RG150 Loud rattle after new piston

[Jase W]

I finished putting a new piston in my RG today, and when I fired it up it had a very loud rattle at idle which goes away when you start moving (or you just cant hear it). Is this normal? This is the first time i've had it running so i dont know what they should sound like. Doesnt sound normal to me. I think it may be a leak at the exhaust port - would be a fairly sizeable leak though. Whats the best way to tell?

Cheers

[FilthyLuka]

"could you describe the ruckus?"

or the rattle in this case.

Does it go away when you pull the clutch in? Put it in gear? rev the throttle? rub your stomach whilst eating an icecream and operating the choke lever with god knows what?

[xwhatsit]

I heard something similar once with a guy with an old Honda twin; the cylinders had been overbored once or twice, because of wear, and then he'd put original-sized pistons in. I read this on the net, so it might be horseshit, it seems to make sense to me, but I'm an idiot.

[howdamnhard]

Piston slap?:slap:

[Grub]

I think xerxesdaphat got it.

My post was going to say, the barrell has been bored and you've bought a standard piston instead of an oversize one.

[howdamnhard]

I think xerxesdaphat got it.

My post was going to say, the barrell has been bored and you've bought a standard piston instead of an oversize one.

resulting in piston slap.

[Jase W]

Crap. I was afraid of that. I was under the impression that it had never had a top-end done. I got the cylinder honed but neglected to ask wether I would need a std or oversize. Im guessing piston slap will eventually result in some breakage?

[xwhatsit]

Well, it'll root your bore again amongst other things.

Is it possible to ring that shop back up and ask them how far they honed it? I.e., did you need a bigger piston?

It's funny, you said you'd never heard it running before. I don't mean to be rude and treat you like a complete newb, but have you heard many two-strokes before? Many of them do have a kind of metallic ticking/popping noise at idle and on the over-run that disappears when you wind the power on. It's not super loud, but it is noticeable.

[Sketchy_Racer]

Also another thing to remeber is that quite a few RG150s have loose power valves, its amazing how many rattle at idle.

And if you know anything about motors you'll see straight away if the piston is the wrong size to the bore. easy way to check, whats the ring gap currently?

[Jase W]

xerxes: i havent heard many 2 strokes. this is definently a very loud knocking noise that seems to disappear once the revs are up.

sketchy: then i know nothing about motors because it seemed like a perfect fit to me, and i have no idea what ring gap is. this is the first time ive replaced a piston.

Is it likely that the piston will be fine? its cheaper to get a replacement bore than it is a piston (if the place that advertised them still has them available)

[Sketchy_Racer]

Before you do anything to quick, just simply get someone with a decent amount of mechanical knowledge to have a look/listen. (preferably someone with RG150 experience)

Other wise any decent mechanic will be able to tell you if the sound is 'bad' or normal.

[Jase W]

back to your original point though, how would it be obvious if the piston was the wrong size? it seemed a pretty snug fit when sliding the cylinder over (although once it cleared the rings it slid over without too much trouble - i had oiled the piston & bore though).

I guess its still possible that it could be a leak right at the exhaust port - i have a sneaky suspicion it might not be sealing properly. Definently doesnt sound like its coming from the bottom end anyways.

[Sketchy_Racer]

Generally for the RG150s oversized pistons come in 4 over sizes (as far as i know)

.25mmOS
.50mmOS
.75mmOS
1.0mmOS

If you say that the piston seemed to be a snug fit, i would think that it should be right. (although that depends on your definition of "Snug")

As for piston ring gap, If it is anywhere in the range of 0.25mm - 0.4mm that should be fine.

How hard was the piston to get into the barrel?

I also assume you know about the piston ring pins. They're there to stop the rings spinning in the grooves and jamming on the ports. did you align them?

What about the gudgen pin clips? Remember to put both of them on?

(sorry for all the silly questions, but i've seen some very experienced hands makes silly mistakes like these before)

A leak at the exhaust port wont cause a knocking, and if it is leaking at the exhaust port it'll be leaking around the whole bore. it would just cause bad running

Where in wellys are you mate?

[Jase W]

Yep I aligned the rings to the pins. By snug I mean it slid over relatively easily (once i got it past the rings - bloody fiddly bastards) but it wasn't sloppy (ie bore had to be square over the piston for it to slide in). Im in Petone.

I definently did put both circlips in.

Ring gap - what is that a measurement of?

I took it for a ride once I had started it (probably a bad move, i'm well known for doing silly things like that) - it rode sweet as, had plenty of power, powervalves were definently working etc. The knock also went away once the revs were up (either that or you just cant hear it).

[Khaitronic]

where can i get new rg piston?