Hello,

I could use some advice on the proper size to drill the EGT probe I am going to install on my truck soon. I did searach on here and found some good info, but still a couple details to iron out.

1. I am installing an Autometer pyrometer. the instructions from them specifically state: drill 5/16" letter R(.339) hole and tap for 1/8" NPT.

Now, 5/16", according to my drill chart, is equivalent to 0.3125", this is nowhere near the .339 they recommend. But also on this same chart, i see that drill size "R" is 0.3390", so this makes sense.

2. According to what I find on Starret.com, their drill/tap chart shows that I should use a tap that is 1/8-27 NPT. For this size tap they recommend using an 11/32" drill. 11/32 equals 0.3438"

So which is it? I tend to think that the folks at Starret would know more about these things than Autometer, however, I'd hate to find out the hard way!!


What final drill-size seems to have the most success?

Thanks in advance for any and all replies!

Matt

 

11/32

 

I always use the bit size engraved on the tap.

 

Thanks for the replies!

Infidel, my tap doesn't have a drill size engraved on it unfortunately..

I guess we are only talking a difference of less than 0.004" in hole size.

On one side, I tend to think that the smaller hole (0.3390) would be better because you can always bring the hole up larger. But on the other hand, perhaps that would leave too much material and might risk breaking the tap off in the manifold..

So, most people have had good success using the 11/32 drill?

I wonder why Auto-meter even put the instructions with 5/16" in it at all..

 

Now things make sense I've installed a few gauges that need a 1/8 pipe thread and I've always used the 5/16 drill bit that was supplied with the tap. Now I know why it's always been such tough sledding. Thanks for bringing this subject up. I also discovered that 1/8 BSP and 1/8 NPT are 28TPI and 27TPI respectively, I always thought they were the same as I never used much pipe thread.

 

5/16" is either a pilot drill or a mistake.

A "R" drill is correct, but not easy to obtain in a lot of places, so the 11/32", which you correctly point out is only 4 thousands of an inch oversize, is commonly substituted. The Tap is tapping a taper anyway, so it makes no practical difference, your hole is more about not tapping too deep than how large the hole is ( to a point ).

 

I used 5/16" and it worked fine. Only need a few threads into the tapered tap anyhow. If I was going into something over 1/4" thick I'd get the right bit.

 

So I presume you've also read on what to do when drilling?
If not, make sure to read up on it so the metal shaving don't remain in your turbo. IE...loosen turbo and slide piece of cardboard in between the turbo and manifold + blow it all out clean and you should be good to go.

 

Tapping too deep?
Please explain this.
I threaded it in pretty well on my manifold.

The few Auto Meter pyro probes I installed didn't look tapered to me.
I treaded it in, then tighten up the lock nut when I got the probe where I wanted it.
I also took the turbo completely off so I had no worries of metal shavings in my turbo.
It has been fine for 8,000 miles or so.

I used my drill and tap set I bought from Geno's.

 

Sure, NPT threads are tapered so they can form a seal. I don't think it is a huge deal with a pyro, but for liquid or gasses there is a set gauge point that we tap to that assures full engagement without running out of threads to seal.

This explains it much better than I can.

http://en.wikipedia.org/wiki/National_pipe_thread

 

Patdaly:
Thanks for the explanation and the link.

I have only tapped a few pyro probes in 12V manifolds, and a few Ram head ornaments.
The Ram heads ended up as my keg tap handle in the shop and the other for my 4x4 shifter on my old Ramcharger.

 

I've installed dozens of pyros and have never done anything at all special in dealing with drill and tap chips.
Zero problems. First time you start the engine they shoot right out the exhaust.
The way some folks get **** about it you would think they were tapping into the intake side of the turbo.

 

The last pyro I tapped, I was swapping the motor from Non - intercooled to intercooled using a HX-35 turbo anyways.

I figured while the turbo was off was the perfect time to do it.

 

for you guys worried about the drill chips falling into the manifold, start the drill in the manifold, and just before it is all the way through, stop drilling and start the engine. let it idle as you finish drilling the last bit through the manifold. the exhaust WILL Blow ALL the chips out. also let it idle as you run the tap into the manifold. no chips will get in the manifold. wear safety goggles as you do this, `cause the crap WILL be blowing towards YOU. been doing it this way for over 30 years and NO problems with chips in the turbo.

 

High Bid: Yep, been doing some research on here and think I will go with the engine running method as jtamulonis describes.

I have tapped several other item with similar concerns over FOD (foriegn object debris) getting into delicate engine interiors. What reminds me the most of this was running a tap to fix the threads on a sparkplug hole. Just go slow with grease on the tap. Then I would turn the motor over several times with the sparkplug out and watch the debris fly out of the hole.

That being said, I see no harm in the method that some choose to remove the turbo, and/or loosen it and block it off with cardboard. We can talk all day about if that much care is needed or not, but bottom line is that it doesn't hurt anything.

For me, This isn't something brand new, and I am too lazy to remove parts that I dont think I need to.

Speaking of the depth of the tap,, i was under the impression that the depth of the tap ewas to help position the probe tip correctly. What I mean is, that you dont want the probe obstructing exhaust flow any more than neccessary. Is that not the case?

Does this even block exhaust flow enough to worry about?

I had thought about just running the tap all the way through, then controlling my depth with the probe/jam nut by doing some measuring prior to installation. Thoughts?

thanks again for all the helpful replies!