Recommended maximum difference between 'old' and 'new' pistons at rebore is 1 1/2oz.(42.52 grms)...The BSA pistons average out at a difference of .458 oz.(13 grms)..so well within tolerance...So, at 514 grms the pattern piston is overweight if the current size is std. or +.020" and just inside if the current size is +.040"....Though some weight reduction would be advantageous...I really don't understand why pattern piston manufacturers don't make them the right weight.
....Ian
email (option): ian@wright52.plus.com
Material can often be removed from inside, under the 'crown' of the piston. Crown thickness can be reduced to approx. .175" thick without problems. I have run them down to .150" experimentally.. Many pistons are made from a common 'blank' and still have the additional material inside to accomodate the valve pockets in the top face for OHV applications. Of course you don't need those in a sidevalve, so all of that material can be removed as well. Pattern pistons frequently use heavier gudgeon pins than original (with parallel bores)..Original pins were tapered internally and are lighter, so fitting one of these will usefully save some weight. If the skirt forms a full circle, a chamfer can be machined around the inside edge to form a more or less sharp edge which removes a little more material....Ian
email (option): ian@wright52.plus.com
Hi Douglas...Same amount (roughly) in the middle as the parallel pin, but tapering outwards from there to each end and getting progressively thinner..and lighter....Ian
email (option): ian@wright52.plus.com