custom pistons anyone?

I am rebuilding my ancient 1988 Kawasaki KX250. Back in the late eighties I damaged the top end and had a sleeved big bore kit installed. That turned it into an awesome woods bike but now it's time to redo the motor and of course the pistons are unobtainium. Wondering if it would be possible to find a similar piston and machine it to size. The easiest thing would (maybe) be to find a four stroke piston without the port cutouts and modify it. The other options are to find a used 250 jug and go back to standard bore, or stick the old piston back in with new rings. It is a little loose and neither option is appealing. Or order the minimum 20 from wiseco.

Open to suggestions.

I don't see why you couldn't, if you can find a piston in the right diameter and pin height/diameter, or close enough to machine, with enough skirt on it to modify.

How much would 20 from Wiseco cost and how quick to do you think you could sell 16-18 of them?

Cast your own like Burt Munro?

Cp Pistons I know does custom piston for not a bad price and there is one more I can't remember.

JE would be another source.

Are standard bore pistons available? If so you could have it resleaved to std size.

Here is a possibly better idea. You said that you could get rings right? Have you actually measured the bore? Is it still round enough to not need repair? If so, and you said the piston was just a bit sloppy, but not trashed, send the piston out to get a low friction coating.

Swain tech, a GRM advertiser, offers this service. IIRC, they can adjust the thickness of the coating to your specs for this exact purpose.

So I have learned a bit about pistons now. They are not round. I did not know that. They are oval, and tapered to allow them to expand in different directions at different temperatures. So much for custom modifications. But the referral to Swain coatings was very helpful. They can build up the piston to reduce piston to wall clearance so that may solve my problem for considerably less than a custom piston. I am going to have the piston and bore measured more accurately today to see if that will do the trick.

I too had a '88 kx250, last year I did a top end rebuild. eBay had brand new Pistons. I cannot remember the brand.

Trackmouse wrote: I too had a '88 kx250, last year I did a top end rebuild. eBay had brand new Pistons. I cannot remember the brand.

Kx 250 pistons are a dime a dozen. Kx 300 pistons not so much....

I believe they offered them in overbored sizes. I remember I had to select "no .000" or .020" or etc.

So what's the bore size of your piston?

They are much larger than stock. 73.5 mm. I believe I have found a replacement but it is $300.00. I am probably going to go the coating route as it is quite inexpensive and has performance advantages.

Try somebody like Eric Gorr. He may of know of something that works

bearmtnmartin wrote: So... pistons... They are not round. They are oval....

Say what? As a machinist (specializing in pistons, valves, spools, shafts, and all sorts of other round parts) I'm curious as berkeley how the hell a piston is machined out-of-round?

Source?

How do you think they machine camshaft cam lobes?

I suspect they're round, but they're definitely not cylindrical. The skirt shape is fairly important.

Before spending any money on your current piston, check the ring-to-groove clearance to make sure it's in spec. if it's too wide, it'll get REALLY wide fast and you might break a ring.

Two stroke pistons are 'all over the place" as a machinist just told me. They expand at different rates from side to side due to the greater mass of the pin boss,and front to back is different again because the port side gets much hotter thanks to exhaust gas contact every stroke. But hotter on the bottom of the skirt than the top where there is more material. The pistons are made taking all that into account so that when they reach optimal temperature they are sort of round. And that is probably why my old air cooled YZ 125 would seize any time I tried to trail ride it. Poor heat control.

In reply to jimbbski:

Oh yeah. Duh. I had brain fade.

Well by chance I have an old air cooled YZ, a 76 YZ125 to be exact and out of curiosity I measured a new piston and a used one. Now keep in mind I did a quick measurement with a digital caliper. Above the ring grove appears to be round but down on the skirt there was a .01 MM difference.

As for the topic at hand I'd check the piston to wall clearance and if it was close put a fresh ring or rings and run it. If the coating gets it close then great. If it seizes then get a barrel off eBay and put it back to a 250. My KTM 300 was reeeeeally loose I ran fine. I sold it off cheap and the guy who bought it put a lot more hours on it before rebuilding it.

Now I do not know what the deal was with your air cooled YZ but mine will plod along all day long even when it's 106 out. The only danger with doing this is drowning a plug.

Tom