Engine/Transmission (1998.5 - 2002) Thermocouple Install?

[kbond]

I'm getting ready to install my egt gauge and was gathering tools when I ran accoss something wierd in the "Pyrometer Installation Guide" over on the www.dieselpage.com. It says to use a 7/16 drill bit to make your hole in the manifold and then to us a 1/4 npt tap to thread it. 7/16 is a whole lot bigger then 1/4 so I am thinking this is a typo. Does anyone know for sure what tap I need to use for the EGT fitting? It's an Isspro gauge if it makes any difference.



Thanks for the help.



-Ken

 

[d90boy]

The instructions you read are correct, for a 1/4" NPT fitting you drill a 7/16" hole. The thread diameter is much larger than 1/4". Remember this is not a 1/4-20 type thread but a 1/4 PIPE fitting size. If your pyro probe is a 1/4NPT use the 7/16 bit, if it's a 1/8NPT it takes a letter R size drill bit which is . 3390" A 21/64" bit is . 3281", a 11/32" bit is . 34375" so you could use either of these if you can't find an R drill.

 

[illflem]

Common question. As far as I can tell pipe size designations don't relate to any measurement on the pipe unless it's the inside diameter of thick wall pipe from long ago.