Richard,
It is south of the pin boss I think and on a diesel I believe south of the last land if it has oil control below the gudgeon.
These are oval for splash oiling the cylinders below the pin ends proper, I believe this is why the stock pistons have splash oil holes fore and aft (minor thrust face), because those are not tapered faces and the holes are their only means of oil.
Talked to a buddy at work who still is into racing, but now its grass drags on snowmobiles. He asked if these were cast or forged pistons, (also new or used). I told him cast-used just for kickes, and about 2 inches in diameter. Off the top of his head he figured we would need to trick the lathe into thinking it was turning something around a 10 or 11 inch diameter to get the proper oval shape, and even then it would not be as good as a factory piston. I guess those are more of an elipticle (sp?) shape- much smoother then we could do on a lathe set-up. And doing hand touch up would be difficult, and you would never get them exactly identicle- which would make balancing harder depending how far off of identicle you are (among other problems like having to much clearance).
After talking a while, he told me we got away with hacking up pistons turning a taper on the entire skirt because we didnt really expect them to last that long, and nothing did- be it a piston or rod or valve spring or whatever, something was always going south.
Then he started talking custom pistons, he says he has paid 6-8 hundred for a set (snowmobile) and I might pay much more IF I could find a company to do it. I imagine the price has a lot to do with how many sets of a certain type of piston they sell- I cant see +10 pistons for a BSA golden Flash being a huge seller, so the price may reflect that in a big way.
After the talks with you guys, and the guy at work I know I am better off just going stock +20 even if it does waste a little barrel- I want a decent running cooking grade engine- and I want it to last. I think spending $800.00 (at least) on a good set of +10 pistons trying to save a $500.00 set of barrels is going backwards. But the discussion was interesting!
Now for another odd "what if"- the oil pump on these bikes runs at 3:1 right? If this could be changed to 4:1, would this increase oil pressure just because of flow restriction? Like trying to jam 10 gallons of water through a drinking straw in a hurry, or would you just have a faster flow rate? What about seals and gaskets, if the flow/pressure is increased would these handle it? Or, does the ratio not matter, its all in the impellor size?
(the things I think about when NOT thinking about stripped out bolt holes!)
Lee