Rebore ST70 Clearance is a bore!

Hi all!! took me a while to find like minded souls cool Dax rock.
Anyhow, im in the process of rebuilding an early ST70E engine for my 2nd dax and as it needs a rebore (v bad scored barrel!) i was wondering if anybody knows the piston to bore clearance for a 6v engine to allow for piston/bore expansion etc? Im fitting a 2nd oversize piston, and is it correct that later 12v dax pistons dont fit 6v engines Thanks. Ade

You're correct.
A 6v piston have the crank mounting holes on a different height as the 12v pistons.
In some cases it might work (with the correct type of head, adding gaskets,...) but it's safer to use the correct piston (barrel).
As for the clearances, if I remember correctly, each oversize adds 0.01mm, so for the second oversize you'd have to increase the original cilinder with 0.02mm. (Not sure!)

Original bores with the 41.4mm stroke:
50cc: 39mm
72cc: 47mm
85cc: 51mm
88cc: 52mm
106cc: 57mm
....

Hi thanks for that mate, so i think what your saying is it should be for bore to 2nd oversize from standard bore should be 47mm+0.5+0.02mm =47.52mm.
0.02mm clearance seems a bit tight. cast Iron barrels and ally pistons dont expand much then?

Thanks Ade

Second bore for a cilinder is indeed + 0.5mm.
So, 47mm +0.5mm = diameter of you cilinder.
The piston (barrel) is always somewhat smaller so it fits the cilinder perfectly.
So... if you've got the right piston, the cilinder diameter should be bored to exactly 47.5mm.

Once the space between piston and cilinder is >0.10mm, the piston/segments/cilinder is worn.
The genuine Honda (or Japanese brands) will expand very little. Aftermarket parts will expand more (and thus wear out more quickly)

So I mistyped a bit in the previous message concerning the min. clearance.
But I tend to remember that the clearance should be between 0.01 and 0.10mm (not sure about the first, but the 0.10mm is correct for Honda parts)

Hi Waynus
Thanks for that info, all is now clear. Time to have a chat with my local engineer then i can put the head back on my engine.
ta alot.