Scooterotica

I'm hoping someone can offer advice re. an unexpected problem with fitting an oversize piston to a rebored GT186 kit.

It's a 64.75mm Mitaka, so on its second rebore (it limped on with a massive ring gap for the past 18 months).

Problem is, the bottom of the piston skirt now hits the crankcase on the down stroke. I'm nonplussed. 60/110 crank. It ran fine with the earlier 64.5mm piston.

Has this happened to anyone else? What's the best way round it? A thicker base gasket/packer or taking a small amount off the skirt? What effect might it have on port timings?

Thanks in advance!

if you have your old piston,measure from the centre of the gudgeon pin to the bottom of the skirt.and compare it to your new one,
if you are going to use the new one you will have to shorten the skirt to suit,adding packers will only,raise the barrel,strange that it is hitting the case with a 110mm rod as you have 3mm to play with normally,is it a gt piston supplied by them cheers

Sounds like the wrong piston to me

Where did you get the piston from? The standard piston has is 0.75mm clear of the crank case with a 58 stroke & 107 rod. With a 110 rod the clearance will be an ADDITIONAL 2mm from standard. It sounds like you have the wrong piston to me. If you send me pictures or details I have 64.75 pistons in stock.

Thanks Rich. You were right, I had originally ordered a wrong piston. I re-ordered a 64.75 piston from Dez Askill, which you kindly delivered to him place last Friday. It's still just catching very slightly in the downstroke, which is a bit of an arse. Is it Ok to machine off 0.75mm or thereabouts?

Yes, it is fine to trim the bottom of the piston skirt up by 1mm but with a 110 rod it should be a couple of mm clear in any case