TST says to go 5/16".
http://www.tstproducts.com/pdffiles/instpy25b.pdf

Piers Diesel says to go 11/32".
http://www.piersdiesel.com/TechPyroInstall.htm


Which did you go? Which is right? It's only 1/32" difference, but still...

 

Q or R drill, .332 or .339 for 1/8 NPT

 

I used TST's instructions and it's still working 2 years after the fact.

bradnon.

 

Thats fine but its not spec.

 

You can use 5/16, 21/64, or 11/32" drill bit, it really doesn't matter much for 1/8NPT, all will work. Any good tap and die set will give drill bit size to use on each individual tap. Because it is taper thread, and exhaust manifold is not very thick I prefer to use smallest bit(5/16). Grease tap, but don't cut threads down too far, or probe will be too far down. Use Vernier calipers to check depth measurement(do only little at a time), use magnet,follow rest of instructions. Hope this helps.

 

I also prefer to make the tap do a little more cutting. I use a 5/16" for 1/8NPT. No problems and good fit.

jlh

 

Let us know how it went, hopefully we didn't beat a to death with info.

 

5/16" = .3125
11/32 = .34375

"Q or R drill, .332 or .339 for 1/8 NPT"

So the answer is "neither" Hrmmm....I'll just go with 5/16" then...depends though I might need to use my 1/4" adapter so that I can space the probe out some...

 

Use a 5/16. I just did it on Friday and it worked fine. Tap called for a 21/64. I used the tap to cut the threads to fit. Just take your time. If this is for the boost guage. You will probably be drilling aluminum. If so, use grease and take your time so that you don't strip the threads.
Chris

 

On a side note you don't have to drill into intake, you can use existing pipe plug hole on side of intake. Just remove and use a reducer to plumb in boost line.

 

That is true. If this is on the 97, then it has a test port that is already there and fits the boost fitting fine. It is on the side right in front of the intake horn above the Injection pump. Looks like a little plug.
Chris

 

Yepper...I used that one for the boost gauge already. This was for the EGT gauge.

I guess I'll just put a clock in the 3rd hole until I get a FP gauge.