Ok, this is my first post, but not my first rodeo. I'm seventy something and putting an engine together out of a box of bits. ( I thought it was all from one engine, it wasn't) ) Been doing this for 60 years. But I've run into something I've not seen before. I have a '67 crankcase and an earlyish (single plug) crankshaft, just had it reground. Nice new original pistons.(7-1 probably) Lovely late rods. so all together it's gonna be a nice engine. I'm balancing the crank as I write. The factory recommends 70% balance factor. OK.
So I weigh the reciprocating part of a rod ( 90g) + piston complete. (387g) x 2 = 954g. Then , 70% of that ( the factory recommended balance factor) = 668g. I make balance weights tape them on the crank and WOW I need to remove 300+ grams from the bottom of the crank to make it balance.
That's a LOT. a 3/8" bar of cast iron a foot long is that much!......Can anybody see where I might have made a mistake?.......Could it be that I just have very light pistons? Or did the crank that I have just have some very heavy pistons on it? I cant really drill that many holes. I'll have to mill the sides of the flywheel....Any comment are very welcome

I posted this in the wrong forum, It looks much more at home here.....