Here is the situation: I recently purchased an 89 250R and I am in the process of rebuilding everything on the quad. The only thing that I have not taken apart is the motor. When I brought the quad, I did not have a chance to test ride the machine. The clutch cover had a crack that the seller had welded, but not bolted on. All the gears look fine and I replaced the clutch itself since I had it apart. The guy told me that it ran fine, and if it did not run after I started it up, then the price would go down. And I trust him, but anyways…

This is my first time diving into a 2 stroke motor on a quad. Since I still have the motor out of the frame, I would like to check the condition of the top end. I hope to replace the pistons and rings, but wonder on where to start. The seller told me that it was bored 80 over, I think, but I remember him telling me it was the largest bore that it allowed in a 250 class.

Once I have the head off, what should I look for?
I have heard there are numbers on the top of the piston that tell the size.
If so, how do I know what the numbers mean?
How do I tell if I can hone the cylinder without needing a bore?

 

More than likely you will not find any identification on the top of the piston. At least I've never seen it. Usually part numbers, manufacturer, etc. are found underneath the piston.

With automotive pistons you'll find a manufacturers part number underneath. To which you can contact and figure out its bore, material, etc.

Otherwise, since you're going to be taking it apart, have it measured somewhere. If the bike were previously raced I would recommend having the cylinder re-sleeved.

I would plan to have it all disassembled and inspected / measured to figure out which parts are still salvageable. Plan on buying a complete gasket and bearing set as well.

For help with performance parts check out the following links:

www.rageperformance.com

www.mfgsupply.com/catalog/atv/

 

actually, wiseco pistons (only kind I've ever used) have the overbore size stamped into the top of them. sometimes they are hard to read if the piston is old and has suffered damage

 

Wiseco and Niks (Pro-X) pistons will the part number and the size, for Wiseco's not only in the part number will you find the piston size, but they also list the actual dimension(.25mm, .50mm,...)

If it's a Wiseco, it'll be a part number 562P* the asterisk after the P denote the number of oversize that it is.

PS= Standard bore
P2= .020"over
P3= .030" over

And so on until the maximum size available. As mentioned it will also list the dimension oversize in millimeters usually underneath where the P/N is stamped on the top.

You'll have to have micrometers to measure the bore and the piston. Perhaps a local machine shop could measure them for, hopefully one that is familiar with two stroke cylinders.

Another good gauge of general engine health is the static compression reading. Under 120 psi, I would reccomend a rebuild. Stock should be 175 psi, whereas you may see as high as 200+.

Good luck and ask questions if you them...

 

I will place a bet that the piston is Wiseco, so I went to the site and got a list of pistons and the numbers.

I have a compression tester and was planning on testing the motor once it was back in.
I have not tested a quad motor, only cars. When I tried to test the compression with the motor on the bench, I could not kick it over fast enough with my hand.

How do you guys test the compression with the motor out?

My next guess would be to place the motor on the floor and try to kick it over with my foot. I just had the frame powdercoated so I really don’t want to put the motor back in to test, then have to pull it out again to fix.

 

nothing to do with topic


atvbbs

 

Not sure what type of comp. checker you're using (whether it just has the rubber plug and you need to hold it in or what). But they make one that screws into the head like a spark plug and retains pressure easier. This could free up a hand or person to keep the motor stationary.

Or perhaps try and make a makeshift frame, anything to bolt it down to, to hold it while kicking it over.

I hope this helps, I can't really think of anything else.

 

The compression testing that I am using is the screw in type. I think it is a Craftsman.

I will try and strap the motor in place some how, but if that does not work, then I guess I will just have to put it in the frame to kick it over.

Oh, 300exkiller – the Frame, Footpegs, Swingarm and A-arms were $250. I think the guy told me the frame was $175. About 95% of the frame was sandblasted before dropping it off. But the guy told me that they would do it anyways, just to make sure the job was correct.