Ive recently removed the pistons from a 1960 engine. the only visable mark on the crown is a letter B. No markings to say they are a plus oversized. Anyone know what the letter B is. There is a very slight concave in the crown, is this normal.
thanks John.
B on piston
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Your second point - slightly concave crown
This is not normal - it suggests the engine was run with the ignition too far advaced.
The Wipac Advance/Retard mechanism was not very robust, and the bob weights quickly got sloppy as the pivot pins wore - letting the ignition over-advance.
For this reason, even at the time, you were advised to set the ignition to the fully advanced setting.
These days, fit an electronic ignition which employs an auto-advance system, so that you dont have to worry any longer.
As for your pistons, I cannot advise reusing them. But you are free to make your own judgement call on this.
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Hi John,
B on a piston is it's size grading.
I think you will only see a size grading on pistons fitted at the factory. Not every new piston was exactly the same size. Not every cylinder had exactly the same bore.
By stamping the pistons to identify the oversize or undersize, they could be selectively matched to oversize or undersize cylinders. We are talking about tolerance differences, but it made for more reliable piston fits and controls running in and seizure problems.
I do not believe you will see a grading letter on an oversize piston. But it will be marked with the oversize dimension. Logical really.
If I'm slightly wrong on this, I'm sure someone will say. But for the lightweights I'm pretty confident.
Peter