Mad screw threads in Moulton Mk3

[Brucey]

I'm in the middle of rebuilding a Moulton Mk3 fork. Unsurprisingly the screw that holds the top out stop (and retains the fork mechanism) didn't want to unscrew and had to be drilled out.

This screw is about 10" down from the top of the steerer and has (allegedly, I've never seen one intact....) has a Phillips #2 head. Usually the screw head has spent its whole miserable existence sitting in a small puddle of water and has corroded to blazes.

I expect I shall end up doing the same thing as I have done previously which is to weld repair the screw and make a more generously sized head on the screw, with a big slot for a flat blade screwdriver.

My instinct has always been that a 3/4" length (it goes into a blind fitting so the length is critical too) replacement screw might be difficult to find (if it was 3/16" whitworth or 3/16" BSF for example) but this time I bothered to measure it and it was worse than I feared. As far as I can tell the screw thread is 3/16" x 28tpi, a size that is so obscure that I don't see it for sale anywhere and it isn't even listed in most standard thread tables.

28tpi.....?? No mistake....

I measured it using verniers and still didn't believe it, hence I broke out the thread pitch gauges. Needless to say the pitch (~0.9mm) is so close to M5x0.8 that tapping the fitting out will result in a rubbish screw thread, so this isn't a favourite repair route.

I have seen a lot of things that have been built with weird thread specifications but this is possibly the worst example yet; there were at least four or five other screw threads which could have been used in ~1970 and any of those would have been easier to source screws for; what were they thinking of...?

cheers

[gazza_d]

One word

Raleigh

[philvantwo]

Two words
Shopper bike.

[iandusud]

I'm in the middle of rebuilding a Moulton Mk3 fork. Unsurprisingly the screw that holds the top out stop (and retains the fork mechanism) didn't want to unscrew and had to be drilled out.

This screw is about 10" down from the top of the steerer and has (allegedly, I've never seen one intact....) has a Phillips #2 head. Usually the screw head has spent its whole miserable existence sitting in a small puddle of water and has corroded to blazes.

I expect I shall end up doing the same thing as I have done previously which is to weld repair the screw and make a more generously sized head on the screw, with a big slot for a flat blade screwdriver.

My instinct has always been that a 3/4" length (it goes into a blind fitting so the length is critical too) replacement screw might be difficult to find (if it was 3/16" whitworth or 3/16" BSF for example) but this time I bothered to measure it and it was worse than I feared. As far as I can tell the screw thread is 3/16" x 28tpi, a size that is so obscure that I don't see it for sale anywhere and it isn't even listed in most standard thread tables.

Image01774.jpg

I measured it using verniers and still didn't believe it, hence I broke out the thread pitch gauges. Needless to say the pitch (~0.9mm) is so close to M5x0.8 that tapping the fitting out will result in a rubbish screw thread, so this isn't a favourite repair route.

I have seen a lot of things that have been built with weird thread specifications but this is possibly the worst example yet; there were at least four or five other screw threads which could have been used in ~1970 and any of those would have been easier to source screws for; what were they thinking of...?

cheers

What is OD of the screw? A 1BA screw is 28.2 tpi.

[Brucey]

the male thread actually measures 4.6-something mm, so is what I'd expect from a nominal 3/16" (~4.76mm) thread. But it is also close to 2BA diameter.

So it might be a BA-based screw thread, but with 1BA pitch and 2BA diameter.....

cheers

[Carlton green]

I'm in the middle of rebuilding a Moulton Mk3 fork. Unsurprisingly the screw that holds the top out stop (and retains the fork mechanism) didn't want to unscrew and had to be drilled out.

This screw is about 10" down from the top of the steerer and has (allegedly, I've never seen one intact....) has a Phillips #2 head. Usually the screw head has spent its whole miserable existence sitting in a small puddle of water and has corroded to blazes.

I expect I shall end up doing the same thing as I have done previously which is to weld repair the screw and make a more generously sized head on the screw, with a big slot for a flat blade screwdriver.

My instinct has always been that a 3/4" length (it goes into a blind fitting so the length is critical too) replacement screw might be difficult to find (if it was 3/16" whitworth or 3/16" BSF for example) but this time I bothered to measure it and it was worse than I feared. As far as I can tell the screw thread is 3/16" x 28tpi, a size that is so obscure that I don't see it for sale anywhere and it isn't even listed in most standard thread tables.

Image01774.jpg

I measured it using verniers and still didn't believe it, hence I broke out the thread pitch gauges. Needless to say the pitch (~0.9mm) is so close to M5x0.8 that tapping the fitting out will result in a rubbish screw thread, so this isn't a favourite repair route.

