Thread type model 8

[Ozmadman]

26tpi on my 1966 Model 14 (using a thread gauge). Looks like 1/4" BSF to me...

Still think it is 1/4" BSF as I got a 5/16" Helicoil set to replace three threads in my cylinder head for the rocker cover bolts and they were definitely BSF. I haven't seen any whitworth bolt or stud on my lightweight anywhere and I have had it completely apart inc engine and gearbox.

[Stuoyb]

AMC policy/practise was - BSF into alloy, all other iron/steel fasteners BSC, small electrical BA.
Confusion comes in 1/4" sizes when BSF & BSC are both 26 tpi albeit different thread forms.
Definitely 1/4" BSF on the engine covers.
A favourite "bodge" on the engine covers is to simply re-tap 6mm when the 1/4" thread has eased.

[Janet]

Isn't it great when someone comes along who really knows what he's talking about.

I do seem to recall Biscuit once saying that Burman gearboxes used Whitworth and made the comment, "at least they know what threads should be used in aluminium". This inferred that others (presumably AMC) didn't.

[Stuoyb]

When you look at other AMC fasteners you can see this policy - particularly on forks.
The 5/16" studs on the spindle clamps are dual thread - BSF (20tpi) into the slider, the cap retaining nuts BSC (26tpi).
The same applies to the studs holding the brake anchor. The screws holding the mudguard to the slider are also BSF-20tpi

[Ozmadman]

When you look at other AMC fasteners you can see this policy - particularly on forks.
The 5/16" studs on the spindle clamps are dual thread - BSF (20tpi) into the slider, the cap retaining nuts BSC (26tpi).
The same applies to the studs holding the brake anchor. The screws holding the mudguard to the slider are also BSF-20tpi

Yes I noticed that on my forks when I had to replace the studs, mudguard to sliders. Must admit from back in the early 70's when working in an Engineers supply's, where we had a basement full of nuts/bots of every kind you can mention , I always had the understanding that a coarse thread such as Whitworth or UNC was the required thread when going into alloy for a secure fixing and with less chance of stripping.

[Janet]

My excuse to not knowing all this techy stuff is that I'm only a girlie.

Although it turns out in this instance that I, too, was right.

[Ozmadman]

My excuse to not knowing all this techy stuff is that I'm only a girlie.

Although it turns out in this instance that I, too, was right.

A quick explanation then, Aluminium is a soft metal compared to steel/iron etc so when it is threaded to take a bolt the thread needs to be strong enough so that it won't strip when the bolt is tightened. If a "fine" (more threads an inch )thread is used there is less metal around each thread for the bolt to grip on and being a soft metal any hamfistedness could strip the thread. A "coarse" thread has less threads an inch meaning that there is more metal around each thread therefore this type will withstand a bit more tightening before it strips. This is why thread inserts fitted to replace stripped threads are often stronger than the original threads as they are made of stainless and if fitted correctly will be better than new

[Andy51]

Hate to be picky, but 5/16 BSF are 22 tpi. Andy

[Janet]

Paul, thank you for a clear explanation.

[kayaker]

sorry not the Turbo model Stannah alas
I'm all for standard threads but why put a design faux pas back in, when it clearly is incorrect in the first place a fine thread in aluminium !
the helicoil insert surely would be a fine thread into the alloy also?
I can see the wisdom of whitworth but since all can be replaced and reinforced with a coarse thread I'm of a mind to go UNC.