1975 Mk 3 bolt size

Please excuse a dumb question from a Newbie but, Are the fasteners on this bike Whitworth or what? Thanks.

dfndr said:

Are the fasteners on this bike Whitworth or what?

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Many people outside the UK tend to think of all "British Standard" fasteners as being "Whitworth" (BSW) when, in fact, many of them are not Whitworth at all.

http://www.britishfasteners.com/threads/

There are in fact, very few Whitworth (BSW) fasteners on a MkIII Commando, as the factory had mostly changed BSW to Unified coarse (UNC) threads for 1972.
UNF was already in common use, but certain Cycle (BSC) threads were also changed over to UNF at that time, but they kept the Whitworth hexagon sizes so various fasteners kept the Whitworth/BSF size hexagons, such as the head bolts and smaller base nuts, even though they were no longer British Standard threads.

However...there are many different threads used on Commandos, including the MkIII, such as:


Below are all the known thread types, with some application examples for the '72-on models:

UNEF = extra fine threads
UNF = many nuts & bolts, caliper bolts, hydraulic brake fittings
UNC = many threads into alloy -but not all
BSW = some odd fasteners
BSF = some threads into alloy
BA = carb screws & electrical component screws
BSC/BSCy (CEI) = carb banjos, rear head nut and stud (nut end), rocker spindle extractor thread
ME = carb cable adjusters
BSP = fuel taps/petcocks

The spark plug threads and spin-on filter threads are metric.



So a pitch gauge and caliper are useful items for identifying the many different threads.

There is a website that identifies many Norton fastener sizes and tpi/thread types by part number: http://stainlessbits.com/link12a.html

Basically all the chassis fasteners are "Amurrican" UNC or UNF with gearbox cover bolts and most engine fasteners in CEI (26 TPI). That will cover 98% of the fasteners with 2% being other odd threads.

As Les points out some engine fasteners retained the British style hex (requiring "Whitworth" wrenches) but actually have 24 TPI threads. An example being the case to cylinder studs..

I would advise investing in a small set of "Whitworth" sockets and combination wrenches and a thread pitch gauge.

:lol: :lol: :lol: :lol:

I feel the pain of a newbie trying to wade through all this.
It just goes to show how demented Commando ownership is.

Carry on.

Thank you for the advice. Not so dumb a question after all it appears. Sounds like I'm going to be spending a lot of time with my new toy. Thanks again. Rick

So, basically, treat every fastener thread as an unknown quantity until it has been measured and identified from the thread tables. Also many Whitworth and UNC fasteners have the same tpi, but Whitworth threads are 55 deg, while UNC is 60 deg. Any fastener could have been replaced before, and it's common to find that POs have replaced Imperial fasteners with metric ones in the UK. :x

http://www.britishfasteners.com/threads/bsw.html
http://www.britishfasteners.com/threads/unc.html

Ron L mentioned the gearbox cover fasteners being "1/4 CEI" (old BSC) thread (most British Cycle threads are 60 deg, 26 tpi, regardless of diameter) however, but I know that John Hudson refers to the outer cover screws as being "1/4 BSF" (55 deg. 26 tpi) in his gearbox rebuild video?

Ron L mentioned the gearbox cover fasteners being "1/4 CEI" (old BSC) thread (most British Cycle threads are 60 deg, 26 tpi, regardless of diameter) however, but I know that John Hudson refers to the outer cover screws as being "1/4 BSF" (55 deg. 26 tpi) in his gearbox rebuild video?

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I'm sure John was correct. I measured the TPI, but not the thread angle.