Oil filter for my 1971 Norton

Should I drill the two holes in the engine cradle for the later oil filter or should I purchase a add on external oil filter?

Start a drillin.

Old Britts has a kit with everything you need. It's at the bottom of the parts list for your model year.

Cheers

I vote for the regular factory type kit and location but getting them two holes just so in such cluttered space is a real test of mechanical hand done machinist ability. I once fretted on getting them just right and pretty much did but now realize no big deal if a bit sloppy as just makes easier to install and get nipped up and locked up neatly, so only you and those inheriting your bike will suffer the pangs of knowing it wasn't exactly factory perfect holes. There needs to be a bit of spacing off the cradle via washers between the mount or the oil canister make foul cradle a bit. The depressing reality of oil filters is about the only time they are doing much is on initial start up to catch big stuff that could block passages but nil protection for the wear size particles we have in mind on fitting filters. There is a reason for the jubelie clamp provision you don't ever want to discover why they provided that annoying extra feature, ugh. If Old Brits provides a good template to nail down the holes positions jump on it as way worth their premium pricing that's not ripping any one off just enough to keep em around. Just being realistic only a special bypass filter can snag the stuff we wish the spin on things did. Sludge traps and tank and sump settling are about all that keeps Nortona from chewing themselves to trash, sooner than later.