Piston slap

[Josh|RD125LC]

What on earth is it? I posted a quick vid of my bike engine running on the RD forum concerning a knocking that can be heard. Its not affecting the bike in any way that I know of, but just sounds more knocky than it used to. I was told it sounded a bit slappy, so Im assuming that person was reffering to piston slap. What is it and is it bad?
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Honda MBX50 | Yamaha RD125LC | Suzuki RG125F Lucky Strike | Suzuki GSF600S
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[Josh|RD125LC]

Heres the link to the video:

https://www.zippyvideos.com/2576067882949196/mov000011
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Honda MBX50 | Yamaha RD125LC | Suzuki RG125F Lucky Strike | Suzuki GSF600S
RD125LC Pictures

[Kickstart]

Hi

No sound when I looked at the video.

Piston slap would indicate a worn bore and / or piston (or a rubbish rebore job). Fairly easy and cheap to sort on an iron bored 2 stroke.

All the best

Keith
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[mchaggis]

Piston slap, often when there is too much clearance between the piston skirt and the cylinder wall, normally the result of excessive wear or some clown putting a piston a size too small in. The main problem is the explosion pushing the piston downwards; the piston doesn't go straight down, but is whanged into into the side by the force of the explosion.

The RD is a twin, isn't it? By shorting out one plug at a time, you can see if the knocking sound changes. It won't eliminate everything, but it'll unload the piston without the explosion (which would normally be pushing the piston into the side of the cylinder). If the slapping isn't there when that cylinder isn't sparking, you can probably rule out anything to do with the bottom end or any ancilliaries. That is assuming it'll run on one cylinder.

The piston normally needs replacing and the cylinder reboring/ honing to smooth it out and get the clearance right. If the cylinder has warped into an oval (unlikely really though), it could end up needing to be scrapped if the walls are too thin to bore it circular again.
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[Josh|RD125LC]

I could hone it but that means fitting a larger piston?
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Honda MBX50 | Yamaha RD125LC | Suzuki RG125F Lucky Strike | Suzuki GSF600S
RD125LC Pictures

[Kickstart]

Hi

Yep, would need a new piston of the right size and either a hone or (more likely) a rebore, if it is piston slap.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Josh|RD125LC]

Hmm. Sounds like more money spending on the damn thing. At times I wonder whether its worth it.

If it is slapping, will it do any damage? Because I don't fancy fixing it at this present moment.
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Honda MBX50 | Yamaha RD125LC | Suzuki RG125F Lucky Strike | Suzuki GSF600S
RD125LC Pictures

[Jamie.]

the link is dead now, is it possible you could host it again so i can hear it? as i was told today my bike may have piston slap too, alough mine sounds more like a tapping noise

[Kickstart]

Hi

Well, ultimatly it could cause major damage to the piston, skirt cracking off and dropping into the engine.

Not likely though.

Think RD pistons are cheap and a rebore should not be more than £20. Gentle few hours work.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Josh|RD125LC]

Heres a new link to the vid for Jamie.

https://www.zippyvideos.com/1105661262993186/mov000011/
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Honda MBX50 | Yamaha RD125LC | Suzuki RG125F Lucky Strike | Suzuki GSF600S
RD125LC Pictures

[Jamie.]

ahh yes, mine sounds the exactly the same as that , alough your knocking noise is slightly louder than mine

[Josh|RD125LC]

Just been thinking, but when the engine was apart, I checked the clutch basket, and it had quite deep wear marks where the plates have been wearing away at the sides. I filed out these wear marks and used the basket again. Now Im just wondering whether the knocking Im hearing is the clutch plates rattling, as there is bound to be a tad more play where I have filed away at the basket, bigger gap so they have a bit more play in and rattling?

If its not that, then its most likely piston slap, and I don;t know what to do, Im putting the bike up for sale once I have a car and have sold my 50 so I want to get it sorted really!
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Honda MBX50 | Yamaha RD125LC | Suzuki RG125F Lucky Strike | Suzuki GSF600S
RD125LC Pictures