Perkins Temperature Sensor Thread Size ?

[SV Demeter]

I am in the process of rewiring the Perkins 4-154 and installing new gauges and instruments. The supplied coolant temp sender is 1/2" NPT. The existing sender is a VDO and appears to be straight thread because it has a sealing washe. It is mounted in the t-stat housing. I put my calipers on the existing sender and thought I was getting 15mm but now looking around at Metric to NPT adapters I am seeing M16 x 1.5 and thinking that is the right size. Anyone been down this road before? Obviously I can order the adapter and see if it fits but figured I would check here first.