"nothing lighter than a hole" apparently, so bought a 1 1/2" HSS carbide tipped hole cutter especially for the job. but on centre punching the first indent (of 6) on the inside of the SLS drum on my BSA A65, a cap just popped out - so i guess the easy provision for some extra fresh air was already built in??? weighing the 6 caps = 10.4 grams so not much of a diet there.
added some budgie mesh to keep critters out.
on to the rearhttps://s1378.photobucket.com/user/...944s_zpsgm7u2kh1.jpg.html?sort=3&o=8https://s1378.photobucket.com/user/...938s_zpskum4njdp.jpg.html?sort=3&o=9
the area available on the rear is smaller so i was planing on smaller holes. no indents on the rear so i made a template with 6 holes marked equidistant between the spline bolts. again, bought a brand new drill, fully expecting to burn it out on the cast iron. centre punched, then starting with a 4mm drill as a guide hole, i was surprised to have it bore straight through with little effort. jumping to the next size(s) up i noticed the metal wasn't producing the expected pasta curls of swarf but odd chips. the depth of the hole/thickness of the vertical face of the drum is only 5mm (the front is 7mm) but i expected a tougher material. it almost feels/looks like bakelite with a few iron filings thrown it just to make it metallic.so finally my question: what is the rear drum of a 1966 BSA A65 made of?
i'm not confident to drill the now 14mm holes any bigger for fear of weakening the structure.