YZ250 Full Motor Rebuild

I don’t remember Yamaha ever having a full bearing/seal/gasket kit for the bottom end when I worked at a dealer. I always go OEM transmission bearings and seals, replace any circlips and snap rings, and replace all the springs in there if the bike is relatively high hour, this really makes a difference in making an older bike feel nice and tight after a rebuild. I wouldn’t shy away from a quality aftermarket Piston like a Vertex, but an OEM Yamaha one is fine too. OEM top end gaskets and wrist pin bearing for me.
 

Rocky Mountain has always been the cheapest for me, but I’ve also got some really good deals on NIB Pistons, clutches, etc from guys cleaning out their garages on here. Nice to throw them in the box for the next time you put it under the knife.
 

Other than that make sure you clean the power valve components well and replace anything worn in there, have a machine shop measure your cylinder or if the bikes got some time on it send it out to get replated with a new piston. Check the transmission components very carefully as those YZ250s almost always run into problems with those. I’d replace the primary bolt too for a piece of mind because I’ve seen those do some damage on the YZ250s as well. 

Budget for a 3rd gear lol jk. 
I don’t know what the cost of an oem crank is, but getting yours rebuilt with a wossner rod kit wouldn’t be a bad idea and could save you some money. I have boyko do my cranks. He also has some good porting specs for those cylinders 

The part number for an OEM "C" top end rebuild kit is 1P8-WB033-00-C.

If you want an "A" kit, then replace the last letter with an "A", A thru D. These kits do not show up under the '21 model year. They are only listed on the '22 and up parts lists, but they are the same for back to at least '05. 

Yamaha has a bunch of new parts kits available that can save you a lot of money over buying all the pieces separately. You can get this kit for about the same price as a piston by itself. 

Yamaha 1P8-WB033-00-C0 - PISTON REBUILT KIT | Partzilla.com

Be careful with the top end kit.  The kit came with a "B" piston while i have a "C" cylinder (or something like that) so i bought the correct piston and have a brand new piston with the wrong size sitting in my drawer unused.  I don't think you get to pick which piston size with the top end kit.   Something to think about....

BTW, Wossner now has, or will soon, a complete crank for the YZ250 for $300. I hear you on the OEM thing, but that is a smoking price and definitely a quality product.

https://wossnerpistons.com/products/cs2028

Parts Unlimited shows next week as the expected-by date for this crank to be in stock.

Have heard lots of good things on the Wossner crank from the people that replaced their faulty 22 KTM cranks with them. 

You get to choose, different part numbers for all of the sizes.

Wouldnt stress too much about what letter pistom you have. I cant remember off the top of my head but its only a handful of microns between an A and a D piston. If you legit measured your cylinder with bore gauges that were mastered then you could make a call on what piston you need. Had a friend tell me he "mic'd" his cylinder, come to find out he was using a digital caliper lol

I always used the C or D piston with cylinder that has 40hrs plus on it….difference is very slight but the larger OD of piston takes up some cylinder wear. 

If you’re capable of rebuilding the top end I’d say you’re capable on doing the bottom end too. Buy the right tools, bag everything the way it comes off, take plenty of pictures, be light with your loctite and heavy with assembly lube!

Yes.

One tool that you should get is this tranny jig--   https://factorywrenchtools.com/shop-tools/ols/products/yz-250-transmiss…

The YZ shift forks are pinned to the shafts making transmission installation more difficult than other bikes. This jig holds everything in place so you can drop it in. WELL WORTH THE MONEY!!!! They make a beautiful aluminum version for working mechanics but the plastic one works for us once-a-year guys/

Get a good heat source. I prefer a good heat gun over a torch, myself. For the cases, I use a hot plate I picked up for $6 at the thrift store. Case on the plate, cover it with a steel pan, and I can slowly bring it up to temp there in the garage while working on other stuff. 

If I can do it, anybody can.

If I can offer another newbie tip, because doing my first motor is still fresh in my mind  . .. 

This will sound weird but hear me out- The part that makes us all nervous is not knowing if everything works till it's all back together and you take it for ride up the street. We are probably all good on doing a clutch and top end but it's inside the cases that is the all new territory.

So what I did before final assembly is, leaving the crank OUT and no sealant on the cases, I put the tranny in, cases together, shift star and linkage in, and then spun the mainshaft and made sure it went through all the gears. (might have to use a socket on a drill to spin it fast enough.)

Then I took it apart and did it again. And again. I did it about six times before dropping in the crank so that I was 100% sure it was right and it put my mind at ease. I figured, I'm doing this for the first time, what's another hour or two? I found it well worth the time. 

The difference between an A piston and B piston is ~.0002", two ten-thousandths. A - D is at most .00066" difference. One thing you should replace is your power valve seal if you haven't already. They will start to get a little bit of slop and wear out the hole that they sit in. 

Not much point in measuring a plated jug. If it's not damaged your GTG. Always go with the larger piston for a rebuild on a used cylinder.

Until it starts to run funny or it sounds like Charlie Daniels is playing the devil went down to Georgia in your cases.

90 hours

I put 100 on my 125 cranks and they always come out in spec. I wouldn’t hesitate to push a YZ250 to 130-150 if you’re not riding it like you’re trying to kill it. 

It's all about good 2-stroke oil and good air filter maintenance.  You do those 2 things and a bottom end can last a long time.  I stick with Bel-Ray filter oil because it just plain works and does not let dust past.  Every time you have the top end open on a YZ250, take a good look at the crank stuffers and make sure the screws aren't coming loose.  And inspect the rod bearing thrust washers for wear and measure side clearance. 

Assuming you keep a clean air filter & run good fuel/ oil, you will know when to get into the bottom end when the transmission starts to fail.