Actually - I just found this very helpful post by Mecchanica
"Those nuts are 1/4 CEI, or 1/4 X 26. It is a Cycle Engineer's Institute thread and uses a 1/4BS or 3/16W wrench. You can't get it at a hardware store. You won't find any metric threads on fasteners on British bikes of the late sixties. The sparkplug threads are metric and perhaps the drains in the late concentric carbs and the main jet cover on the monoblocs are the same 14mm, or a near British equivalent.
Essentially, you will find that 68 and earlier bikes are mostly CEI threaded nuts and bolts, and mostly BSF bolts into alloy....like case bolts, but the cover screws are CEI (26 tpi from 3/16" [or was it quarter?] through 1/2" shank size. These can be replaced with the "U.S." sizes if you use both nut and bolt. Fine thread is preferred for vibration resistance.
The headbolts remained CEI through the end of the 650s, changing to NF with the short rod 750s in 73. I believe the rocker box nuts and studs changed in 70.
From 69 on, most of the fasteners were UNF (SAE fine)....particularly any nut and bolt combinations.... and the bolts into the case alloy were NC (including the cover screws). These you can find at the hardware or marine supply store (if you require stainless....look for grade 316 instead of standard food grade 308). The heads can be easily domed to remove the lettering and then polished, looking as good as chrome and never rusting.
The stock "U.S." bolts are identifiable by a circular depression in the top of the hex. Fasteners without a hex head sometimes have a groove cut on the side of the hex. The nuts are usually full finished, as compared to the CEI nuts which were half finished (lead-in bevel only on one face). Some components also are marked....such as passenger footpeg abutments, the later ones with NF threads have a groove cut into them at their base.
The carbs and the points systems kept the British threads, generally. The rectifiers switched to NF centerbolt and nut and the zeners kept the british thread, but fitted a nut with a 1/4 CEI threaded bore with a 7/16" hex....a hybrid. The emblem retaining screws remained British Association thread. Other odd electrical fittings remained Brit, for interchangeability.
[ This message was edited by: Mecchanica on 2007-06-15 13:36 ]
