82 cr500 piston clearance . limits ? and how to measure ?

rebuilding my 82 cr500 . was planning on doing piston etc . just beacuse it was an unknown to me .
bike was running fine .

piston measures 86 across and 86.2 front to back
bore measures 86.3 at the top . front to back .

where should i be measuring and with what ? feeleger gauges ? digital verniers? or do i need to dig out the dail vernier calipers etc ?

what is reccomended clearance and at what point would u replace it .
piston is a stock mahle

cheers

take the cylinder to a shop and get it measured and checked for out of round or if not go 25mm down form top of cylinder and measure or place a new ring in the bore and measure the end gap pushing the ring up in the bore with the top of the piston to keep it all parallel, option one is the best choice.you would have to look up the recommended ring gap on the mahle web site.machine shops may take off slightly more around the exhaust port to allow for extra heat/expansion.

The owners manual shows putting the piston in the cylinder and sticking a feeler gague between the skirt and cylinder wall. I think it has something about the end gap as well. Pretty sure at least an 1983 owners manual can be downloaded from the appropriate section on here. Pistons sure measure out tapered for me. I think I brought this up before and got agreement and no disagreement so just stating a measurement of piston diameter and nothing else is vague.

I was told pretty much buy the guy I bought my first 500 from that the upper end can outlast the lower end, 500 specific here. I tend to think if you don't ride in a pack with dust and keep from having a ripped air filter or need maximum performance that advice is not too far off. I did eventually switch out my top end for a better one someone else had done on a parts bike but the starting gears and then the crank went bad not too long after that. The replacement pistons are not the same. The distance from the crown to the ring slot is different (more pronounced difference for the 430 ones), they are lighter, and the cut outs on the sides at the bottom don't quite fit the actual features of the cylinder as well. They are lighter like the water cooked mahle ones. At least the ones I have held.

With respect, only some of the above is correct.

All pistons are deliberately made oval and tapered with the largest measurement taken at the very bottom of the piston skirt front - to - back.

For 'say' a 500 the std piston dimension of a new piston at this point would be 85.92mm

The piston manufacturer has allowed for the clearance, thus the bore would be bored to exactly 86.00mm

As the engine warms up, the piston expands to become 'round' and the gap reduces - hopefully a point where it is a perfect running fit in the bore...

Andy Elliott.

It still ran for a while . I found the picture in the manual detailing limits . I put a cheap run in it and it snapped after a day of racing . That's the 3rd ring that piston has had .
I'll rebore it . Going to need at least an 87mm .
Where does everyone buy pistons from currently . I see plenty of wossners on ebay for 170 odd usd which I'd consider an acceptable price

Frankly, I really only trust using a dial bore gauge. Find someone who has one, if you own a lot of bikes invest in one. Have a nice
fowler now.

Please beware: I did not have one and guess what got a bad bore job and yes it seized. Thought I could trust a local race/machine shop to do it.
So now I check all my bores for out of roundness, taper, square and concentric etc. even after any machine shop work.
Also great see how our bikes age. Seeing not much wear as one thinks.

Because of Texas heat I have a bit more clearance added. So instead of .004 have 00425 or 0045 on the big bores.

Please see the Husky spec sheets for rebore and compare to piston makers specs too.
I prefer a little more.

Forged piston expands more. I put my 390cr at a .0045" clearance and the first easy ride she lightly seezed I opened the bore to .0055+" and she was ok. Probably closer to .006". I firgured at first the tighter the better, the longer it would last. I can't open the husky specs, help?

One must measure the top and bottom of the bore to be sure it's ok.

We have an old boy who does the boring and knows everything about everything two stroke . He can do whatever numbers he feels suits . I'll let u know what he ends up with