...is 1/8" BSPT.
Most aftermarket gauges come with 1/8" NPT senders, there's a 3-way adapter on eBay that allows you to keep the stock sender or you can just get an NPt to BSPT adapter from here:
http://www.airlines-pneumatics.co.uk...derCode=103359
Took me a while to find this info the other day so thought I'd share
...don't forget PTFE tape!