I decided to remove the rear suspension and have been instantly confronted with a problem. The bolts that are safety wired and hold the safety strap to the front of the radius arm need to be replaced. Barrett says they are 7/16 - 20 UNF. Does anyone know if that is correct? They were partially stripped upon removal so I want to re-tap the captured nuts in the mount. I did check and they are 7/16 - 20 but my thread gauge is SAE so don't know the thread angle. Before purchasing a tap to clean up the threads I want top make sure I get the correct one.

 

SAE stands for "Society of American Engineers".
So find 7/16 fine thread on your gauge - U.N.F (union national fine) or 20 TPI (threads per inch) and that will be correct.

SAE _ UNC/UNF don't use thread angles like the metric system, it's just threads per inch.

There is an angle to threads, but I don't know what the standard is, but it's not measured that way anyway.
Get a tap tap that says 7/16 20 UNF.
If there is room, get a "taper" type tap, it will be easier to start as apposed to a "plug" or "bottoming" type tap.
If there's not enough room, then go with the plug type.

 

SAE and UNF are exactly the same. I think that when the US tried to rationalise their thread standards, the old SAE was adopted as UNF.

 

Craig (Jagboi) says the bolts are UNC ~ He has sent me a wonderful book on the subject so I can learn coming from a metric country. ~ This might only apply to the engine. I would check with him. He knows the S Type backwards.

 

Refresh Page for S Type comment

 

The Radius arms, top of bridge-piece and brake calipers are wired.

 

Bridge Piece = Cradle in some speak.

 

7/16-20 is UNF - fine thread. If it was UNC it would be 14 threads per inch.

As a general rule on these cars, any thread into steel will be fine thread, anything into aluminum is coarse thread. You see coarse in studs into the cylinder head. The SU carbs and fuel pump along with Dunlop brakes are Whitworth/ British Association thread forms, everything else is SAE fasteners.

 

It is my understanding that Jaguar went to SAE threads (UNC and UNF) with the introduction of the MK7 in 1951.
This was to satisfy the American market which was at the time their main market. Lyons had done his homework on gripes over servicing previous models.
The previous MK4/5 series were full of the old BSF, BSW, BA which were a pain to work on with different spanners etc.(I did have a MK5)
Not every part made the transition as suppliers like Moss, SU, Lucas etc. still clung to their old English thread systems.
However, any parts manufactured by Jaguar in the 1950s and 1960s should use SAE.
I can't speak for the later car series as I have found metric creeping in.

 

Not exactly, but close enough. The (slight) differences between SAE and Unified National thread forms are the radius of the fillet at the bottom of the root of the thread and the number and tolerance of classes of fits. For all practical purposes they are the same and completely interchangeable.

For even more pedantry, the thread forms are SAE and Unified National, and within those exist fine and coarse thread standards. Thus UNF means a Unified National thread standard, Fine series.

The Unified National came from an agreement in 1949 between the USA, Canada and the UK to adopt a standard common to the three countries.

 

JB has it spot on. Incidentally, UNC is close to BSW (Whitworth). But the thread form and diameters changed a bit. It's a strange thing that when people write standards, they always have to change things a little.

 

UN series threads have a 60 degree angle, BSW is 55 degrees. As a historical note, when Packard built the Rolls Royce Merlin engine under license in the USA they used the BSW fasteners of the RR original drawings to maintain complete interchangeability between Packard built parts and Rolls Royce built parts.

Once major difference is that UN/SAE fasteners always have an exact fractional head size, or AF (across flats) designation. For example a 5/16" shank diameter bolt will always have 1/2" across flats head, BSW does not do this. The wrench for UN series fasteners, which is most common wrenches and sockets, always uses the across flats designation. Thus a "half inch" UN wrench (spanner) is 0.5" between the jaw of the wrench. BSW uses the fastener shank diameter, thus a BSW "half inch" wrench actually has a jaw opening of 0.920".

 

Thanks JB. I knew you would clarify the matter. I've got lots of interesting reading to do (27th Edition Machinery’s Handbook).

 

https://www.reddit.com/r/Engineering...ead_standards/

The photo at the top of the link is a nice illustration of some thread types.

One of my old colleagues worked on threaded connections. I was initially surprised when he mentioned that the first two thread rings carried about all the load - the rest are passengers, but it's true. He also explained that almost everyone uses lock nuts incorrectly. I'll stop before my internet disappears again ...

 

Pete no!! You can't leave us in suspense like this. Please tell us about the lock nuts Pete or I will not be able to sleep tonight.

 

Most lock nuts don't actually do much in the presence of vibration. They add a bit of friction, but they are practically useless. An appropriate grade of thread locker ( Loctite is the best known brand) works much better. It has an added benefit, in that it fills the inter-thread spaces, so stops water entering and stops corrosion. A lock nut won't do that. I use loctite on every fastener on the car.

Peter: Should we get into the differences between a bolt and a cap screw?

 

It was a long time ago and (engineering snobbery) I really couldn't work up that much enthusiasm for threaded connectors even if the main objective was for connections that went down oil and gas wells rather than nuts and bolts. There was a load of stuff about what sort of locknut was best for an application. But his big issue was telling the fitters on site that they shouldn't use jam nuts and if they do they (if I remember this right?) the thin nut should go on first instead of last (which most of them preferred).

JB, Doesn't the difference between a bolt and a screw depend on what country/continent you're living in?

 

No, they are the same thing! It's called a bolt is when the threaded fastener is used with a nut to make the joint and the nut is turned to tighten. A cap screw goes into a tapped hole and the head of the fastener is turned to tighten. However, the fastener itself can be identical, but called different names depending on how it is used (nut or no nut).

 

We have an argument down in the west country about whether you put Jam on a scone first then cream on top. (Devonshire) or cream on first like butter then jam on top of the cream. (Cornish). I am a cream first man even though I live in Devon and I get some funny looks in the tea houses.

 

No No, jam first, it's stickier then cream so it can be spread into the scone (also fills the holes in a crumpet as well)
Then cream...
If you try and put jam on the cream, then you get cream all over the knife and if you need more jam, then the jam jar gets full of cream.
Also, if you try and spread the jam around on the cream, the cream will just get all over the place.
If the jam didn't set, then I suppose...

How could it possibly work well with cream first and then jam ?