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MG Midget and Sprite Technical - UNF v Metric Threads

It is a pity Spridgets threads are UNF. I replaced all the rusting fastners under the bonnet of my modern MG TF with metric stainless ones purchased from Toolststion for pence.

Jan T
J Targosz

Plus one Whitworth, one BSF, and several BSP.
Alan Anstead

What are the 4 propshaft to diff flange bolts? I always thought unf.

When I was at the morris minor centre a few weeks ago, the bloke there informed me they are not unf, but something else. Can't remember what he said now.

Anyway, prop bolts are unf aren't they?

anamnesis

Yes.
But early Minors may have predated the change to UNF. TD/TF were still BSF in the body.
Paul Walbran

Ah right, thanks Paul.
anamnesis

Jan. If the old, carbon steel, nuts and bolts rusted, there is a good chance your "stainless steel" nuts and bolts may rust. The various rust resistant steels have been around for close to 100 years now and we have found that the resistance to rust (iron oxide) depends on a number of factors, including the specific formulation of the stainless steel. Any stainless steel that can be hardened has a significant possibility of rusting, depending on how it is used and kept up. A good example is allowing "stainless steel" to be in contact with carbon steel which has rust present. That can cause the stainless steel to begin rusting. Tests have shown that hardened stainless steel will rust when a single grain of table salt is place on it in a climate having high humidity.
Stainless steel is a good choice, in some circumstances, but you have to understand its limitations and best uses.

As to "UNF", the Unified National system came into effect in the late 1940s after the Allied experiences in the second world war. The British received large amounts if "lend/lease" war materiel which had nuts, bolts, and threaded systems that the British had no tools to work on. In certain other areas (e.g. the Rolls-Royce Merlin engine), the US received "English" products and lacked the tools to work on them. Something as simple as a lost bolt could cause a machine to be non-operable until the non-standard part could be obtained. As a result of this sigificant problem, the US, GB, and Canada established a commission to examine and determine a series of bolt sizes and thread pitches, then to examine the tooling used to make the nuts and bolts and to work on the nuts and bolts. Called the "Unified National" system, it defined a "coarse" thread and a "fine thread" for each size of bolt. (E.G. the 1/4" bolts have a coarse thread of 20 turns per inch and a fine thread of 28 turns per inch. Non-standard forms, of older design, include 22, 24, 25, and 30 threads per inch.)

The US used mostly the coarse pattern (Unified National Coarse or UNC) fasteners because, in mass production work, they assembled faster. (E.G. A 1/4'-20 bolt requires twenty complete turns to move the bolt one inch into the material, while the "fine" 1/4"-28 bolt requires twenty eight complete turns to move one inch into the material.) Thus, US made products required less assembly time, when used with a coarse pattern thread, than would be needed using a fine pattern thread.

The Brits often preferred a fine pattern thread because it required less torque to give the same bolt stretch. (Bolt/fastener stretch is what you are trying to achieve when you use a torque wrench, set to a certain setting, to tighten a fastener.) The bolt thread is like an inclined plane (a ramp), in that the steeper the ramp (lower threads per inch) the more energy it requires to go up it. A gentle ramp (more turns per inch) is easier to walk upwards on than a steeper (less turns per inch) ramp.

The metric system has a number of thread pitches available, some of which will interchange with the Unified National system thread pitches. Certain metric sizes/pitches may be interchanged with UN nuts and bolts.

There is no reason that nuts and bolts of the UN pattern cannot be made up in some form of rust resistant steel, except the single criterium of cost. Thus, if someone in the UK, or elsewhere, wanted "stainless steel" nuts and bolts, the primary user of that system is the US, and that is where one would start first to obtain them.

Les
Les Bengtson

That's interesting stuff Les.

Thanks for explaining that.
anamnesis

The other issue with stainless fixings is that they can stretch and not recover. My engineer friend tells me not to use stainless bolts for anything structural.

Regarding interchangeability, I find that 2BA (used for cockpit trim on Sprites) also works with 5mm. Useful as I have replaced all those extremely annoying loose nuts with 5mm rivnuts.
Les Rose

I would second Les's comment in that stainless fasteners should not be used on an steering, braking, suspension or other load bearing application as even the so claimed high strength versions will only ever achieve half that of the standard UNF Grade S bolt but which must be used in conjunction with a compatible grade of nut. A couple of locations use even higher grade bolts e.g. flywheel, crownwheel.
Stainless should be limited to trim only applications.
S G KEIL

This thread was discussed between 18/04/2023 and 20/04/2023

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