Hitchcocks Forum

Hi Guys. There’s a funny noise when starting to accelerate my 1996 535cc, just like piston slap. It is only there if I use more throttle than I need to hold the speed constant on the flat. And it doesn’t seem to matter if it is just a millimeter of throttle movement or WOT (wide open throttle). And it makes the same noise however much throttle I use and at any revs and whether engine is hot or cold - so long as there is a just a bit of acceleration. If I keep a constant throttle, the noise disappears until I next accelerate.

So I removed the head and the barrel and found the gudgeon pin is slightly loose in the piston, looser than when I installed it ~1000kms ago. I can see movement between the gudgeon pin and piston by the layer of oil squeezed out as I rock the piston (see comparison image below) and it feels like there’s ~1/4 millimeter slop (image in my photobucket account Chrisd200).



There is no discernible play in any of the other bearings though. This all happened after ~1000kms with a badly unbalanced engine, now fixed.

So, the question is this - is that wear critical? If the piston is worn where the gudgeon pin fits, perhaps caused by the shaking from the unbalanced engine, can it lead to catastrophic failure of the piston? (NB I have our hosts steel conrod and needle roller bigend and this is our hosts 535cc high comp piston).

Any comments? Yes, the best treatment would be to camera capture the noise and post that – but the bike’s in bits right now because of the noise.
Cheers, ChrisD

Chris you've obviously spent a lot of money on the bottom end in the hope of gaining reliability and performance, though how the balance was overlooked i'm at a loss to understand..

I think the piston and pin are now toast due to wear you describe and eventually the pin may break out the groove holding the clip..

Check again that the rod is absolutely central to the bore and theres no side strain to the piston/pin also small end eye on the rod is true..

Good job you didn't ignore the noise hoping it would get better..

1/4mm that's 10 thou which is a lot, I find this hard to understand some pistons have a floating pin and some makes are a tight fit but normally wear at this point is very rare, it would be the little end bush which would fail before this happened, the resulting noise it a very light tinney metallic tap which is barely discernible, is the piston a split skirt or solid skirt type?. What you describe sounds like the normal noise from a solid skirt piston.

Dennis and others, thanks for your input. The sound is not typical of a little end failing – I’ve had that in old Ford’s and VW’s I used to drive, so I well know what it sounds like. The piston is a solid skirt and is undamaged. The rocking movement I note is along the length of the gudgeon pin, so it isn’t much and there is no discernable vertical movement between pin and piston because of the oil layer. There is zero play between pin and little end.
I’m now thinking of putting the whole lot back together, riding it and recording the sound. I have the 10th motorcycle Century ride next Sunday, i.e. 100th day, 100kms, 100 years bike+rider for club points (which I won't score), starts at Century City near Cape Town) so I need it on the road – hope it holds out!
Cheers, ChrisD

Chris, this has always been a problem with the larger solid skirt pistons, because they have no expansion split they need a greater clearance and are prone to either slap as yours appears to or seize in the bore, I know which I would rather have. You may just have to wear ear plugs.