Saving Old Seagulls

I have the manual on order, just waiting for Postman Pat to get fed up with delivering Chrissy Cards and deal with the important stuff (like Seagull manuals!). In the interim, can anyone advice me on the diameter of the flywheel nut for a 1961 Century Plus?

At the moment I have no spark, would be good to try the drill-on-flywheel trick to see if I can recharge the magneto.

Any advice very welcome, I tend to serve up virtual beer and pizza as thanks (not very filling, but calorie free and hangover free: everyone's a winner!).

Al.

Pretty sure it's 1/2 Whit which is the same as 9/16 BSF. Mind you there are a variety of nut sizes and mine have been mixed and matched.

Cheers Hugo.