Question about 86 250R - Bad News - Good News

[waterfowler]

Well the bad news is that I had it idling this morning and bam...it just stopped abruptly. Uhh ohh. I kinda thought piston/rings by the sound of it. I tore it down this morning and sure enough it siezed the top ring and the piston was cracked at the skirt. Went through the crank and bearings and all look OK. I pulled a couple large pieces of the piston skirt from the motor. Doesn't look like much damage occured. Good News. The piston and cylinder are still on the original bore. hehe. Now here comes the question part. The cylinder looks OK, no scratches or gouges, I'm thinking I can just lightly hone it and but in a stock Wiseco piston/rings. What do the experts think?

[TimSr]

A cylinder will wear egg shaped, with the center being worn out wider than the top and bottom. It will do this with normal, and may leave the cylinder looking perfectly smooth and scratch free. This is something that needs to be checked in addition to loking for blemishes on the cylinder walls. An easy non precision laymen's test is to take one of teh rings, and see that the ring gap is exactly the same in the top, cneter, and bottom of the cylinder. If your cylinder is worn, you shoudl have a wider ring gap in the center.

[ATC'R]

Tim is right I have found that just because it "Looks" ok doesn't mean the cylinder is still true and round. If the bike is on the original bore it more than likely has some cylinder taper or out of roundness which honing will not remove. I always check my cylinders with a dial bore gage with every tear down. I actually always check a fresh re-bored cylinder as well. I have received a poor bore job from CT racing once and have had to send back my cylinder for rework. Just a note that even if the cylinder was still straight and round it would still require additional clearance to install and run a Wiseco forged piston...

[Kilborg]

Tim sr got it dead on. If you don't have any dialagauges, use piston end ringgap and measure with a micrometer.

[waterfowler]

Thanks for the feedback, it will be put to good use.
Thanks again,

[200x Basket]

for about $40 you can have a new bore and have it done right. why bother doing it half assed?

[ATC'R]

Using the piston ring end gap clearance to check for taper and out of roundness in a cylinder is very crude and virtually impossible to do with any degree of accuracy. That's like using your thumb for a measuring tool in carpentry. The Honda service limit for Cylinder taper and Out of roundness on your 250R is .001 of an inch max. There is no way anyone will accurately measure .001 of an inch or less using feeler gauges. Checking and "adjusting" the piston ring end gap with feeler gauges is another thing, as that's a static measurement and the service limit clearance is .020 of an inch. How are you going to check the cylinder bore diameter and it's service limit using the rings? The cylinder bore service limit on your year bike is 2.601 of an inch and should also be checked using a dial bore gauge or at the very least some telescoping gauges and a micrometer. For what it costs for new pistons, rods, gaskets, etc (not to mention ones time). Its best to due things right the first time when assembling your engine. It will run better, longer and probably save you from unnecessary headaches and expense down the road.

[ATC350X 85]

Dont do that. I tried to salvage the original bore on my 86 250r cause it was a low hours machine. I honed it and put a new wiseco in and when you would rev it up it would slap. I measured the bore, the piston etc and it was close so i tried to salvage it but it wasnt as close as i hoped. So I tore it all back down and got it bored .25mm (.010")over with a new Honda piston. Now it doesnt slap, it just detonates but thats another issue of bad gas and the wrong spark plug.

Justin