Piston movement

[10 inch Terror]

Morning Gents, I have just built an Imola engine and there is a rattle coming from the top end when the engine is running. With the exhaust off i can put my finger in the exhaust port and push the piston and it is rocking in the bore. It is a mark 2 kit and i seem to remember reading that there were some of these sold with piston / bore clearance issues, will the piston expand went it gets up to the correct temp or if i run the kit any distant will it wreck it? Any help or comments very welcome. Thanks in advance Pete.

[bristolmod]

certainly sounds like "Piston Slap" (excessive piston to bore clearance)

A lot of 2 strokes exhibit this "problem" especially Lambrettas (listen on the start up at the next ride out!!)

This is probably due to OEM pistons not being available any more thus forcing owners to use pistons with a different coefficient of expansion than factory built engines, which means that the bore to piston "gap" has to be larger than normal, which results in a "rattle"

Generally the rattle disappears when the engine heats up

I would be inclined to run the engine up to normal operating temperature and report back your findings- it may disappear

Chris

[10 inch Terror]

Thanks for that Chris. Cheers Pete

[Donnie]

what was the clearance when you built it?

[10 inch Terror]

what was the clearance when you built it?

Thats the problem i did'nt check it, the barrel and piston were sent to be tuned and i was told by the tuner the clearances were all fine. I may be worrying for nothing and it may be all ok and the rattle will go away as it warms up. I just did'nt want to risk wrecking it by taking it out for a long run. Looks like i'll have to bite the bullet and take her for a run and see what happens. Cheers Pete

[bristolmod]

As an aside, remember the old SR185 kits from a couple of years back? They used to rattle like buggery, due I think, to problems with the taper on the supplied piston (used to fly though!)

On the last few rebores I've done I've taken the piston/ bore clearance down starting at 3 1/2 thou (rattled when cold): 3 thou (still rattled but not as bad)

Running a rebore at present at 2 1/2 thou (shock horror), with no problems and no rattles whatsoever) This is on a cast iron block and a 64mm piston giving around 186 cc's. The only "mod" I've done is to drill 2 small holes (2mm) either side of the piston where it sweeps the area either side of the exhaust port- theory being that it allows a cold charge of 2T mixture to cool and lubricate that hot area. Seems to work (famous last words!!)

Giving it a "fast" run in with lots of short sharp runs (and lots of welly with the throttle when cold/cool)

Let us know how you get on

Chris