need rebuilt again in the future.

edit: I think you may be able to get it replated with a newer bore size up .5 or 1mm or what ever is needed and this may be the most cost effective way to get it running again.

Hey David thank you very much for your detailed response! I got to talking to my buddy's bike mechanic earlier today and decided to just bring him the complete motor and let him do it. I don't know what I'm really looking for as far as things that may need replaced, and luckily I brought everything to him. He noticed right away that the jug/cylinder head was damaged, and that there was a broken piece of aluminum from the skirt of the jug broken off down inside the motor.

He's going to tear it down completely tomorrow morning and see what all I need and get back to me since he can order all the parts at a really good price. And big thanks on the heads-up about doing a larger piston/bore. I would have certainly thought some noticeable power would have been gained from that. I think I may just stick with doing an FMF pipe and muffler, and maybe a jet upgrade on the carburetor to pick up a little more oomph. I don't even know what this bike is capable of in stock form, but regardless of what everyone keeps suggesting to me I just don't think the bike it going to have enough power for me with a couple little mods. Haha. It needs a header pipe anyhow, so I don't mind spending $200 on a fatty.

I just ordered new handle grips, a clutch cable, front brake lever, clutch lever with perch, and a fuel tank cap. That takes care of pretty much the few broken/damaged/worn parts that need replaced.

One thing I haven't been able to find anywhere for this bike is a kick-stand. I guess these things didn't come with one originally apparently, but I'd like to put one on it instead of always having to lean it up against something when I get off of it. Any leads on a universal kickstand that works on this thing??

Thanks again!

you are welcome.

yeah kick stands and mx bikes dont mix very well. its the harpoon effect that makes them less desirable to the manufactures .
parts unlimited is a distributer that sells through motorcycle dealers and their moose line they have a clamp on kick stand you may be able to modify to fit , it is made for newer years.

mx bikes came with a triangle kick stand that locks into the rear axle for holding the bike up during storage , I usually just lean it up against a tree while I'm sucking down gatorade . lol

:lol yeah like Dave said, kickstands are deffinately not for MX bikes, they can get in the way they get worn out and start to fall and they always fall when you really dont want them to :fact

the rear swingarm triangle is awesome all the time, i used those on all my mx bikes, they are small enough that you can fit it into your backpack too when you ar on the trails and don't really trust the tree to hold your bike :)

http://us.st12.yimg.com/us.st.yimg.com/I/pitposse_2038_108761407

I don't even know what this bike is capable of in stock form, but regardless of what everyone keeps suggesting to me I just don't think the bike it going to have enough power for me with a couple little mods. Haha. It needs a header pipe anyhow, so I don't mind spending $200 on a fatty.

what is your height/weight? yz 125 is pretty snappy, i think you will be surprised at the power. you also might want to throw in some aftermarket reed valves. one of the best mods for the money i ever did for my yz250. you will most likely need to re-jet though, so dont mess with the carburetor until after if you plan on doing reeds.

after you have ridden it about, you can also change sprocket sizes if you want more power in different locations (ie, better pull off the bottom, or higher top end)

Thanks for the replies fellas!!!!!

you are welcome.

yeah kick stands and mx bikes dont mix very well. its the harpoon effect that makes them less desirable to the manufactures .
parts unlimited is a distributer that sells through motorcycle dealers and their moose line they have a clamp on kick stand you may be able to modify to fit , it is made for newer years.

mx bikes came with a triangle kick stand that locks into the rear axle for holding the bike up during storage , I usually just lean it up against a tree while I'm sucking down gatorade . lol

Yeah I was wondering if I might be able to modify one of the clamp-on ones... maybe have someone fabricate me some brackets to clamp on the swing-arm.... but if these things DO start fall down on their own then yeah, not for me. I don't think I really plan on doing much jumping and all that wild stuff with this bike.... I'll probably use it more for trails. But all it takes is one jump for a kickstand to fall probably, so I better not. Everyone else tells me they just use a tree, too haha.

