My question is this. It says in the Defi manual that the fitting is 1/8PT. I know PT stands for pipe thread but what is the 1/8 stand for? I assume it's not diameter in inches becase the drill bit needed to drill the hole is 21/64" which is much bigger than 1/8". Can someone please explain this to me?

Thanks

they may have done a metric to standard conversion on those sizes. 1/8NPT doesnt sound right. id doube check the metric sizes before i went drilling anything.

1/8 Pipe Thread is right. Thats what is for Greddy (and I assume most other) EGT gagues.

If you look in any tap and die set (metric or standard) there is almost always a tap labled 1/8 pipe on it.

One thing I should mention with a pipe thread tap is the farther you thread it is, the larger the hole becomes, so, keep test fitting your probe so that you don't go too far.

But what is the 1/8? 1/8 of an inch? That can't be right. The fitting that the EGT probe slides into is much larger than 1/8".

Nobody knows?

EDIT: because I dont know what I am talking about...

Wow, so many 'mechanically inclined' people here and no one knows what pipe tap is? Designated PT sizes are the amount of diameter change per inch of thread. 1/8" pipe tap means that there's a diameter change of 1/8" per inch of thread. Larger hole size allows a good solid start, and then the narrowing makes it at the least water tight.

^^^ damn.. ninja edit wasnt fast enough... :\

Thanks for clearing that up spar..

At least someone answered, but wrong. Standard threading on 1/8" PT is I believe 24TPI (threads per inch), although anything is possible, it all depends on the tap you use. Tool and die shops will have almost certainly a 36 TPI 1/8" PT. With a CNC lathe you could cut whatever TPI you want. There are applications where it needs to be air tight that go over 40TPI. There is no hole size designation because the hole size changes. 1/8" pipe would effectively be a 3/8" - 24 in terms like 10-32 or 1/4-20.

I suppose I should add that it is indeed the size you want to use, not whatever anyone would recommend or the metric equivalent or anything like that, if the installation guide says 1/8" PT then that's what you use. The post about checking it was right, check every half turn of the tap once threads are established all the way through whatever you're tapping. If you're cutting into something ~1/8" thick you'll probably be properly cut as soon as you're past the entry on the tap. (section with smaller diameter and no real cutting edges.) Use tap oil. It'll keep you from cross threading and really screwing things up. After each half turn back it out and run it in and out a few times before checking, this will remove any burrs and keep you frrom over cutting the hole.