WR 426 Top End Overhaul Questions? What to replace?

I am getting ready to do a Top end overhaul on my 426 and am wondering what all I should replace? It is a good running bike with no troubles, it has just under 7000 kms on it. I plan to buy a piston and ring as well as a timing chain and gasket set for sure , but I am contemplating valve seals, springs and keepers as well. It doesn't seem to be too worn out but I haven't measured the springs or anything yet, I just thought it would be good preventative maintenance and could save a broken spring and dropped valve in the future and I thought the more aggressive Hotcams that I have might be a little harder on them. Any suggestions would be appreciated. Thanks


I wouldn't touch a well-running bike's top end. If the compression and leakdown tests are within specs, I'd leave the top end alone. I've got 20k + miles on my bike and all the cylinder head parts (valves, springs, etc.,) are all original.

But, if you insist on doing it, Yamaha recommends also replacing the cylinder when replacing the piston.

Same boat here, got an '01 that (knocks on wood) runs awsome. Never shimmed a valve, never done anything more than change the oil often. Compression is still very good too. I was just in that mindset that it "feels" like it should be time to do these things.........but............have been told a number of times to leave a well running bike be.

I just don't know, I really don't want to just ride it until it grenades. I think at the minimum I am going to put a fresh piston and ring in it with a new timing chain. I will try to do this fairly soon probably by the time I get the bike up to 8000 kms. I don't think it is going to hurt anything, like disturbing a good running bike is going to cause problems. I am plenty qualified to tear this motor down my only hesitation is on spending money that could be better spent elseware. I guess I should do a compresion test and see what it looks like,that will give me some idea of wear. A leakdown test would be better, unfortionatly I do not have any leakdown test equipment, but maybe I will buy some as you can never have too many tools in the toolbox. I was hoping to have a few more opinions on this but if I don't get any more I think I will go with my basic plan of piston and timing chain. Thanks for the feedback.


Leak Down is the way to Go. you will need an adjustable regulator so you can feed in the air pressure since it will push the piston down. then make a hose fitting to connect to the cylinder head. you can braze a hose barb on an old spark plug after removing the porcelean. tear down your cylinder head and check inspect and mic everything probably everthing is good install new valve seals and you will feel better

If you have good compression, there should be no reason to touch the piston and rings.

unless your just hell bent on a rebuild. If I was gonna tear mine down for a rebuild I would probally hop it up with the big bore kit and 13:1 piston :banana:

Just dont forget that the cylinder is nickel plated and if you just swap out piston and rings, you still have to run a glaze breaker through it to get your cross hatch in so you get maximun break in for superior ring seal. Im really not sure how thick the plating and/or how they hone it :ride: . but you can get it bored and replated.

A big bore kit and high compression piston sounds like a good idea too. I could always use more power. What kits are available and has anyone had good results with any kit in particular. I thought that the piston would be starting to wear by now with such a short skirt. I used to replace the pistons regularly on my two stroke with no honeing of the Nikasil coating. Just a quick shotch-briting by hand and install new piston. I still think that a fresh piston, ring, timeing chain and valve seals would be a good idea. I am surprised more people haven't had their engines apart by now on a bike this old to have more opinions on this. I guess it just goes to show you how tough these motors are on the yamaha especially the 426.


Replace chain, valve seals if you remove valves for all round check, and measure piston to cylinder clearance as per manual, also measure each ring clearance and all other stuff, that is recommended.

I'm into doing mine 99 400..all, except oil rings, is in spec, so I just polished valves, grinded valve seats, will also replace chain, rings and piston pin, because it really looks 7 years old :ride:

I'm with the majority, "if it ain't broke, don't fix it".

Definitely replace the valve springs, valve seals, and cam chain when doing a top end job. Its a good thing to freshen up the bike with a new piston and rings. Don't listen to these guys that say "don't fix it if it isn't broke"....they are the ones who will be parting out there bikes when an $8 valve spring fails and does $1500 worth of damage to there engine.

Mine is an 02 and I've freshened it up twice now, and put new valves in the second time. Nothing was too worn, but its better to be safe than sorry. :ride:

What do you guys recomend for preparing the cylinder wall before putting the new piston and rings? All I ever did for my CR250 two stroke before a new piston was a light scuff with some scotchbrite pad wrapped around a cylinder hone and a few runs in and out on the drill. I have been told by others to leave the cylinder alone and just put the new piston and rings in.

I used a scotchbrite pad and some WD-40 to clean up my cylinder. :ride:

Hey there I caught your post. Don't overlook all wearable parts. Check the oil pump and make sure the clearances are still in spec, its the heart of the engine. If the oil pump is beginning to fail your engine will be following quickly behind it. Many people tend to ignore this part. Like you said these pistons are short skirted so piston to wall clearance is always a concern so it is important to check. But whats more important is how round the cylinder is from top to bottom and if it has any taper if you are seeking maximum HP. I am currently going through my YZ400 that I bought for next to nothing because it had a bottom conrod bearing failure do to an oil pump being worn out. Theres nothing wrong with tearing down and checking clearances and dimensions of parts because when these things let loose they do some serious damage. I been working with my very experienced local motorcycle mechanic/machinist/engine builder and I have lost count of how many yzf's I have seen just trashed do to worn parts that would have been caught by preventative tear down to check the wear and dimensions. Changing the cam chain is a must, everyone seems to have caught on to this one. Don't overlook the valve springs either they are cheap and they do a very important job in keeping your valves under control and not colliding with your piston. Don't forget these are race engines and they can and do wear out parts much faster than the old generation 4 strokes. Theres nothing wrong with tearing apart a good running engine to check its internals it is usually the best way to save yourself from a $1000 plus repair bill. if you want any other advice feel free to pm me. I am going to end it here because there is just too much to cover.

Thanks BNIO I appreciate the input. I will go through everything I can to check for wear in the top end and I will also look at the oil pump thanks to your suggestion. I don't think I will look at a big bore kit but I might look for a 13.5 to 1 piston. I may look at some minor port and valve bowl clean up while I have the head apart with the valves out. I have done minor porting and casting clean up on several V8 engines over the years as well as my old FZR750 all will some improvement in performance. I am sure there is a little clean up that could be done on my 426 head. I am trying to order all the parts I can before the tear down so I have as little as possible down time.I may end up replacing parts that could still have life but the convenience and piece of mind on reliability will be worth it.


I put a Luke's Racing big bore kit (13.5:1 Wiseco Piston) on my '01, and holy shit what a difference! Bnio is absolutely dead on about the oil pump, too. If your valves aren't tightening up, I wouldn't replace them, just check to be sure the stems aren't bent, and see if you can find a shop to dye penetrant or x-ray inspect the areas where the stem meets the head looking for small cracks. You can't eddy current these valves, as they are Ti. All who recommend new valve springs are right on, too. I would also replace the keepers, as they are really cheap. Be sure to take a look at the gear teeth on the cam drive gear (machined into the crank half) for wear as well. A new cam chain won't last too long running on dorked up sprockets.

I disagree with the majority! My 2002 WR426 broke a valve after I owned it for 2 years. The valves were checked three times in that time and were in spec every time. The valve broke one month after I had them checked. Yamaha uses titanium valves which are great because they don't stretch and need much adjustment like the steal valves. The down side to titanium valves is that they are not made from one solid piece. Each valve is two pieces that are friction welded together. That is where my valve broke. It cost me about $1,700 to fix once it broke.

I now have a 2005 Wr450 with a 2 year extended warrenty. I am a firm believer that the WR is much better than the Honda 450X when it comes to the amount of maintenence, but I like having the extended warrenty anyways.

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