Metric thread

[Speedy Builder5Participant @speedybuilder5]

dimensions

[Speedy Builder5Participant @speedybuilder5]

I have been asked to make some (Male) screwed hub dust plugs for a vintage Bugatti.

These screw into a female thread which is 47mm inside diameter and 1.5mm pitch. When I look at various charts, the inside diameter of a 1.5mm pitch screwed nut is D – 1.786mm . This would make the nominal diameter 48.786. This seems a weird size. I had a go with 'Thread Pal' as posted last week, but that didn't help as it would not 'fit' on my notebook screen

Any ideas folks ?

BobH

[Brian WoodParticipant @brianwood45127]

Hello Bob,

You might find some help in this website which includes French Metric systems and early French motor threading

I don't yet know how to post links but this should get you there

http://www.ac2litre.com/fasteners.html

Regards Brian

[Chris TriceParticipant @christrice43267]

I can’t imagine it would take very long just to turn and thread some dummy ones of slightly different sizes out of ordinary bar stock you could offer up to test the fit.

[JasonBModerator @jasonb]

The reason it seems an odd size is because the thread forn in not a straight V form, the vallys are rounded and the crests flat, if you are cutting a full form thread then thos efigures should be correct but if you are just using a pointed tool ground to 60deg then you will need to cut deeper.

[Speedy Builder5Participant @speedybuilder5]

Thanks for the site ref, but it doesn't go near to 48mm dia threads 1.5mm pitch (or thereabouts) Would it be reasonable to think that with thread rounding, the thread OD would change from 48.786 to 48mm But is this just wishful thinking.

Problem is that the car is not next to the lathe, but some distance away and I would like to be able to produce a test piece somewhere near correct at the first bash.

BobH

[JasonBModerator @jasonb]

Use the sizes for a standard Metric Coarse thread of M10x 1.5 from any chart and add 38 (48-10) to the diameters. Its the pitch that affects the depth of thread not the nominal diameter

Can you get a spare wheel to the workshop fot easier test fitting

 

 

Edited By JasonB on 12/09/2014 16:19:06

[Robert DoddsParticipant @robertdodds43397]

Bob H,

one method that would get you closer to a first time fit would be to wrap a piece of wire of known dia into the thread groove, long enough to overlap and then use an inside vernier caliper to measure "inside the wires". You would then need to do some calcs to work out the effective diameter and from that decide on your OD

Piano wire would be best but you could work with a piece of copper or soft iron wire as long as it had no kinks in it

Bob D

[JAParticipant @ja]

Bugatti had some strange but fine engineering practices. It is possible that the thread is special to them and only them. I would not assume it is a standard thread of any period.

Lots of technical books have been written about Bugatti and there is the Bugatti club at Prescott, Gloucestershire, who have archives and a library if you want further information.

JA

[Neil WyattModerator @neilwyatt]

Turn a bit of old softwood to about 49mm diameter with a taper on the end. lubricate it with a bit of beeswax or even 3 in 1. Wind it in and out again, and there you have a 1:1 model of your thread to take all the measurements from.

Neil

[Les Jones 1Participant @lesjones1]

Hi Bob,
I have a suggestion that might work. Get two short lengths of 10mm (1.5 mm pitch) threaded rod. Get someone to hold them inside the thread that you want to measure 180 degrees apart and measure the distance between the outside of these rods with a caliper just where they come out from the thread you are trying to measure. This is just an idea. I have never tried it.

Les.

[Keith LongParticipant @keithlong89920]

Bob

You could try taking an impression of the thread using "polymorph" thermoplastic. Softens in hot water, you can knead it to shape and when it cools it retains its shape and hardens to become like a softish nylon. Plenty strong enough to take accurate measurements from. From memory I bought the tin I've got from Maplins for about £12. It won't be wasted, you can re-soften it and use it again and again until its either too disgusting to handle or too full of bits picked up.!

[chris stephensParticipant @chrisstephens63393]

When working on old cars it would be a mistake to assume a continental car is all metric or that a Brit one is all imperial. It is also foolhardy to assume that a metric diameter has a metric thread. As an example Lancia petrol fittings can be 20mm by 20TPI and not 20 x1.5mm , the voice of experience speaking. Old Morris cars use metric thread on their engines but have BSW head sizes and back to Lancia, early ones use unified threads but have Metric spanner heads. It is a bit of a minefield for the unwary.

On the part in question, if you are making a replacement for a lost item, is there one on the other side of the car you could copy. If you can borrow one, you can use the wire wrap method to match new to old. On a thread that large don't go looking to work to tenths, a half millimetre tolerance on dia,might be close enough! ~Better a bit too small rather than too large.

Good luck and don't forget to charge accordingly, never heard of a poor Bugatti owner.

chriStephens

[Howard LewisParticipant @howardlewis46836]

Yes, do be wary of making assumptions about fasteners. (It is easy to mistake 0BA for M6; same OD but the thread angles are different).

And as already said, manufacturers do mix threads. Remember some Morris cars used Hotchkiss engines, many years ago.

A little more recently, the Bristol RE bus used Gardner engines, with BSW and BSF threads. On manual transmission models, the clutch was bolted on to the flywheel, using 5/8 A.F head bolts with 5/16 BSF threads! AND the Bell Housing was retained by three 3/8 BSF studs and nine 3/8 UNF studs.

The Leyland Leopard, of the same vintage was to Unified standards, but used the Leyland 0600 and 0680 engines with BSF and BSW threads. The later ones could use the metric standard 500 Series engines.

So with fittings being read across from earlier versions, you can imagine the care that needed to be taken when in for repair.

The BMC A Series engine was to Unified standard, BUT the valve clearance adjusters were 1/8 BSP because of the fine thread.

And occasionally, you come across a fine thread variant of a thread form, such as UNEF (used on the Transfer Pump Rotor of the CAV DPA Injection Pump)

Someone once said that ASSUME makes an ASS out of U and ME.

Just look at the posts about the threads used on Myford 7 Series lathes, to realise the need to consider all possibilities.

Wherever possible try to check with thread gauges and a Micrometer or Calliper (Vernier or Digital), or a known thread, and even then be careful.

The Mandrel on my lathe puzzled me for a time, 8tpi Whit form, but no obvious Imperial or Metric size.  Eventually, seeing an ad for backplates with 2 1/4" Whit  x 8tpi thread, the penny dropped!

Howard.

Edited By Howard Lewis on 19/09/2014 14:59:46

[ANDY CAWLEYParticipant @andycawley24921]

Check that they are not left hand thread on one side and right hand thread on the other. The wheel nuts are definitely different on each side. I did some of those dust caps ages ago and I can't remember for definite.

Edited By ANDY CAWLEY on 19/09/2014 21:00:19

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