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What difference does thread angle make?

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What difference does thread angle make?

Posted by patrick_mullen at August 23. 2017

I wanted to replace the fixing screws with the chewed up heads on the points base plate of my 1960 Jubilee. No real problems in doing this. But what is the actual thread ?

The old screws are 3/16" 32 TPI .  I at first assumed they were BSF or Cycle thread but on checking my stock of screws I find that UNF and Metric (M5) are also 32 TPI.  So if 3/16" BSF, Cycle, UNF, and M5 are all the same TPI and screw in easily does it make any difference which is used ?. I have used a couple of new UNF screws which fit and tighten up nicely. It is only the points base plate.

Patrick

Re: What difference does thread angle make?

Posted by david_evans at August 23. 2017

The thread for most British bike electrics will be a BA (British Association) thread. BA threads have a unique thread angle of 47.5 degrees, Metric, UNF, UNC and Cycle use 60 degrees, Whitworth and British Standard Fine use 55 degrees. Just because your threads have a similar angle does not mean the pitch for a given diameter is the same. Imperial threads work on a threads per inch system. ( many Cycle threads are 26 TPI irrespective of the diameter) Metric threads work on a pitch e.g. 8m x 1.25, where 1.25mm is the distance between thread crests.

You mention 3/16" threads. the nominal outside diameter of a 3/16" thread is 0.186". the nominal diameter of a 5mm thread is 0.196" thats 0.010" bigger. If you are happy bodging your bike with incorrect screws that will maybe stretch the threads in the alloy casings. Fill your boots.   Standards sir.

Re: What difference does thread angle make?

Posted by ian_goodhall at August 23. 2017

It's probably a 2 BA originally. 2 BA is 0.185" diameter and 31.5 TPI. They are rather odd sizes compared to most British fasteners but a lot of BA is very close to metric threads so for practical purposes your M5 will probably be fine.

For future reference BA is used a lot in anything electrics related.

Ian

Re: What difference does thread angle make?

Posted by patrick_mullen at August 23. 2017

Thank you folks.  This is on a spare engine and the previous owner(s) obviously only had a limited toolkit - like a club hammer and a pipe wrench.

2BA seems the right size for fixing the points to the base plate so I shall keep that in mind as being the norm for electrical items- they are tiny little screws. I have used 3/16" UNF screws to secure the base plate into the alloy engine housing. Presumably 3/16" UNF is effectively identical to 3/16" Cycle as they screw in with fingers and feel right.

I admit that standards here are pathetic; 2017 Irish air in the Spanish tyres as well as metric washers(possibly from China)!.  But the engine runs.

Patrick

 

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