Do I need to replace my piston?

[indiandunes]

Do I need to replace my piston? YZ 426. It has little vertical scratches on it No blowby marks. I have no idea how many hours on the engine.

Thanks,

[MNellis]

If you can feel the scratches then it probably needs to be replaced. The piston to wall clearance is pretty important in these motors so be sure to measure it if there is any doubt. If that piston gets to rocking then you'll tear up the nikasil (sic) coating and wear away that cylinder faster than you can believe. It'll show up as sludge type of material in the filter screens. I think the manual recommends checking the piston every 5 races and replacing "as necessary". They don't tell you what necessary means though. ?

I've got the whole motor torn apart right now replacing the crank, piston, cylinder, 3 intakes and upgrading to the 450 cam. It's pretty expensive.

Be sure to check the small end free play (side to side movement). Mine was at .055" indicating some probable big end bearing problems.

Indian Dunes. Damn! I cut my teeth at that place around 1972. I met Gene Hackman there and used to watch them film the Black Sheep Squadron. Cool.

[SSpeeDEMONSS]

i would replace it. i have my motor apart replacing the bearing on the big end of the rod. i looked at my piston and it has a few little marks and i thought it was fine. well the guy i take my motor to told me to replace it. he said that it would probably be fine for a little while but if it breaks it would probably split in half and tear up my cylinder. better safe then sorry.

[Wayne_Thompson]

I would replace it. Just did mine and it also had minor scratches. My motorcycle guy said it would be a good idea to replace it now before I had problems.

[MNellis]

i would replace it. i have my motor apart replacing the bearing on the big end of the rod. i looked at my piston and it has a few little marks and i thought it was fine. well the guy i take my motor to told me to replace it. he said that it would probably be fine for a little while but if it breaks it would probably split in half and tear up my cylinder. better safe then sorry.

You mention replacing just the bearing on the big end? Are you doing this yourself or having someone press the crank apart? What are theyl charging you to press it apart and put it back together?

[SSpeeDEMONSS]

i had the crank pressed apart, put in a new bearing, pressed back together, and then trued. i dont remember exactly how much that cost. i can find out but, i also got a complete gasket set, both crank bearings, another bearing for the case, a front sprocket, a chain, and something else i think. it all cost me 500.

[MNellis]

i had the crank pressed apart, put in a new bearing, pressed back together, and then trued.

That's interesting. keep us informed as to how that works out. I didn't think I could trust anyone locally (certainly not the service dept at my local dealer) to press apart and retrue the crank so I went the other direction and just ordered a whole crank assembly.

What happened or what indication did you have that you needed a new crank bearing? In my case, the small end of the rod had about .055" side to side free play along with 3 bent intake valves and enought front and rear piston clearance that it sounded like a tamborine at a Gypsy wedding. ?