Featherbed engine mounting bolt thread sizes?

... 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.

As this post contains a very handy table, I've decided to ask my question here to complement the original question.

I can imagine these bolts should be shanked and only have thread at the very ends that bear no load. When sourcing BSC bolts, the diameter and length are specified (as in the table above), but how do I know how long the shank is?

Would these bolts normally be fully threaded or is there a standard that dictates a certain length of thread for a certain lenth of bolt?

Here are some details I have on record, hope this is right, if not someone will correct it.

BS 1083
The thread length of BSW or BSF bolts manufactured to BS1083,
with diameter d and thread length t,

t = 2d
I believe that BSC/CEI are the same

BS 1768
The thread length of UNC or UNF bolts manufactured to BS1768,
with diameter d and thread length t,

t = 2d + 1/4" up to a length of 6", where the formula becomes t = 2d + 1/2"

DIN 931 & DIN 933
The thread length of Metric bolts manufactured to DIN 931 and 933,
with diameter d and thread length t,  

t = 2d + 6mm up to 125mm;
t = 2d + 12mm from 125mm until 200mm; 
t = 2d + 25mm at 200mm and over. 

Hope this helps,
Stan

The general thinking was the finer the thread, the more resistant to loosening under vibration. Cycle thread standards were regularly adopted. 

A bolt with a shank is always better than a bolt with full length thread, better known as a machine screw. A bolt or stud that goes through, say, two engine plates and an engine mount between, would have a plain section equal in length to engine mount plus two plates. The fully assembled bolt or stud would have a washer and nut and have two full threads showing outside the nut. 

Norton and AMC in this era made many of their fasteners in house. Particularly studs. Turning and plating departments had to be fed with work. 

To answer Gérardo above, the designers often specified a generous rounded end to the studs. This allowed a certain amount of hammering on the stud ends, to help when fitting in "normal" tolerance situations, without damaging the threads. Copper faced hammers would have been used - occasionally. 

The rule of using the longest shank possible, which makes a much better control of assembly position, carries through to all bolts and studs on AMC Norton of the era. 

To most people the above is just ridiculous, but there will be a few like me who worked in the engineering industry at the time. Always the thought that in-house had to be cheaper, and quicker, than buying-in. The workers (your mates) and tools were downstairs, impatient for piece work and overtime, so use them and keep the unions happy too. 

Most of the workforce in the era had experience of wartime manufacture, and those vastly different times gave a different mindset to today's global manufacturing practices. 

Peter
 

Well, that blew life in an old thread! Sorry, I couldn't not make that one...

Very handy information Stan, and very interesting Peter. Thank you both for adding that. It makes this post increasingly informative.

I think it's very valuable to not only know the figures, but also know the reasoning behind them. It helps with understanding, instead of just repeating.

Tim

Both 26 tpi but 55 and 60 degrees. Not always compatible. Had problems with that on a BSA brake rod. BSA had a habit of mixing BSW, BSF, cycle threads and on late models also UNC and UNF. Also some BA. With few exceptions Norton had cycle threads. Some BSW in aluminium. Until the Commando which gradually got more and more UN. .