I'm rebuilding a '98 RM250 and think I might have a problem with Piston to Cylinder clearance. I had the Cylinder replated and honed by US Chrome, I sent my new Wiseco piston so they could hone it to the correct clearance. The piston box says Clearance: .0015 in. I measured ring gap at .008 and .009 in. which might be a little tight, Wiseco says .004 in for each inch of diameter (.004 X 2.61 = .010). After installing the cylinder and rotating the crank it feels smooth but I can hear a noise that sounds like the piston is making slight contact with the cylinder. I hear the sound as the piston gets close to the top of the stroke. It kind of sounds like a file on metal, I think the skirt is contacting the cylinder. I tried measuring the clearance with an outside micrometer and a telescoping gauge but I'm not too good at it, I keep getting different readings. This isn't my first rebuild but I'm no expert either. I don't remember hearing any noises like this before. This Wiseco piston has some small grooves or ridges going around the piston and I think that when the skirt makes contact I hear the filing noise. I don't remember pistons I've used on other bikes having these grooves, I think they were smooth. I took the cylinder off, cleaned and relubed everything then put it back on but get the same noise. Now what? Any ideas? Could this possibly be normal?
Piston to Cylinder Clearance
- Thread starter jdon88
- Start date
Did you lube up the piston and ring when you installed it. I'm going to guess no, that noise is the ring being "Filed" by the cross hatch that's on the cylinder. That is what it is supposed to happen. If you have any concerns take the piston and cylinder to a machine shop and have them measure it. Maybe they will show you the correct way to measure with the telescoping gauges. here's a link for some help with those gauges. guide for telescoping gauges The grooves on the piston are from the machining process by wiseco.
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If the new piston had the proper clearance(check with a simple feeler gauge at the skirt) then it should be good to go. You also need to insure proper clearance at the exhaust bridge, 2 to 4 thousandths.
- Thread starter
- #6
Is this how you check with a feeler gauge at the skirt? I got it from a different website. http://i44.photobucket.com/albums/f30/xr75kid/002.jpg
Kind of. You need to check it where the piston rides. Put it almost to the top. The only time I have it out of its normal position is checking the bridge. My most bent up feeler gauge, .002
- Thread starter
- #8
I haven't checked clearance with a feeler gauge yet, I don't want to remove the piston from the crank unless absolutely necessary....I guess I'm still hoping for someone to tell me this noise is normal....I will have a bike mechanic friend listen to it and see what he thinks but that won't be until this weekend. Its not really a scraping sound, kind of like a nail file on fingernails sound. I took a closer look thru the reed valve area while the piston goes up and down. When the top of the piston gets about and inch from the top of the cylinder, I start hearing the noise, I think it is the skirt making slight contact with the cylinder. I think the grooves/ridges on the piston is what makes the sound. Maybe smooth pistons do not make this sound? Is it normal for the piston skirt to make slight contact with the cylinder at the top part of the stroke?
The skirt rides on the cylinder wall, minus oil.
jdon88 said:
haha don't do that
you should pop your cylinder off and take it to any automotive machine shop, have them stick a bore gauge in there and measure it accurately to a tenth or half thou
you can't accurately measure a bore with feeler gauges, outside micrometers or vernier calipers.
a bore gauge will also tell you about the cylinder condition, out of round, taper...etc...
I bought a cheap chinese one (about 100 bucks), have used it a ton, one of my more prized purchases. all u need to use it is a micrometer at least as big as your cylinder and a nice clean bore!
You would not want to bet on it would you?. Measuring tools are all in the hands of the person doing the measuring. The feeler gauge works fine for almost anybody, cheap and readily available. If the clearance is excessive, you have issues. If it is too tight, same thing. If you have issues, then get the bore gauge, or just send it to get fixed. The title did say piston to cylinder clearance. A simple feeler gauge will do.
robbieisurfriend
Member
- May 1, 2007
- 85
- 0
Well thats alot better than seizing up your bike during break-in. Paranoid is a good thing to be with an expensive investment.jdon88 said: