Piston help

[tiny]

My '89 EliteR(se50) needs a new piston and I will need to up size it and the rings to +0.25. Does the cylinder need to be bored out to compensate for the new piston? Or is there enough leeway not to need to?

Thanks

[90GTVert]

I'm kinda confused on how you "need to up size it" by 0.25mm if you don't know the cylinder measurements. If you can find a service manual for your Honda, it should tell you what the service limits are on the cylinder bore and piston diameter and give you specs on the acceptable piston to cylinder wall clearance.

Chinese manuals tell you to measure the bore about 20mm deep in the cylinder to get the bore size. Yamaha uses the more common method for engines that also gives you the most info. They want you to measure the top, center, and bottom of the bore on both the wrist pin axis and 90 degrees off the pin axis. Then you use the largest average of the 90 degree off measurements in one of the three spots and subtract your piston's diameter to get a piston to cylinder clearance spec. You can also see how far the cylinder is out of round and tapered. The piston should be measured 90 degrees off the pin axis and about 15mm up from the bottom... basically just below the pin level. Pistons have a taper to them because of the way they expand with heat, so measuring other ways is not accurate.

Standard piston to cylinder clearance in the Yamaha manual (40mm bore) is 0.035-.0040mm (~0.0015") with a service limit of 0.10mm (~0.004"). Again, check your manual to be sure, but that should be a decent guideline for most similar sized engines.

You can also use feeler gauges and slide them between the piston and cylinder to get an idea of clearances, but the above is the preferred method. You have to be careful with feeler gauges more than a few thousands of an inch thick, because they don't conform to the shape and may make you think you have less clearance than you actually do. Use 2 thin feelers if you're checking and clearance is too much... though by the time you need to break out multiple feeler gauges, you're at least pushing service limits most likely.

[tiny]

Sorry for the confusion. The standard piston for my SE50 engine is no longer available but I can get one that is +0.25 larger in diameter. And I'm pretty sure that the cylinder will need to be bored out to compensate for the larger piston.

[90GTVert]

Oh, OK. Yeah, that's too much for a standard bore unless it was really worn... and if it was worn to where 0.25mm/0.010" oversize fits with acceptable clearances, then it prob needs to be worked over anyway.

[maddog21]

Yes it needs bored out

[tiny]

Thanks y'all a little assurance is always helpful.