marshg246
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- Jul 12, 2015
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We (especially those in the US) tend to call all British threads Whitworth. In truth not much BSW (British Standard Whitworth) is used in British motorcycles. BSC (British Standard Cycle)/CEI (Cycle Engineers Institute) are much more common (26 tpi). Then there's the BSF (British Standard Fine) which is more course than BSC. Before the Brits attack, I'm no British thread expert – this is just (hopefully correct) background for what's coming.
I've seen the two places where BSW is used in a Norton be confused pretty often. The eight 1/4"x 20 rocker spindle cover screws (06.3124) are BSW for all years. In theory, the three screws that secure the inner primary to the engine are BSW through 1972 (06.0375) and UNC (Unified Course) 1/4” x 20 (06.2669) from 1973 on – I have no idea when they actually changed but it certainly was not 1/1/1973. I'm guessing when they started building 850s they used UNC and switched the 750s at about the same time.
So, BSW and UNC are both 1/4" x 20 but they are not the same. BSW has an included angle of 55 degrees between threads and UNC is 60 degrees. Also, the depth and tips of the threads are slightly different.
In the head, using a UNC screw will re-cut the threads without much trouble and if the correct screws are a little loose it not a terrible idea to run a 1/4" UNC tap through them and switch to UNC. That also opens the choices of screws available significantly but can confuse the next guy pretty bad.
When I use “helicoil” below, I mean whatever HeliCoil-like thread repair.
The inner cover screws are a bigger problem. A 69S that I'm working on had (factory installed?) BSW helicoils but the last person to work on it forced in 1/4" x 20 UNC. In the process, the helicoil was damaged and when I unscrewed them to remove the inner primary the helicoils were ripped part way out. When I took the crankcases apart, I was able to (with a LOT of effort) to get the helicoils out without doing more damage. When I tried to install new BSW helicoils I could see that they would not be tight enough in the cases, so I installed 1/4" x 20 UNC helicoils which went fine. Hopefully whoever works on it after me will realize that UNC is installed.
I've seen the two places where BSW is used in a Norton be confused pretty often. The eight 1/4"x 20 rocker spindle cover screws (06.3124) are BSW for all years. In theory, the three screws that secure the inner primary to the engine are BSW through 1972 (06.0375) and UNC (Unified Course) 1/4” x 20 (06.2669) from 1973 on – I have no idea when they actually changed but it certainly was not 1/1/1973. I'm guessing when they started building 850s they used UNC and switched the 750s at about the same time.
So, BSW and UNC are both 1/4" x 20 but they are not the same. BSW has an included angle of 55 degrees between threads and UNC is 60 degrees. Also, the depth and tips of the threads are slightly different.
In the head, using a UNC screw will re-cut the threads without much trouble and if the correct screws are a little loose it not a terrible idea to run a 1/4" UNC tap through them and switch to UNC. That also opens the choices of screws available significantly but can confuse the next guy pretty bad.
When I use “helicoil” below, I mean whatever HeliCoil-like thread repair.
The inner cover screws are a bigger problem. A 69S that I'm working on had (factory installed?) BSW helicoils but the last person to work on it forced in 1/4" x 20 UNC. In the process, the helicoil was damaged and when I unscrewed them to remove the inner primary the helicoils were ripped part way out. When I took the crankcases apart, I was able to (with a LOT of effort) to get the helicoils out without doing more damage. When I tried to install new BSW helicoils I could see that they would not be tight enough in the cases, so I installed 1/4" x 20 UNC helicoils which went fine. Hopefully whoever works on it after me will realize that UNC is installed.
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