Mechanical Water Temp Gauge NPT Size

[cebra]

I am looking to pick up a fitting for my LS motor but I do not know what fitting size a standard mechanical water temperature gauge is. In my SBC, I had to put a fitting in the block to step it down a tiny bit. If I can figure out the SBC hole size I could get a really good estimate of the gauge fitting size or is there a standard size for mechanical water temp gauges? I have searched a lot and cannot seem to find this, the LS conversion fittings come in 12mm x 3/8", 1/2", and 5/8" NPT options. Thanks.