:lol yeah like Dave said, kickstands are deffinately not for MX bikes, they can get in the way they get worn out and start to fall and they always fall when you really dont want them to :fact

the rear swingarm triangle is awesome all the time, i used those on all my mx bikes, they are small enough that you can fit it into your backpack too when you ar on the trails and don't really trust the tree to hold your bike :)

http://us.st12.yimg.com/us.st.yimg.com/I/pitposse_2038_108761407

That triangle one sounds ideal. I HATE carrying gear when I ride anywhere though. I usually just throw on my helmet, glove, work boots, and just hope that that's all I need while out riding haha. No tools, no back-pack, etc. I guess I'll get a triangle one and carry it in a back-pack when I go out for longer rides and stuff, and for parking the bike at the house. Thanks for the picture and all!

what is your height/weight? yz 125 is pretty snappy, i think you will be surprised at the power. you also might want to throw in some aftermarket reed valves. one of the best mods for the money i ever did for my yz250. you will most likely need to re-jet though, so dont mess with the carburetor until after if you plan on doing reeds.

after you have ridden it about, you can also change sprocket sizes if you want more power in different locations (ie, better pull off the bottom, or higher top end)

I'm about 5'9" and 170-175 lbs. I'm not a big dude, but I'm just really surprised to keep hearing people, people bigger than me, saying the 125's are damn-quick. I guess I'm just worried about being disappointed after getting the bike back together this weekend, and then ending up wanting to take the carb back off and jet it or something. But maybe I should just stick with just getting the pipe for now, unless the guy working on my motor tells me that the carb needs to be re-worked.

The first thing the guy that runs the shop that I dropped my motor off with yesterday did was toss my reed valves in the trash can. Haha. So he has every intention on hooking me up with aftermarket reed valves.

Why do you say to not jet the carb until after the valves? Is that because I'd just need to jet the carb accordingly to the valves? I'd probably hold off on the sprockets until mine need replaced, and I get really familiar with where the bike has and lacks power. But we'll see... everyone might just be right and I might scare myself on this bike without any real mods lol.

Thanks for the replies fellas!!!!!
I'm about 5'9" and 170-175 lbs.
Why do you say to not jet the carb until after the valves? Is that because I'd just need to jet the carb accordingly to the valves?


yeah, if you spend time and money jetting the carb and THEN add new reeds, you will have to re-do it all according to the new reed. but if he already tossed your old, get the moto tassarini(sp?) v3 vforce reeds. they are hot. Reeds (http://www.motosport.com/dirtbike/product/VFORCE-3-REED-VALVE-SYSTEM/?catalogId=109666&words=reed+valve&srchHistoryURL=fpg10xd5||words~reed%20valve^parent CategoryID~category_root^categoryID~1^categoryName ~Dirt)

and im like you- i dont carry any gear while riding. i leave that in the truck, either at camp or at the parking lot at the track. if you are in the desert, just us a bush. forest, use a tree. kickstand or triangle is a waste imo.

So... the prognosis on my motor isn't good lol. I was told that I need a new crank, connecting rod, piston, jug, and of course all necessary bearings, seals, and gaskets. The shop found out that their suppliers don't offer any YZ stuff prior to '97, and the only thing that he can really get is a Wiseco piston. He also wants $480 in labor to put it all back together, which I can't afford on top of the price of all the parts I'm going to need. I have a shopping list of what I think I need to order and do all myself, and I was hoping maybe some of you guys here may be familair enough with the '94 YZ 125 motor to know if this is everything that I need:

Piston:
http://www.oemcycle.com/Item/product/1278/

Connecting Rod:
http://www.oemcycle.com/Item/product/3552/

Crank Bearings:
http://www.oemcycle.com/Item/product/1368/

Complete Gasket Set:
http://www.oemcycle.com/Item/product/600/

I didn't have any luck finding a new crankshaft, and I'm not sure exactly what is wrong with the current one, if there is anything wrong. The shop owner mentioned the crank in the list of parts, but he may have mentioned that just because he felt it was something I might as well do if I'm doing everything else. The current rod is rusty up top from being exposed to weather without the jug installed, but moved completely freely along with the crankshaft, so I'm not sure that there's any damage to the crank shaft.

However, a piece of one of the skirts on the underside of the jug was broken off, and apparently fell into the lower engine. Not sure if that damaged the crank in any way or not. I'll try to get the engine back within the next few days and take some good pictures of the parts to see if maybe the jug is still useable, because I can't find one of those anywhere either. Mine already has a sleeve in it, but it will need to get replaced if I were to re-use it because the shop owner measure the bore to be something like .4mm (or maybe .04?) out of round.

I have also been unable to find a reed valve for the '94 model year YZ 125. The V3 valve is only available for the '95+ apparently, and everything else I see is pretty much for '97+.

Any good help you guys can give me on this would be highly greatly appreciated!! And let me know if I'm leaving anything out on this bottom end and top end rebuild shopping list that I have going on.

Thanks again,
Andrew