2-smoke question

20 replies to this topic
  • acarter920

Posted July 25, 2002 - 09:56 PM


I am checking the top end on my Rm250 and need to know what the piston/cylinder clearance should be. It is about 45 thousandths (.0045) of an inch when i push the piston all the way to one side. I dont have a manual, and was wondering if anyone knew what the clearance should be and if im good to go.

It was making like a knocking sound, but the top and bottom end bearings are tight. I noticed that when i push the piston hard up agains the cylinder, it makes the culprit noise... but that's where i got the .0045 measurement from, so its not hitting the cylinder.

Also, the cylinder is perfect, no scratches or scuffs or anything. Could the noise just be normal engine noise?

Any help is appreciated, thanks!

[ July 25, 2002: Message edited by: acarter920 ]

  • sirthumpalot

Posted July 25, 2002 - 10:15 AM


Be sure to measure the piston width at the skirt and the cylinder width across the area where the skirt rubs. Compare against the specs for your piston (is it stock?) and cylinder. Unfortunately I don't have any specs for you. Anyway, if the piston is lose, especially if the skirts or cylinder are worn, it can rock and make some noise.

It could be normal noise, I have no experience with that motor so I don't have any idea what noises it makes.

  • Uncle_Rich

Posted July 25, 2002 - 11:45 AM


if you checked it right(like sirthumpalot said), I think that is too much
best thing to do is call a Susuki Dealer or Piston Dealer (wiseco etc.)
I would GUESS .0025 to .003 inches


  • acarter920

Posted July 25, 2002 - 12:14 PM


If the clearance was too much at .0045 compared to .0025, then that would be the opposite of wear. I dont think my engine is growing... :D

Because as the ring wears, the clearance would be less when the piston is pushed all the way to one side. Did i just confuse myself?.. or did i confuse everyone else also?

Lol, I think my best bet would be to call a zuki dealer. I never even thought of that. :)

  • J.Hall

Posted July 25, 2002 - 01:32 PM


Im not confused, you are talking about ring end gap and they are talking about piston to cyllinder clearance. The ring end gap could be good and the piston skirt worn and sloppy so check both. Buy a manual and it will make life much easier, a clymer is about $15. I dont know what year your bike is but the 96 to 2000 with the internal waterpumps made a funny enguine noise. I had a 96 and it had a slight nocking sound to it at idol but that didnt change with a new piston. Just measure everything, you might be ok and just slap some new rings in and be good.

  • J.Hall

Posted July 25, 2002 - 01:36 PM


oh yea .0045 is 4.5 thousanths. .045 is 45 thousands. If it was that much I dont think it would run.

  • moto_madman

Posted July 25, 2002 - 03:34 PM


I have a 98 RM 250. It makes all kinds of sounds at idle but as soon as i rev it it all goes away. The local mechanic has an 00. It sounds likes its gonna blow up idleing but as soon as he revs it all noise goes away.

  • acarter920

Posted July 25, 2002 - 05:48 PM


I was talking about piston/cylinder clearance... that's what i measured. If I measured it at .045, and the suggestion was .025, that would mean the rings need to wear some more for it to get to an .025 clearance between the cylinder and piston. That's why i think he was talking about the ring end gap, because then his number could be correct. Oh well, not a big deal.

The bike is an 00' and it sound like the piston is slapping. But there are no wear marks on the piston or cylinder whatsoever. This is what has me puzzled. It could very well just be normal engine noise.

Maybe i just need to ride it and forget about it... thanks for the help :)

  • acarter920

Posted July 25, 2002 - 05:51 PM


Also, IF the piston skirt is worn a little and that is what is causing the knock, even though there are no wear marks on the piston or cylinder... would this cause any damage that needs to be taken care of right away?

I ask because i already have everything back together and dont feel like tearing it apart again.

  • BigLou

Posted July 26, 2002 - 01:17 AM


It's been my experience that RM's are kinda noisy anyway. *shrug* But as far as the rings wearing and that decreasing the piston to cylinder wall clearance, that won't happen. The top of the piston just "floats" in the rings. As the rings wear, the top of the piston is allowed to flop around more (a few thousandths)while it is still relatively centered on the rings. As the rings wear, the end gap of the rings also increases.
The piston skirt will wear, and eventually be too small in diameter to be within the service spec and should be replaced. But generally speaking, the ring wear and piston to wall clearance are unrelated.

Did that make sense or did I just confuse you more (along with myself!)? :)

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  • John_Lorenz

Posted July 26, 2002 - 02:07 AM


RM = Rattle Machine

  • sirthumpalot

Posted July 26, 2002 - 02:42 AM


If the piston gets too loose it can slap the cylinder and break. It's got to be pretty loose and doens't happen often but can happen. If you don't have any reason to believe that it's unusually worn then maybe you can find someone else with the same bike and compare engine noises?

