Does anyone know the type of thread used on 1973 E-type. Metric or Standard American do not thread easily.

 

They will be virtually all UNF and UNC, so essentially like US. Electric components may use BA (British Association). However modern Jaguars are metric. I don't know when the changeover took place, but I do know that Jaguar put out a warning about brake parts some time in the late 70s as all the brake stuff was bought in, and the manufacturer had converted to metric.

If you want to be certain, you'll need a micrometer or caliper gauge, plus a thread gauge.

 

I bought this "thread check" tool a few years ago. It has been more than useful for my restorations. My old eyes cannot always see if the threads on a small bolt line up with the threads on a traditional thread gauge. I hope this link works.
https://www.ebay.com/itm/38508446098...Bk9SR7DE2bOQYQ