Hi can any one can tell me the correct spark plug for my motorcycle, I have order one and it came as short reach, but this doesn't look correct to me. Thank you for Any help.
Short reach...
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Schools of thought
There has been quite a lot of discussion about this aspect on various forums. It seems that through the post-vintage era, with their iron-head OHV and SV engines, Norton fitted heads with a 3/4" threaded depth plug hole. Despite this, it is quite clear from literature published by Norton, as well as KLG and Lodge that the recommended plugs were always 1/2" reach.
This seems to have been a continuation of the vintage-era practice with integral head side-valve engines, where the valve cap fitted with the plug was taller than the plug reach.
The reasons for this don't seem to be recorded, but perhaps it was to give the plug nose an easier life, or possibly to reduce the risk of a protruding nose causing pre-ignition with low-octane fuel.
Many owners report using 3/4" reach plugs in the 1938 type engines (with 14mm plug as yours is).
This is from the KLG guide for 1958...they show the 1/2" reach F70 for all iron head engines and the 3/4" reach FE70 with the alloy head. I think that you're going to have to make your own mind up...If you go for long reach then certainly not the extended nose type.
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... is the recommended for some reason but I found a long reach (can't remember the model) works much better in my 1952 ES2 - esesntially the same engine.