VB piston markings

[bevanclark]

Hi folks,
Another basic question - I haven't had need to remove the VB's barrel as yet, but when I took the head off some time ago, I found the the stampings 87 20 on the crown. I have no idea what make of piston it is.
I presume that means it is +20 oversize, but puzzled about the 87 when the std bore is 86.4, or is this normal practice to round it up ?
No big deal, just curious.
Cheers,
Bevan

[nevhunter]

Plus 20 is usually shown as +".020".gets a bit thing when the size is metric . If you have an inches micrometer, what its the figure you want on it?. Best measure at the Usual place.(bottom of the skirt at a right angle to the Pin) and add the clearance specified or what in your wisdom is better and bore the cylinder. Honing is OK if you don't have to take out much.Sunnen type hone NOT that lead balls with grit in them jigger. Nev

[Roger Gwynn]

That sounds like a Peter Hepworth piston, a relation to Hepworth of Hepworth and Grandage who was the first to make repro pistons in the modern era. In order to keep costs down the pistons were sometimes made to odd oversizes to suit available rings. 87 20 is 87.2 mm or +.4mm or .016". This is no problem as long the the cylinder is bored to the piston as it should be and not to an oversize.

[bevanclark]

Thanks Roger, I understand your explanation.

I couldn't detect any significant side to side movement when I had the head off and, although I could see where the piston reached the top of its stroke, I couldn't feel any lip in the bore so I'll leave well enough alone . Apart from my current oil pump problems, the bike runs beautifully.
Cheers,
Bevan