When to change out your piston

I'll try and quantify my statement about the 50 hour piston change.

For those who race, and want maximum performance...yes, I'd agree with a 50 hour (or less) time frame for swaping piston's. But for those who don't race (like me) and rarely hit the rev limiter, by short-shifting the bike.....I think a blanket statement about 50 hours is overkill.

There is no way to truly compare the bike brand new, as oppossed to now, unless I first took some measurements (acceleration, etc. ) for a comparison. All I know, is that the bike still likes to keep the front wheel lofted on hard acceleration.

p.s. remember, these are not Honda's.....


Which air filter oil are you using?

I was using no-toil but I switched to Maxima FFT when this started happening and the airboot is staying clean now.

I was using no-toil but I switched to Maxima FFT when this started happening and the airboot is staying clean now.

I use the FFT too. I found it to be the only filter oil that actually traps dirt and dust. I soak my filters in it and squeeze out the excess. The stuff is messy, but works extremely well.

Well here goes, I'm tearing the topend down and gonna put in a fresh piston. Got a buddie/mechanic that's coaching me thru the process too. I'll be replacing the pistons, rings, gaskets and maybe the valve guide seals. Wish me luck cause I've never done this before.

Also I think I solved where my dirt in the carb was coming from. The o-ring on the top plate above the slide (where you go in to acess your needle) was sqashed flat with age and there was dirt on both sides of the seal. I also changed out all my vent lines too.

let me know how it goes.........good luck...........

I wish you luck as well. I'm glad you have someone there to coach you for your first time. I wish I would've had that luxury...lol.

So far, so good. Looks like those frequent oil changes did some good. 300hrs and maybe a .0004 between piston and cylinder. Everything else looks good too; timing chain still has half it's adjustment left, cams are in good shape, no sign of valve seals leaking on piston and no play on the rod to the crank. Gonna clean up and inspect the valves tomorrow. Damn glad I've got supervision otherwise I'd have no idea what I'm doing.


Thought I'd add my 2 cents. If you had dirt in your bowl, it wouldn't be coming from the filter or airboot. That flows right through the carb, into the ports and then the cylinder. It shows up as worn top end. The dirt would be coming from the fuel tank since the fuel first flows into the bowl and sediment settles at the bottom, which in turn can get sucked into the jets. Pull the petcock off and check the small filter or if necessary, install a small plastic inline fuel filter. Lastly, get yourself a CLEAN towel to lay all those engine parts on. The parts will attract all kinds of dirt from that wooden bench top and you'll be doing the rebuild again soon. When building a motor, cleanliness is #1.:devil:

I think I may do this in the spring just save up and get new valves timing chain piston and rings the whole shibang (how hard is it to put all this in on a scale of 1-10 1 being could do it with my eyes closed and 10 being I almost died)

my bike runs great but I have never been into the motor and have no clue how many hrs are on it (bought it used)

as for your problem with the dirt in the bowl are you sure you dont have water in your gas I know the first couple times I saw what water looks like in gas it looked like dirt to me just food for thought

also I know when doing something like this you should give your bike a good wash but no matter how hard I try there always seems to be dirt on the parts I take apart any advice??

Ok I am no expert on the 4 stroke YZ's....and have never been inside the engine of one. However I have rebuild mant street bike engines, couldnt count how many top ends on 2 stroke, and at least a dozen small and big block chevrolets which equals alone 96 pistons.

With that being said the guy who posted pics of his old and new piston...those wear marks on the piston could be desribed as minimal at most.! The vertical marks on the piston and cylinder in the pic most likely occured withing the first few hours of operation and in no way were inhibiting performance on that bike. I may be incorrect..but considering the compression ration of the YZ 4 stroke I beleive the piston is well above 10:1 which leads me to believe that piston is forged. Even though the YZ turns higher RPMs than your average 4 stroke motor,,,,no way except under pro/expert race conditions should that piston need replacement in 50 hours of operation. Consider this: many 4 stroke engines turn RPM's way beyond the YZ motor and runs for thousandsof hours and miles, sometimes years at a time with no piston replacement..usually first rebuild is just rings. I am one of those over maintaining guys and on my YZ426 at best I may replace the rings and possible run a cross hatch ball hone though the cylinder (depending on the wall material as I dont know what its made of yet or if its plated with anything) to create a nice cross hatch for the rings to seat nicely.

Not telling anyone what to do...its always your own decision, and it cant hurt that is for sure...but if you must spend money, Id put it towards more beneficial uses IMO like purchasing multiple air cleaners to exchange between cleaning, ghanging coolant more frequently, changing oil and oil filters more frequently etc etc. I just dont think for 95% of the people who own a YZ 4 stroke need to do this...proof is evident of the people here with 100-200 + hours on original top ends. Again all this is JMO

Now that I've taken the piston, cylinder and head (with valves) to be re-fitted with new parts to the local bike/machine shop we've all agreed this engine could have gone another 150 to 200hrs before this work was eminent.

So for the record books, this '02 YZ had the potential for as much as 500hrs running hard in the woods (staying off the rev limiter and frequent oil changes and air filter servicing) and could still be going.

I am looking forward however, to how much stronger it'll pull now with a fresh top-end along with peace of mind when far from the truck. If I had to do it again knowing only a little over half the top-end's life was gone, I still would just to keep it at optimal performance. :mad:

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