This fitting has 5/8-18 female threads to accept a mechanical temperature gauge bulb. The other side has male M16x1.5 threads and includes a sealing washer.
We also offer a similar fitting for electrical gauges. Our 1/8 NPT to M16x1.5 Adapter (Part No. AM2268) has the same male metric thread, but the female thread is 1/8 NPT to accept most aftermarket electrical sending units.