I have seen a lot of things that have been built with weird thread specifications but this is possibly the worst example yet; there were at least four or five other screw threads which could have been used in ~1970 and any of those would have been easier to source screws for; what were they thinking of...?

cheers

What is OD of the screw? A 1BA screw is 28.2 tpi.

Unless I’m mistaken Brucey gave the OD of the screw as 3/16”

It’s true that the 1 BA pitch is near the approximate 28TPI that Brucey measured, but the OD is over 1/64” bigger so maybe that’s not a goer. The OD of a 2 BA screw is very near 3/16” but at 31.4 TPI the pitch is 10% out ... makes you wonder though.

For supporting data see: https://britishfasteners.com/threads/

[fausto99]

Don’t know about mk3s but mk1s have a 2BA screw which I replace with a M5 cap head and a washer. I have a special very long Allen key, made up for me specially in the model shop when I used to work, to do it up.

[Brucey]

a couple of photos

it doesn't look like a BA thread form to me?

for scale the thread measures 4.60mm externally and is 28tpi as near as I can judge.

the revised version

FWIW I already have a long 4mm allen key, and I could have used it here (I welded the top of an M5 caphead to the old screw) but I went with 'simple' on the basis that I might not be the person who next has to undo the screw. The screw is now half stainless, half high tensile steel.

cheers

[Mick F]

That is weird!
What a strange thread!

Seems to me, that there has to be an engineering solution to this.
Bore it out, and tap a new thread that's a common one, and fit a "normal" bolt.

[Brucey]

it is already 'solved', hence second photo. NB there are at most only about 10 threads engaged; there isn't much there to start with; my replacement screw is slightly longer and may require packing washers but it will get closer to full engagement.

FWIW the screw thread form looks to have a 60 degree flank angle which means it isn't BA.

Alternatives such as boring it out fundamentally changes it (the fitting is blind and it is blind for a reason) and as I mentioned upthread, tapping it out to M5 x 0.8 will result in a substandard thread. So you can 'make it work' in various ways, but few are without compromise.

cheers

[simonineaston]

This screw is about 10" down from the top of the steerer and has (allegedly, I've never seen one intact....) has a Phillips #2 head.

Difficulty removing these screws is a common issue for restorers of early Moultons. The advice used to be to do your best to clean the screw head, then use a bright torch to determine the type of head. Slot heads were commonest in the early years, but there was a switch to Phillips later on. Trouble is, which type was used was unpredicatable - probably whatever was in the parts bins. Moultoneers used to say, "You've got one go at getting that bolt out... don't waste it!"
The Kirkby factory had the worst reputation for quality control including things like the appropriate appliction of lubricant to parts about to be assembled, which obviously affects how readily the retaining bolt comes out, but as far as I know, the mark 3s were not made in Liverpool... Glad to hear the issue is resolved.

[simonineaston]

If you're really interested in chapter & verse on this topic, Brucey, you could do worse than dropping Michael Woolf a line. He's run Moulton Preservation as a non-profit organisation since I can remember, and will undoubtedly be able to comment. PM me if interested in his address.

[fausto99]

Definitely a Raleigh thing as the mk3s were made by them. SA having the same ancestry and i've had similar problems when wanting to fit triggers, cable and brake reaction arm clips to unusual frame locations and needing to use longer screws. I've usually had to replace the nut too because I could not identify the screw type. BTW did you try a SA screw?

[Brucey]

Definitely a Raleigh thing as the mk3s were made by them. SA having the same ancestry and i've had similar problems when wanting to fit triggers, cable and brake reaction arm clips to unusual frame locations and needing to use longer screws. I've usually had to replace the nut too because I could not identify the screw type. BTW did you try a SA screw?

I too have replaced screws (and their associated nuts of course) in SA triggers etc many times but (duh...) it hadn't occurred to me this time that it might be a related problem. I guess I long ago accepted the compatibility of screws on SA fittings as a lost cause; it normally doesn't cause a big problem because they don't go into tapped fittings, so you just change the nut as well.

cheers

[Brucey]

If you're really interested in chapter & verse on this topic, Brucey, you could do worse than dropping Michael Woolf a line. He's run Moulton Preservation as a non-profit organisation since I can remember, and will undoubtedly be able to comment. PM me if interested in his address.

that's where the spare parts came from... the MkIII uses a different spline on the lower fork bushing and (fortunately) MP have both types available. In fact wear on the lower bushing is what has precipitated the whole fork rebuild. One of my chums uses this machine as a 'daily' and it was worn when they got the bike several years ago. Needless to sat it hasn't improved in the meantime!

cheers