Featherbed engine mounting bolt thread sizes?

They are almost certainly bsf threads. If you have no thread gauge, put then alongside a steel ruler, take a picture of both together, and blow them up to count them accurately.

... Cycle (BSCy) rather than BSF which is what my plunger ES2 has but I'm ready to be corrected.

I find a thread gauge and pair of cheap digital calipers essential for checking things like this.

Unless a Sideline is some sort of hybrid cross between a Wideline and a Slimline the engine fastening on a 1959 M50 / ES2 (Wideline) should be Cycle thread (CEI / BSCy).

Diameters vary (5/16, 3/8 & 7/16) but most are 26tpi apart from the 7/16 stud threads which are 20tpi but still Cycle thread.  As David & Ian suggest, you really do need a thread gauge, they are cheap enough,  particularly as the stainless fittings that you mention, unless from a specialist such as Stainless Classics, could be anything.

Hope this helps even if not the instant internet solution that many new Norton owners might seek.

The following is an extract from Ch 38 of the revised 2020 DSNs.  With thanks to Dale M. 

ENGINE PLATE AND MAIN FRAME FASTENERS.

Front plate stud, wideline (3)                                              3/8”       26           4 ½”

Front plate stud, slimline, top.                                            3/8”       26           4 ½”

Front plate stud, slimline, bottom                                       3/8”       26          5”

Front plate to frame bolt, slimline (2)                                 3/8”       26          1”

Rear plate to engine studs, top (2)                                      3/8”       26         4 ½”

Rear plate to engine stud, bottom                                      5/16”      26         4 ½”

Rear  plate to frame bolts (4)                                              3/8”       26          1”   reduced hex heads

Swinging arm stud, featherbed                                            ½”        20        10”

Swinging arm stud, pre-featherbed, twin flat head              ½”        20        7 5/8”

Top spacer tube stud, to 1965                                             7/16”     20         10”              

Rear cross tube stud, slimline                                              3/8”      26        10 ¾” 

Footrest stud                                                                       7/16”     20          10 ¾”

L.H. footrest nut,                                                                3/8” whit hex, domed

R.H. footrest nut,                                                                5/16” whit hex

Pillion footrest stud                                                              3/8”     26           1”

Gearbox top bolt, twin flat head                                          9/16”   20         4 7/8” reduced hex nut

Gearbox bottom stud                                                              ½”     20           5”       

Bottom stud nuts                                                      can be 7/16” Whit or ¾” AF hex.

Gearbox adjuster bolt, 9/16” eye                                         5/16”   26           2 ½” reduced hex nuts

Shock absorber top bolt, wideline                                        5/16”   26          3 1/8”    with lead

Shock absorber top bolt, slimline                                         3/8”     26          1 ¾”

Shock absorber bottom bolt, all                                           5/16”    26          1 5/8”

Mudguard mounting bolt, slimline                                      3/8”      26           3 ½”

Centre stand pivot bolt                                                         9/16”    26          1 ¼”      bored 5/16”

Brake pedal spindle                                                              ½”        20            1”

Excellent table! Does anyone know if the norms of old British screws, indicated that they should have such a pronounced rounding in their threaded ends, (that at present they are no longer observed). I hope the translator helps me and they can interpret it. Greetings.

I think you will generally find the thread tips on commercial cut screw threads are clipped, not rounded, and the grooves a little deeper...according to one of my books. The result is that thread engagement is rather less that the theoretical ideal, but an extra 5% length makes up for 5% loss of theoretical thread strength. Don't take this as Gospel truth if you have a critical need for some reason! A proper thread chaser should meet the Standard. I might try and find the reference I was looking at a few days ago.

... he has Stainless replacement bolts so I would say, measure what you got, some  unscrupulous merchant may have slipped him metric...   As a starting point, you could try your original nuts...

Alternatively, take them to an engineering shop, they will tell you (without measuring if they are  old enough!).

David: I always though UNF were undefined root and crest radius, whilst BSF were specified in the thread form?

Cheers

Jon

CEI and BSF threads are both 55 degree rounded whitworth form. Oddly no BSW standard thread has 26TPI, but 1/4" BSF does. So 1/4" BSF and 1/4" CEI should be the same.

CEI can have several different pitches, but all the smaller ones are usually 26TPI. So it is hardly a standard at all, given that it takes the form from Whitworth, and has different possible pitches for the same bolt size.

I had to tap a Commando mainshaft nut to 5/8" 20TPI "CEI" from 5/8" 20TPI UNF (from memory) for my Dommie, in order to use the Electrex conversion with a belt drive. Mostly to round off the trapezoidal US threads. Close, but no cigar.

I think the tap says "WW Thread form", so what being "CEI" adds to that I don't know.

... doesn't take its from from Whitworth as it is 60 degrees as opposed to 55 degrees. It is a standard regardless of the fact that different options of pitch are allowed for some sizes. If it was not then you could use any old pitch and call it CEI. Information form Machinery's Screw Thread book 18th edition.

TBH I'm surprised that a commercially available UNF 20tpi wouldn't fit on a 20tpi Cycle without using a tap as they're both 60 degrees. A UNF nut does have the crests flattened rather than rounded but this would make no appreciable difference in fit.

Thanks for the correction on the angle Ian.

The flattening might make the minor radius shallower.