Gipsy major, again.

[Terry Lee]

Does anyone know what size the thread is on the oversize exhaust studs? ( the end that inserts into the cylinder head)

[ROB THOMASSON]

Strangley most of the threads on the Gipsy are metric as it was derived from the Renault engine. As no self respecting engineer had metric spanners in those days this gave rise to nuts with metric threads and imperial sized heads! In this case as it's a stud I'd try a metric thread gauge on one.

[Rob Swain]

The metric thread / BS hex is not as uncommon as it might sound.
Engines in Morris 8 cars of the 1930s and 40s had the same trick as the engine was originally designed and built by Hotchkiss en Cie, a French company. Morris built hundreds of thousands of the things. They even ended up in tanks as auxilliary generator units!