Piston/cylinder Clearances

[admacp]

Can anyone tell me the max. clearance between cylinder & piston using a feeler gauge. I have a really annoying rattle which I cannot find. (I don't have access to any proper measuring tools or bike dealers) The bike is a 280 Pro and is a 2004 model.

The barrel & piston look in excellent condition (B size) and I have measured the clearance as 4 thou (which is 0.1mm). Is anyone able to tell me if this figure is ok and if not if it is the likely cause of the rattle (piston slap).

The rattle is not present until a few seconds after starting (first throttle blip) & does not go away - seems to be worse on closing the throttle.

When buying a piston in the uk, do the piston sizes go up in increments of thousandths or millimetres and what is the maximum size they go to (I think they are available up to a D size but I don't know the actual size increase over a B size.

Thanks to anyone who responds

[kevin j]

(I think they are available up to a D size but I don't know the actual size increase over a B size.

USA reader, holding back from any smart sexist comments.

[opdeweegh1]

I'm not convinced that a couple of thou will give an annoying rattle as described from the piston clearance.

My own 03 300 pro recently developed a rattle from the top end also after it over heated due to the fan not kicking in due to a flat spot on the stator.

There is a very good description and some good advice on what to check for these annoying rattles at the following link. it might be worth checking some of the items described before buying new piston and rings etc.

follow this link and read about " spark knock "

http://www.gasgasmuseum.com/PDFs/Jetting/JETTING.pdf

Hope it helps.

[admacp]

Thanks guys. I could do with knowing the recommendations for jetting the carb for the uk at sea level as a starting point - anyone got those?

opdeweegh1 - did you cure the rattle? If so - did changing the jetting help!

I am starting to think about something like jetting as the bike is immaculate and I would be very disappointed with the engine if it is "worn out" already ('04 model)!

I assume that as the piston heats up - it would expand and the rattle would get less but there doesn't appear to be a rattle at startup on idle until a few seconds have passed.

[jtt]

admacp, I agree with opdeweegh1, your piston and cylinder are fine. Sounds like a fuel/jetting issue to me. You say it does it when hot? ....at closed throttle, or just under 1/4 throttle? My guess is it's lean on the pilot, they all seem to be unless your in the mountains. I'd go up at least one size on the pilot, with the highest octane fuel you can easily get. Willing to bet it cures the problem straight away.

[admacp]

I`ll try messing around with the jetting and report back. It starts rattling (definately not pinking but is a similar noise to tapping the exhaust with a spanner) just after returning the choke which is long before it heats up to temperature and is not 100% consistent with engine revs and can be slightly variable. The rattling continues throughout the whole rev range.

As far as petrol goes - I am running regular unleaded which is the same as my mates 250 and his runs with no problems.

[admacp]

I have just checked the carb and all of the jets are as standard spec. and there is no evidence of any dirt or blockages inside.

The only thing I have not checked is the fuel but I doubt if that will make the difference as I previously mentioned that my mate's 250 is fine with the same fuel. I have checked the spark plug and it's pretty black so it doesn't look as if it is running lean.

Apart from piston slap has anybody got any more ideas?

[opdeweegh1]

Admacp. Sorry I never got back to you earlier but I'm offshore just now and really busy.

I haven't fully cured my own bikes rattle as yet due to fact I'm at work, but sounds more like a knocking than a rattle and I'm sure mine is related to spark knock. When I return home I am going to fit my new Keihin carb that I've just bought and see if that makes any difference.

For your bike I would consider looking at the little end bearing. My bike really only starts the knocking noise when running at hotter temps or under serious load on hill climbs etc.

I had a JT 350 a couple of years ago and the little end bearing went on that, but again it was more of a knocking sound also, but I guess it would be worth a look.

Cheers.

[admacp]

I've checked the small end bearing and it's fine. My last hope is that HondaRS's theory is correct and a set of piston rings will cure it or at least help it. I find it hard to believe that so much wear has occurred in such a short time.

HondaRS - I checked the piston clearance at the point you mentioned and that is where the 4 thou size was though I had to be reasonably forceful (shoogle it about a bit but it did go up the gap about an inch maximum at this size!) Is there any way of knowing if the rings are badly worn before buying them? I would have guessed that there would be quite a bit of blowback down the side of the piston, past the rings, making the piston black - but there isn't much sign of it.

Finally, I rocked the flywheel gently from side to side and there is a slight tapping sound with a very small movement somewhere along the line. I've checked the big end, small end etc. and there's no vertical play therefore my only conclusion is that the movement is indeed the piston rocking slightly in the barrel.

[wayne thais]

From my exprience with a used Sherco what you are experiencing is a result of wear, 4 thousands is way too much clearance, check with the dealer, importer, I would guess that the recommended clearance is around 1 thousand, here are the Sherco specs (0,0270mm (.00106 in)). Take the piston and cylinder off of the bike and place the piston in the cylinder and shake it, you will hear your rattle. Replacing the rings will NOT solve the problem. Your piston is rattling around in the cylinder and that is the noise you are hearing. You may be able to get a new piston that will solve the problem, I would suggest that you take the piston and cylinder to a good shop and have them accurately measure them. If a piston will not solve the problem then you will have to have the cylinder replated, I had one done in the US for something like $150. The place that replates the cylinder will require that you send them a new piston when you send them the cylinder.

Wayne