Fork Damper Rod Threads

[kommando]

AN can get carried away with using the wrong drawings, their rims were wrong but useable for a period as they were using the latest drawing revision. That latest drawing never made it into production at Norton. Engineering uses not just a part number on a drawing but also a revision number, each revision number is a variation on the first drawing and I have seen drawings with 17 revisions, but only 2 or 3 of the revisions were ever in production and the drawing does not have this info. The other variations were cast to one side for one reason or another. In production we asked repeatably for all changes to be reflected in new part numbers to avoid the confusion of drawings with different revisions floating around the shop floor. This was knocked back as the extra part numbers would be using too much of the allowable range on the mainframe unless a new part number scheme was introduced which would also cost money.

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[mdt-son]

Les and all,

I stand corrected. Norton/AMC did use Whitform (special size Whitworth thread pitch) in a few instances, as shown. Understanding why is not difficult - in most cases.

NM 19265 & NM 19267 Rear axle and nut - axle is 17 mm, thread is 5/8" x 20 TPI Whitform. Nearest BSF thread is 5/8"x14 TPI, nearest CEI thread is 5/8" x 26 TPI.
026536 / 000230 Engine PTO shaft nut - 5/8" x 20 Whitform.

Axle and shaft are applications which may get abused, and having too fine a tread would make the threads vulnerable to damage and wear. On the other hand, a 14 TPI may not provide sufficient safety against loosening.
For equal preload stress (e.g., 70% of bolt yield stress). the preload increases with pitch. So, for 5/8" the Whitform is a compromise between BSF and CEI in certain areas.

NM 14367, NM 14367 Gearbox top bolt and nut - thread is 9/16" x 20 TPI Whitform. Nearest BSF thread is 9/16" x 16 TPI, nearest CEI thread is 9/16" x 26 TPI.
Application of the Whitform thread to the gearbox top bolt and nut doesn't make sense to me. The specification appears to be a heritage from Bracebridge street - how was their thinking?
It didn't make sense to the Plumstead engineers either (clamping of the gearbox will be improved using 26 TPI), so they specified CEI threads for the G15 series.

-Knut

 

[L.A.B.]

NM 19265 & NM 19267 Rear axle and nut - axle is 17 mm, thread is 5/8" x 20 TPI Whitform. Nearest BSF thread is 5/8"x14 TPI, nearest CEI thread is 5/8" x 26 TPI.

I believe the 19265 axle is 9/16" (same as the pre-Mk3 Commando Unified axle) not 17mm, as the bearings run on the shouldered ends of the bearing spacer, not the axle.

The 19265 axle and 19267 nut thread is 9/16" x 20, not 5/8" x 20.
https://www.vintage-motorcycle.com/en/parts/nuts/norton-wheel-spindle-nut-9-16-x-20tpi

Ashley is going to look into the apparent 'Whit' damper rod thread and nut discrepancy but it's a UK national holiday tomorrow (Monday) so that won't be until Tuesday at the earliest.

Edit:

Application of the Whitform thread to the gearbox top bolt and nut doesn't make sense to me. The specification appears to be a heritage from Bracebridge street - how was their thinking?
It didn't make sense to the Plumstead engineers either (clamping of the gearbox will be improved using 26 TPI), so they specified CEI threads for the G15 series.

...NV then changed it from 20 Whitform to 18 TPI UNF from '72.

 

[mdt-son]

I believe the 19265 axle is 9/16" (same as the pre-Mk3 Commando Unified axle) not 17mm, as the bearings run on the shouldered ends of the bearing spacer, not the axle.

The 19265 axle and 19267 nut thread is 9/16" x 20, not 5/8" x 20.
https://www.vintage-motorcycle.com/en/parts/nuts/norton-wheel-spindle-nut-9-16-x-20tpi

Ashley is going to look into the apparent 'Whit' damper rod thread and nut discrepancy but it's a UK national holiday tomorrow (Monday) so that won't be until Tuesday at the earliest.

Edit:


...NV then changed it from 20 Whitform to 18 TPI UNF from '72.

You are absolutely right. I mixed up with the front axle, sorry. I should have known, as I have one of Don Pender's axles just in front of me! The rear axle diameter is approx.
14.3 mm and the thread is indeed 9/16" x 20 TPI Whitform.

The reason NV relaxed on clamping force for the gearbox top bolt was probably the use of an adjuster bolt locked in place by a crosshead bolt. This ensured fixation of the top gearbox mounting forward and rearward.
For the G15, Matchless engineers provided fixation to the rear only.

- Knut

 

[L.A.B.]

To update.
I received a (rather brief) reply from Ashley today in which he said: " These are Whit form, the old drawing is stamped British Standard Brass, which are all 26tpi but Whit angle. Typical Norton."

I thought he might have posted a more detailed explanation here as it wasn't entirely clear to me exactly what "these" referred to (both rod threads, and both nuts or upper only?) so I replied requesting more detailed information, however, it does seem the rod thread is supposed to be 3/8" x 26 TPI BSB, a Whitworth thread form and as I mentioned previously, 3/8" x 26 TPI BSB (British Standard Brass) and 3/8" x 26 Whit form ought to be the same or at least very similar.

 

[Madnorton]

Norton liked using Whit form threads, and AN will use the correct Whit form if the drawing stipulates it. Just because others wrongly assume it is 26 tpi cycle doesn't mean they are correct.
The comment on the wheel rim needs explaining, we used to sell the MK3 rear rim as a MK3 front, when I joined and queried it, only to be told that is what CWC fit, and yes they are also wrong. It was considered in the 70's and I have the CWC discussion notes with Norton from that period suggesting it, but it never made it to production for a front fitment, only the rear where the MK3 uses the 3:1 pattern rim. I have also asked them for the MK3 rear rim to be made in 3:1 in a wider width, but alas they didn't seem interested. They suggested the 18''rim in 1:1 pattern, considering all current 19'' tyre fitments suggest using rim width other than WM2, why the WM2 is still being made in this size astounds me, and yes they also fit tyres.

 

[L.A.B.]

Norton liked using Whit form threads, and AN will use the correct Whit form if the drawing stipulates it. Just because others wrongly assume it is 26 tpi cycle doesn't mean they are correct.

Including RGM apparently, who thinks the AN 067657 damper rod threads are 26 TPI BSC!

STEEL FORK DAMPER ROD 26TPI FEATHERBED

15801 067657

www.rgmnorton.co.uk

"THREAD: 3/8" X 26 TPI BSC
5/16" X 26 TPI BSC"


It would perhaps help if AN and other vendors gave the thread type of both fasteners and not describe one part as 'Whit' but not the other (the 3/8" nut as just '26 TPI').

A typical example would be the "CYLINDER BASE/ HEAD NUT 3/8" x 26 WHIT FORM"...

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...because if the nut is Whit form then the 26 TPI ends of the base studs, cylinder head studs and sleeve nuts should also be Whit form.

 

[johnod]

Just want to be sure I've got this right before i order, 3/8" x 26 TPI BSB is the thread for the damper rods?

 

[L.A.B.]

Just want to be sure I've got this right before i order, 3/8" x 26 TPI BSB is the thread for the damper rods?

Well, BSB is what it says on the factory drawing according to Ashley but at such a small pitch size I doubt it would make much difference whichever 3/8" x 26 TPI tap/die is used.