Need new piston, help!

[Ohio2stroker]

Alright, so my wiseco .25mm piston is shot in m 97 rm125, an I just had the cylinder sleeve bored out to match the piston about a month ago, my problem is, wiseco doesn't make the .25mm over any more, I can't find anyone who does, but they do make a .40mm and a .20mm ., should I get my sleeve bored again to match a .40mm? Or would I be fine with getting the .20mm? I'm assuming it would cause more damage if I got the .20 since the sleeze is bored to a .25, also, where should I buy a new whole crank assembly at? What brand?

[GEOF]

You might be better with the .4. But you really need the piston to set up piston to cylinder clearances so I am not sure what your engine builder used when boring it out. 

As far as cranks try Partzilla, Rockcymountain ATV or Ebay.

[tim2007]

Why is the piston shot in just a month? Ought to last a lot longer than that. But yes, the 0.4mm would be the size, with boring to match, although are you sure that is the metric sizing? Oversizes tend to go 0.25/0.50mm and .020/.040" 

[Ohio2stroker]

Well I'm pretty sure one of the lower bearings for my rod went out, causing metal debris going up into my cylinder, and tearing up the piston, so the rod is locked in the crank and the piston is trashed.. But I will go with the .40mm over if I can, I'm unfamiliar with how much you can bore a sleeve.. But should I just buy a new rod and bearings instead of a whole crank? There is a shop here in town , so I could have him replace the bearing and rod

[Ohio2stroker]

[Ohio2stroker]

You were correct it is .20/.40''.. How do those compare to the .25mm? Is .20'' smaller than a .25mm?

[GEOF]

0.025mm = 1 thou and you have written 200 thou which is 5 mm or as you have wrtten it .20. I am sure you meant to write 0.020 which is 20 thou which is .5 mm

0.25mm is ten thou and 0.20 thou is 5 mm and 0.020 thou is .5mm so 0.20 thou is larger than .25mm. Anyway that is neither her nor there because what I think you are looking for is your clearances.

You need to measure your bore and allow the next oversize. .2mm - .4mm - .5mm or even 1 mm over. You have a sleeve so honing and boring is a piece of cake. Then allow piston to cylinder clearance of 1 thou per inch of bore size.

Gap the ring, relieve the exhust bridge if it has one, crosshatch the cylinder. Buy a complete crank or have your rebuilt. Make sure your engine builder can true the crank to less than 1 thou runout.

[Ohio2stroker]

Okay, so which size piston should I get then? And then I'll just have the guy at Knobbies (local shop) match the cylinder for me

[GEOF]

The only way for me to answer that is to say.....Mic and measure your bore as it is todate and go with the next available oversize. If you are unsure then post the measurements and I will have a look around and see what is available. The best way to measure is using internal bore snap gauges or an inside dial bore gauge and measure this in MM against a outside mic meter. If you use a set of verniers you will not get an accurate measure as piston to cylinder clearances also need to be taken into consideration. Basically.....What size is the bore now.....at the top and at the bottom? 

[Ohio2stroker]

I'll have to measure when I get back home, I'm over in Nebraska right now

[Ohio2stroker]

I haven't been able to measure it yet, but from this pic, .020" is a bit over .50mm which is a bit over .25mm of my current piston, but I guess that's the next step up that wiseco makes now, so I should be able to buy that,, what all do I need to buy for my bottom end? I'm going to buy a Hot Rods connecting rod, and the bearings and seals, what else? Thanks for all the help so far!

[GEOF]

Your .50mm will do the job if you have to take out .25mm to attain it.  Don't forget to allow for your piston to cylinder clearances on top of this. Your engine honer should allow for this when he has the piston in his hand. He will bore the cylinder out and hone and cross hatch to suit. I would allow a 3 thou bridge relief also if you are using a wiseco piston as it is forged. If it has the cooling holes in the piston you might not need the bridge relief but thats a call you and your engine man will have to make. You also should chamfer the ports after he has taken out the stock of .25mm so the piston ring dosen't run the risk of snagging. Do this before final finishing hone. If he has a BRM ball hone to put on the cross hatch then this will do the job of chamfering the ports. They work well for this.

For your bottom end, (and I take it you are rebuilding the crank) you will need what you have mentioned. Two main bearings, If you use aftermarket make sure they are C4 bearings. Two seals, Install them with a little grease, One bigend bearing. Your crank builder will need to knock it around and true it up to preferably less than 1 thou. You will need a bottom end gasket kit and 3 bond if it does not have a center gasket. I would also recommend lapping the cases so they are true. You may need the bigend thrust washers if it has any. Oil the bigend bearing after installation. That's about it for the bottom end. Good luck and I look forward to seeing your progress.