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[Ben]

Having had a look I think they are all parallel threads. The conduit uses a pipe thread too.

We had the bloke at work last Friday who services all our compressed air and vacuum lines. He looked at the conduit thread and had nothing to fit in his van. We suspect its a size that's fallen out of favour today. The pipe thread sizes can get confusing as Micheal has said, the size listed isn't the thread OD as on a bolt.

Thread cutting on a lathe can produce any thread you desire. As long as you have the pitch and TPI info you can turn an internal or external thread to your chosen diameter. This is how I did the filler necks on the fuel tanks.