YZF Rebuild

Anyone with above average mechanical skills, a decent set of tools and the factory service manual can do a piston or ring change on the Yamaha 250F. This is one of the easier engines to work on. Very intuitive design. Biggest problems most first timers have is getting the cam cap torque correct. The cam cap is also the cam bearing surface on these engines so getting the torque out of whack can gall the bearing surface and ruin the cylinder head. If you take your time and use a proper torque wrench you should have no trouble.

Refreshing the cylinder head takes some specialized tools and knowledge to do it right. I would strongly advise sending this part of the work out to a professional. The titanium valves in these engines have a very short life due to the materials being used and the incredibly aggressive valve acceleration rates used to get this level of performance. As a result the valve service intervals aren't comparable to old school XR type engines or the engine in your family car. Okie's 250F pretty much junked the valves after just 2 seasons, and he is by no means a rev happy rider. That has been true of most of the 250Fs I've had apart.

If the crank is changed as a complete assembly it is pretty straight forward assuming you are familiar with splitting the cases on a two-stroke. The crank main bearings are probably the biggest issue you are likely to run into. The mains can be changed at home without a press but it takes some care and proper use of heat and freezing of the bearings. Mains tend to wear faster on the 250F than I've seen on some other engines so if you split the cases make sure you change the main bearings. You'll also want to pay close attention to the transmission shaft bearing near the clutch pivot arm assembly. They tend to wear quickly. You'll need a special blind hole bearing puller to get this bearing out. Goodson is a good source for the puller (it's about $100). It's unlikely you'll ever get it out with a standard slide hammer style puller. Lots of us have tried in an attempt to avoid spending the $100 for the correct tool. Save yourself the anguish and just buy the tool up front. You can thank me later. ;)

I think you'll find it's a fun engine to work on if you decide to tackle it. Be sureto give yourself plenty of time, don't rush anything and ask lots of questions. If possible, take pictures as you take things apart so you can refer back to them. I also suggest using ziploc bags to logically group parts together along with little notes about anything special that you notice as you are pulling things apart. A note as simple as , "Don't forget to do this" about a specific part or procedure can make a big difference when you reassemble something. There is enough combined knowledge here that I'm sure we can get you through it. So dive in and enjoy yourself.

Good Luck. :thumb:

At this point all I can offer is advice based on my experience, you can decide for yourself what it's worth. ;)

Both options are good choices for the piston/ring part of the work. Any good mechanic can do it.

I would agree that the chances are pretty good that a life of trail riding isn't the type of stress to warrant a crank replacement yet. Okie isn't rev happy but his 250F did spend every weekend being ridden aggressively on a motocross track for 2 years before it dropped a valve at Cooperland (a rev happy track). His crank was moving around enough to let the center valve hit the piston and pop the valve keepers. I guess we waited one week too long to change the crank. :scream:

The only real question is the valve train. If you are spending the bulk of your time riding at moderate revs and aren't seeing the valve clearances closing up beyond spec then you should be able to just pop in a piston and rings , degaze the cylinder, lap the sealing surfaces, pop it all back together and ride with peace of mind for a good long time with. No reason to play with the cylinderhead unless there is clear evidence of an issue. I tend to err on the conservative side with these engines, but having seen what happens when these things come apart at 13,000 rpm it's sort of a survival instinct. :thumb:

If the valve clearances have moved significantly since it was new then you should send the head out to someone who specializes in performance work to be refreshed. I mean no disrespect to dealership mechanics when I say it's unlikely that the local mechanic has the requisite skill or experience to do the cylinder head portion of the work properly. There are a few, but those guys are rare.

I hope this helps.

John - The only choice other than OEM at this point is the Kibblewhite Stainless Steel valves. I've put them in a number of YZF4 engines with great results. They come as a kit with new springs to deal with the slight weight increase. Outstanding quality and workmanship, plus they can be resurfaced down the road unlike the OEM valves. According to Mike Perry from Kibblewhite the 250F parts will start shipping towards the end of the month. The folks at Kibblewhite are pretty fanatical about quality and they always wait till they can do it properly before they release products.

The seats just require a quality resurfacing at a minimum. I prefer Serdi or similar equipment to keep things as accurate as possible and to open up the possibilities of additional work in the throat and cylinder side discharge area.

Originally posted by Imho

Hey Rich,

Is this also the case with the YZ400/426/450F? I take it the revs are less and I don't think the 400 has titanium valves. Also, would running WR timing on a 400 increase the time between rebuilds?

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The YZFs with steel valves are incredibly robust even when they are revved HARD. Most of the YZF engines I've seen went a couple of seasons before they ever needed a single shim changed.

Running WR timing on a YZF is a really BAD idea IMO. There have been a lot of misconceptions about WHY Yamaha set the valve timing the way they did on the WR thanks in large part to our friends in the magazine business who don't understand cam timing.
Anyone who does a little research into engine design will quickly discover that the only logical reason for Yamaha to advance the exhaust cam timing so far in the WR was to start the exhaust blowdown early enough to get the engine to run properly across the rev range with the super-restrictive stock exhaust insert . Once that restriction is removed the exhaust valve opens way too early with the WR timing (23 crank degrees too early) and will tend to flatten torque on the low end, and just kill any hope of making top end power because the exhaust closes 23 degress earlier and reduces overlap by that same amount. The only possible value of the WR timing on a YZF is you'll see an improvement in no-load idle quality, other than that don't waste your time with that nonsense. :thumb:

So I guess to answer your original question, yes it probably would increase the time between rebuilds. Given that it won't make any real power at high revs, and won't be as much fun to ride you'll probably leave it in the garage more and never wear it out. ;)