Date: Wed, 11 Sep 1996 12:55:41 -0400
From: Robert K. Bell

At 06:12 AM 9/10/96 -0500, you wrote:
>I was almost finished putting my gearbox back together Sunday night when I
>found out the hard way not to borrow a cheap torque wrench. I snapped off an
>inner cover stud. It would have been easy to make one except for the fact that
>I don't have the right threading dies. I also discovered that my shift fork
>spindle is bent, it would be easy to make but I don't have the right die for
>it either. If anyone can tell me what all of the different British Standard
>sizes were used on Norton's & Triumph's I would appreciate it, I'm going to
>purchase a set of taps & dies so I don't have this problem again.
>Thanks,
>Guy Chiasson

British Threads:

British Standard Fine: 55 Degree Thread Angle: 3/16-32TPI

7/32    -   28TPI
1/4     -   26TPI
9/32    -   26TPI
5/16    -   22TPI
3/8     -   20TPI
7/16    -   18TPI
1/2     -   16TPI
9/16    -   16TPI
5/8     -   14TPI
11/16   -   14TPI
3/4     -   12TPI
13/16   -   11TPI
7/8     -   11TPI
1       -   10TPI

3/16    -   24TPI
1/4     -   20TPI
5/16    -   18TPI
3/8     -   16TPI
7/16    -   14TPI
1/2     -   12TPI
9/16    -   12TPI
5/8     -   11TPI
11/16   -   11TPI
3/4     -   10TPI
7/8     -   O9TPI
1       -   08TPI

British Association: 47.5 Degree Thread Angle: 0BA 6.0mm-25.4TPI (Approx.)

1BA 5.3mm  -  28.2TPI
2BA 4.7mm  -  31.3TPI
3BA 3.7mm  -  34.8TPI
4BA 3.6mm  -  38.5TPI

Cycle Engineers Institute (CEI) or British Standard Cycle (BSC): 60 Degree
Thread Angle: All are 26TPI 5/16 to 1 Inch

British Standard Pipe: 55 Degree Thread Angle: 1/8-28TPI

1/4  -  19TPI
3/8  -  19TPI
1/2  -  14TPI

Large Diameter Threads: 55 Degree 20TPI

Note: Many large diameter threads on British motorcycles are 20TPI. Some
late Triumph fork top nuts, Vincent exhaust ports etc.

Metric: This thread found on most British Cycles: 14mm x 1.25

Late British motorcycles had most of the bicycle parts attached with SAE NC and NF fasteners while engine internals remained a mix of BA, BSF, CEI and a few BSW threads.

There are a few other screw thread patterns used on British motorcycles such as spoke threads (mostly 32 & 40TPI) and the odd 15mm x 18TPI among others.

If you encounter one that you can't identify give me a call. I might be able to help.

Ken Bell
Bell Engineering
301-622-5015

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32.2 Torque and bolts stretching

Date: Mon, 08 Apr 96 07:47:00 -0600
From: Your Name Here

I read the latest stuff about rod bolt replacement and torquing with great interest.

A few things about materials. If you stretch a bolt past the elastic limit and well into the plastic range, you will permanently lengthen the bolt, but it will relax through the elastic range again. The reason a bolt that has been permanently stretched may fail in use is that the bolt has likely 'necked' down thus decreasing its cross sectional area. This takes a lot of doing to permanently stretch a bolt, especially if the bolt is terminated in aluminium cases.

Torque is used as a 'CRUDE' measure of bolt stretch. I say CRUDE because is a very rough approximation of the clamping force developed. Many items enter into the torque vs clamping force equation. Friction is number 1, friction of the threads (this varies greatly, did you retap your cases? did you polish your bolts? then you changed the boundary conditions!!), friction of the interface between the bolt head and the washer, between the washer and the surface (If you lube to reduce the friction then you increase the clamping force, not necessarily a good thing). The thread pitch also enters into the equation, enters as a friction component.

So why bother torqing at all? Well it works well enough for most applications.

Now why do some manufacturers demand you throw the rod bolts away? They use the material properties to ensure that the clamping force is constant from bolt to bolt. When you torque these rod bolts you are stretching them into the plastic range. This then uses the elastic properties to provide an amazingly accurate clamping force. Once these rods are stretched they do neck down and upon successive uses will neck down enough to fail.

This property is used on many cars to ensure proper clamping force for cylinder heads.

Another method is to measure bolt stretch, this is usually done with through bolts. The bolt length is measured before installation, then as the nut is tightened the stretch is measured. This is very accurate and used on many race engines. The material of the bolt, the cross-sectional area and length enter into the determination of stretch.

Of course some manufacturers demand you replace the rod bolts just to sell you new ones.

If you 'feel' a softening of the 'torque' when you torque your bolts down it is a very real possibility that you have stripped either the bolt or the nut or cases. Ralph is correct in that the torque required must increase right up to the time you reach torque, if it does not, well!!!!!!!

The reason you bolt things together is to hold them together through clamping forces. This helps seal most of the oils in, puts rod and cylinder bolts in a favourable part of the stress strain curve to tolerate cyclical forces.

Andrew Wolf
buncha toys for this boy

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