time for a top end

My '99 yz400f is running absolutely fine, but I've read where some people are replacing their piston for no apparent reason. I'm a "B" MX rider with many hours on my bike (approx. 200 Hrs). The bike doesn't smoke and there is no loss of power. I recently adjusted my valves for the first time no problems. My question is, are some people been experiencing piston failures (i.e. cracking or siezing)or are they replacing the piston for the heck of it. :):D

most i'd say (like me) replace em every year to be safe. and prolly to give em something to do :)

would you rather spend a 150 bucks now and have a fresh top end or have a piston explode 20 miles from home.

why would it be 20 miles from home? he rides MX :)

Oops, didnt see that.

In that case. Would you rather spend 150 bucks now and be set for a while or lock up the motor in practice?

If the piston goes on a four-stroke it might take out some very expensive parts. If you have some time you can tear it down and check the tolerances.

[ May 12, 2002: Message edited by: moto madman ]

These guy's do have a point and it would be wise to listen. However, just for info, I have three years of hard riding on my 99 YZ400 on the completely stock engine. I ride this bike harder than most (high rev wide open desert and wide open sand duneing) and have had no problems what so ever. I have noticed a loss in compression so I think I need some new rings and a piston but it has been 3 years. I've estimated between 5 and 8 hundred hours on the motor with frequent oil changes and cleaned every trip air filters.


Just do a cylinder leakage test. This is the best way to evaluate a 4-stroke's top-end health. All it takes is a $100 tool and some compressed air--or you can get your local bike or car speed shop to do it for you. It only takes about 5-10 minutes; you just replace the spark plug with an air hose and remove the magneto nut cover so that you can hold the crank at TDC with a LONG wrench.

A brandnew bike, after break-in, will leak at about 4-6%. If you're still less than 10%, don't worry about it. Over 15%, you need to fix it--but, is the problem the piston/rings or the valves? That's the cool thing about the leakage test, you can locate the worn component by listening to where the air is blowing out:

Bad cylinder/rings= air is coming from crankcase

Bad intake valves= air is coming from carb

Bad exhaust valves= air is coming from pipe.

Remember, don't assume that you can just replace the piston/rings, you may also need to get the cylinder head/valves reconditioned, i.e; valves replaced/ seats refaced/ spring tension reset.

Good luck,


Thanks for the info on the leak down test Steve. I plan on having the valve job done along with new rings and a piston. Would it be a good time to replace the bottom end bearings?


Bottom end bearings should last a very long time on a four stroke. The exception would be if you have a major lubrication and/or dirt contamination problem.

My advice: check the main bearings for any perceivable play when attempting to move the crankshaft up/down; check that the rod side clearance is within service manual specs. If both are ok, leave the bottom-end alone.


Thanks Steve. I ride almost exclusivly in sand (Glamis sand dunes) and can't be breaking down in the dunes. You can't tow a bike in the sand very well.

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