Hi all,
The saga of my B40 rebuild continues.
The bottom end is together with the correct non end "end float". New bearings, sludge trap cleaned out, dizzy 'refurbished'.
Now to the top end. The motor came with a barrel and piston fitted. The piston, which is of the low compression variety, looks new and is marked as 1mm oversize and the barrel does measure 80mm.
Q1 what should the piston ring gap be ideally (in mm and inches please, I'm not good at converting!)
Q2 is the gap the same for all three rings or is it not so important for the "oil scraper"?
The bike also came with a box of bits including several old pistons and another barrel (with a broken bottom fin), two of the pistons are marked as +40 thou and one has a solid top ring then two very thin rings in the second groove and finally the oil scraper ring.
Q3 Are the two very thin rings a bodge or is the idea to position them so that in effect there is no gap for oil to pass through? They do seems to fill the groove very precisely.
Many thanks in anticipation.
Generically, ring end gap is .003" per inch of bore.
So if you have a 3" bore then end gap is .009"
A 2.5" bore is .075" or 7 and half though.
This is a ballpark figure and manufacturers have their own , but this is a good starting point
:) :) :) :)
I often wonder how my old engines ran when I was much younger and didn't know about such things as ring gaps.
Mike 8) 8) 8) 8)
Don't forget to place the gap on the top ring opposite the plug and then , working downwards 120 degree round the piston for ring 2 and then 120 roundwards for ring 3