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Geoffrey Healey said:

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jlaird

jlaird

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"After the second World War, we changed over to a unified system of screw threads, fixings, nuts and bolts that was compatible with the Society of Automotive Engineers' standard of use in the USA. This change, a wise one , cost a lot of money but it did birng a degree of standardization to the ndustry and resulting benfits to the customer. All Big Healeys and Spridgets were built using unified SAE standards, while the Jensen Healey was metric.

Somehow I missed that in the past. No wonder I had trouble figureing out fastenings.
 
Yes.
But some older (pre-war) parts that continued to be made after WW II(especially some SU carb parts) continued to use the Whitworth threads. And the wrench sizes were continued on some older-designed parts for a while too. Whitworth threads use a 55 degree thread angle so they are almost impossible to tell from the normal 60 degree SAE (and ISO) threads.
 
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