  • Kirtwell

Posted July 26, 2002 - 02:46 AM


Most 2 strokes require .002" (two thousands)
clearance between cyclinder and piston to prevent seizing. If you have more than that a small rattling noise will develop. Then the noise will accelerate due the excess movement inside the cylinder. Eventually the skirt will stress fracture and break off. Then your motor will either seize or worse grind it up and you will need to split cases to clean that crap out.
My advice is for you to take the cylinder off and get the exact bore measurement to the .0000" Then buy the piston accordingly.

This is speaking from experience :)

  • moto_madman

Posted July 26, 2002 - 05:16 PM


two thousandths clearance? That sounds a little tight.

.0045 should be fine. RM make noise new or old. When i did the top end on mine it made the same knock that it did with the old piston.

The only RM that didnt rattle, was a 96 i saw that had a PC motor in it. Very smooth sounding.

  • Guest_blatham489_*

Posted July 26, 2002 - 05:23 PM


Are you sure that's not .045mm??? That would be almost .002", which sounds about right. .045" would be enough to see daylight!

  • acarter920

Posted July 27, 2002 - 04:44 AM


BigLou... as the rings wear, the clearance will decrease between the piston and cylinder. This is because i pushed the piston all the way to one side as far as it would go. Because of the round cylinder, the ring should wear evenly all around. So if i had a fresh pair of rings, then the ring wore and i pushed the piston to one side and then measured the clearance, it would be less than it would be with a fresh set of rings. I see what you are talking about though. The ring end gap wouldnt decrease, but the cylinder/piston clearance will.

Blatham, you are right, .045" clearance would be a little much, lol. I meant to put .0045" but forgot to put in the extra zero :)

I went to a local bike shop, and they said the Rm's are real noisy, so I think everything is fine and dandy. Thanks for the peace of mind everyone!

  • Uncle_Rich

Posted July 28, 2002 - 10:54 AM


Ring gap and Piston to cylinder clearance are 2
different thing.

When the rings rub up and down against the cylinder they wear. as they wear they open up more thus more end gap. too much end gap low compression.

As the piston rubs up and down against the cylinder it wears and becomes smaller thus more clearance between the cylinder and piston.
Too much piston clearance and the piston slaps against the cylinder thus breaks the piston skirt off. When the skirt breaks off the parts lock up the motor. You dont want this to happen.



Posted July 30, 2002 - 09:08 PM


Moto Madman- You mentioned you have a 98RM250. I have just went through the exhausting task of replacing the impellar pump seal on mine( splitting the cases) and although it was a lot of work it gave me the chance to learn a lot about this engine and to check everything out for wear. I have a problem though, my RM will not rev cleanly at above low( estimating 3 or 4k) rpm under a load. I am trying to find ideas as to what could be causing this. The top end is new, and I was able to check the cylinder for wear as well. It is all in speck. I also chesked the power valve for sticking and it operated smoothly. This is a problem that just came about on it's own, therefore it is hard for me to see how the jetting could be the problem, it's always been jetted perfectly before and nothing was changed. Now I am considering maybe a reed problem because this is the only thing I have not checked. Could a worn reed cause this. By the way this problem existed before I tore the engine down, so I don't think I screwed anything up.

As I said before the prob is that it will not rev cleanly under power( revs fine in nuetral), and it just WILL NOT come onto the pipe.

If anyone has any ideas, thanks in advance, if not, sorry for wasting space on 2 smoke probs...hehe

  • moto_madman

Posted July 31, 2002 - 07:40 AM


Turn the CHOKE off. Ive done that before. Talk about scary in a whoop section.

It sounds like a spark problem. Try a new plug. Check the gap. If its too big it could cause a situation like that. You can gap it as low as .020. The gap could be too small too. I forgot what the manual reccomends but .024-.028 is usually about right for most 2-strokes.

You can check the spark too. Take the plug out and put it back into the boot. Ground the electrode to a head nut. Kick the motor over and look at the spark. It should be large and blue. If its yellow or small its a sign that there is a problem somewhere in the electrical system.

Check all the electrical connections and grounds. Make sure they are clean and tight. Wraping them in electrical tape will keep water and other crud out.

If that dosent work it pull the magneto cover off. If there is junk inside or water got in the stator coils could have been damaged.

Wait i forgot one. Pull the plug boot off and check the wire going into it. There should be about a 1/4 inch of bare wire. If not trim the sheilding back a little.

  • Shawn_Mc

Posted July 31, 2002 - 07:50 AM


Its not coming onto the pipe because the reed is probably dead.

Do your spark check and all, but my money is a reed that isnt up to par.
Twostroke guys should automatically do the reed when the cylinder comes off. They're usually only 15 bucks. Cheap insurance. Id go around the whole thing and make sure you didnt do something silly too. My buddy put the piston in his CR500 backward. It happens. It ran, but wouldnt pull second gear.

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