i know these should be 1/2", 20tpi full nuts, but are they small hex ?

what size should they be AF ?

email (option): chris.astinbarker@btinternet.com

Hi Chris, mine are 20 mm hex, Good luck, Michiel

email (option): m.wijbenga@hotmail.com

Chris they won't be AF! The thread is 20tpi BSC. which I think is a Whitworth form. Although it's very close to 20 tpi UNF and I have used UNF taps and dies to clean a thread. But the spanner size will be Whitworth/BSF.......I'm no engineer! Ron

email (option): ronpier@talk21.com

Ron I dont belive you,take a close look folks at rons work ! andrew.h.

email (option): warbikes@gmail.com

hi,they wont be 20 mm hex ,all sizes on british made machines are imperial.
the exceptions are,and i do not know why,the spark plug thread (14)mm,one or two of the bearings,i.e the gear box main bearing (sprocket),and of course the bore x
stroke.
during the war,to save metal the old whitworth spanner sizes were reduced to the one below,i.e. 1/2'' became 7/16''bs (british standard).
without digging into reference books to state what the difference was ,i can asure you that there is a considerable difference.
there is a common myth floating around the world of spanners,people readily
assume that af stands for american fine,in reality it stands for across flats
the same as all the metric spanners,sockets and etc.
the guy who invented the system of interchangeable parts (threads),also laid down
the dimensions of the relative hexagon as compared with the material propreties of the day.
about a hundred years later,when needs must,the system was changed,so now on a whitworth spanner it will state,1/2'' whitworth,7/16 b.s
hope this helps
cheers rick

email (option): richardholt@rocketmail.com

Ron, I think Chris meant Across Flats ! Easier way to see what spanner you need...

email (option): viaconsu [at] planet [dot] nl

Yes Of course Hans. I always associate AF Spanners with unified nuts and bolts.
I did say " I'm no engineer"

Ron

email (option): ronpier@talk21.com

There are a few more 20 mm hex bolt and nuts on my M20, e.g. 2 frame bolts (saddle and front down tube) and also the rearstand pivot bolts. And I strongly believe they are all original. ('39-'40).

Also found 20 mm hex on earlier BSA's so think this must be an exception to the standards...

Regards, Michiel

email (option): m.wijbenga@hotmail.com

hi hans, ron et al;

it was the accross flat size i was after, as there are several companies producing stainless ( god bless it ! ) in the correct thread form, but with different hex sizes.
if anyone has an original fotrest nut, please measure it and let me know !

email (option): chris.astinbarker@btinternet.com

The valve lifter fitting in the barrel and the crankshaft pinion nut are also this size...Ian

email (option): ian@wright52.plus.com

Hi all,

The "normal" across flats size for 1/2 BSC nuts and bolts is 20,83mm. Due to wear and tear this dimension could be a few tenths of a millimeter smaller by now... Maybe that's why Michiel measured 20mm...?

The imperial system sometimes also uses nuts that are one size smaller than the "normal" size. In case of 1/2 BSC this should then be 18,03mm, which is the across flats size for 7/16 BSC.

Hope this helps...

Jan

email (option): wd.register@gmail.com

I have repro nuts that are made of 20 mm hex and after a while they look very (rusty) original.

Henk.

email (option): ahum@quicknet.nl

thanks everyone...

henk, how about adding a new page to the technical info, then as these questions get answered we can add the answer...

email (option): chris.astinbarker@btinternet.com

Hi Jan..My first thought was an Imperial size...but .032" (.83mm) wear is a lot of wear and this size (20mm?) occurs in a few places. One of them is the valve lifter fitting that is screwed into the barrel..That piece is hardened, so a high degree of wear doesn't occur there. Likewise with the crankshaft pinion nut, that was also hardened....Ian

email (option): ian@wright52.plus.com

Hi Jan, thanks for your suggestion but it is not wear for sure.

Also the nuts on the brakeplate for holding the fixed pins, the brass plugs with the filters in the fueltank and the nut on the doomed clutch plate are 20 mm. I actually use my 20 mm spanners quit a lot, Michiel

hi,here is the standard chart for BSC / CEI,i dont know why BSA bastardised the
hexagon in these cases.
http://www.motalia.com/Html/Charts/cycle_chart.html
cheers rick

email (option): richardholt@rocketmail.com

Ian Wright

Hi Jan..My first thought was an Imperial size...but .032" (.83mm) wear is a lot of wear and this size (20mm?) occurs in a few places. One of them is the valve lifter fitting that is screwed into the barrel..That piece is hardened, so a high degree of wear doesn't occur there. Likewise with the crankshaft pinion nut, that was also hardened....Ian

email (option): wd.register@gmail.com

Hi Jan..It's a puzzle. I'm still trying to work out why
BSA used two Metric main bearings and two Imperial ones in the M20(with matching 'odd' shafts).Also why the main gearbox bearing is Metric but the one on the other end of the mainshaft is Imperial, again requiring the manufacture of a Metric sleeve gear...
All at a time when we had a fully Imperial system for most applications...Ian

email (option): ian@wright52.plus.com