This is one of the exhaust flanges: It is undersized
that boss on the top has to be bored out 12.7mm and it is starting out at 13.8mm which does not leave a lot. Having given the back a rub on some abrasive paper to flatten it off and using the boss to hold the flange by it was stuck on a bit of old steel in one of Mr Silky's collets.
It was horrible, snatched and came off the glue mount at this point I decided that that wafer of metal around the flange was not enough and it would need correcting. So knock up a couple of new bosses at 15mm diameter, silver solder into place and then use that boss to hold the part while turning the backs.
Before I drilled the mounting holes I decided to check with the size nuts the drawing asks for and
look at that no way is a nut that size going to fit, even one size down is too tight
So I decided to go for 7BA studs rather than the 5BA the drawing shows which means they need to be drilled 2.5mm not 3.5mm and as I have not done the heads yet the drawing has been updated for those as well
I will need to think about the inlet castings, they are all solid without any obvious means to hold them
Jo