Drilling and tapping exhaust manifold for EGT thermocouple

[TurboSandman]

I have my Autometer boost and EGT guages in an A-pillar mount, lighting and boost functional, but now it is time to drill and tap the manifold for the thermocouple. I bought a new, sharp 7/16 bit and 1/4 NPT tap but made the mistake of comparing threads on the thermocouple bushing and the tap; WAY DIFFERENT!

Anyway, my handy thread pitch guage give the pitch on the thermocouple as 27 threads per inch (27 ?!!!) while my tap is 1/4 NPT-18. No big deal; just buy a 1/4 NPT-27, right? Well, who has them? My search on MSC only turned up 18 tpi in a 1/4 NPT tap.

I know this is a problem with me as everyone else has already been through this, but where did I go wrong? Advice for the newbie please.




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'97 2500 SLT, 5 sp, 4X4, 3. 54 LSD, Camper and Trailer Packages, Autometer Boost and Pyro, NRA LIFE MEMBER

 

[HEMI®Dart]

Turbo,

If I remember right, Drill & tap a 1/4" NPT hole, then get a 1/4" NPT to 1/8" NPT reducer bushing, then screw the T-couple into the reducer. Use never-sieze on the bushing threads. Don't overtighten. I'm pretty sure the threads on the T-couple are 1/8" NPT. I found my reducer I needed at Home Depot.

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[This message has been edited by HEMI®Dart (edited 05-24-2001). ]

 

[EMDDIESEL]

Try www.mcmastercarr.com they sell alot of odd ball taps and related stuff. If you dont see it onlne, call them. If you cant find it, email me and ill send you the right drill bit and tap. I have extras. Just let me know the EXACT size you need. Good luck

 

['956Wheel]

WOAH! There is no such thing as a 1/4-27 pipe thread! There is a 1/8-27 pipe thread and the tap drill is . 332 DO NOT DRILL A 7/16" HOLE AS IT IS TOO BIG FOR A 1/8-27 TAP.

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9535hundred

 

[TurboSandman]

Thanks HEMIDart, EMDIESEL and Statland; I took a closer look and the 1/4 inch tap is significantly larger than my thermocouple bushing. I suppose it must by a 1/8 inch bushing! Duh, as my teens would tell me.

Statland, you are sooo correct; a standard 1/8 NPT is 27 tpi. Now, is everyone else's thermocouple twice as large as mine? I feel so... inadequate.

 

[LSMITH]

LOL@Turbo!!!!!!! I have an 1/8" thermocouple too if it makes you feel any better.

 

[hammong]

I have NO IDEA why they call a 1/8"-27 NPT fitting 1/8" --- it's closer to 1/3" on the outside diameter, and 1/4" on the inside. Go to the store and get yourself a 1/8"-27 NPT tap and compare it to your pyro fitting, they're probably identical.

My tap said to use 5/16" drill bit. If you already did drill your 7/16" hole for the 1/4-18 tap, you'll need to get a 1/4-to-1/8 NPT reducer and put your pyro probe in through that... it may not go in deep enough though.

Greg

Pictures of my probe, hole tapped and installed at <url>http://www.rints.com/hammond</url>

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Y2K Dodge Cummins QC 2500 SLT Laramie 4x4 Long Bed, 4. 10/Auto, Forest Green/Driftwood, 265x75R16 Michelin LTX, 6 speaker CD, Camper/Tow/Plow Packages, Westin Black Nerfs, Mopar bedliner, Ultra bright diamondplate toolbox, K&N Filtercharger, Autometer Ultralite EGT and Boost gauges in 2-gauge pillar w/tweeter, DD Stage II injectors, boost module, and boost elbow. Yee haw!
http://www.rints.com/hammond for photos!

 

[TurboSandman]

Hammong, my research indicates that for NPT (National Pipe Thread) taps, the size (1/8 or 1/4) refers to the size of pipe which is to be threaded. That is, a 1/8 NPT tap threads a hole to accomodate a 1/8 inch I. D. pipe, not a 1/8 inch hole. Other thread types such as national fine (NF) or national coarse (NC) refer to hole sizes, either in inches or mm, or in drill number sizes. Then there are different thread profiles and I sort of lost interest after that.

I hope this helps; some of it may even be accurate.

 

[BFaoro]

I just installed my Autometer boost and pyro. Yes you do use the 1/8 NPT and drill with a 5/16 bit. Go to the TST web site and use their instructions for the drilling and tapping for the pyro. My pyro works great, it moves instantly as you accelerate or let off !!

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96 2500 4X4, SC, SLT, 12 valve,lost kitty, silencer ring gone, 5SP 3. 54. dana 80, Pac Brake with 60 pound valve springs, line-x liner, Auto meter ultra lite boost and pyro gauges on A pillar,

 

[illflem]

TurboSandman, after working with pipe most of my life I've yet to figure out where the pipe size relates to the dimension. The ID number may be the closest but that depends on wall thickness. The only consistent dimension in one pipe size is the OD and it is nowhere near the named size. The next monkey wrench is when you get into copper pipe then the dimensions change again. One thing I've found to be rare though is metric sized pipe, US National Pipe sizes seem to be the world standard.

 

[TurboSandman]

Thanks all!

I just got back from test driving my now functional pyrometer; works great, just like you said, ROCKYMNT,really responsive. From a steady 50 mph showing 500 degrees to 800 degrees in 3 or 4 seconds or so at WOT. Cool down was similarly quick,leading me to see the need for a post-turbo thermocouple as I don't think the turbocharger really cools that quickly. I am impressed with the boost at 24 lbs in an unmodified (at least by me) '97. Is this typical for a stock truck?

Illflem, I agree; there seem to be several approaches to naming thread sizes but how NPT relates to pipe size (or was that tubing size?), I am fuzzy on. I did a search on google.com and found a website with an overview of thread terminology, but I have no personal experience to go on.

Anyway, now that I have instruments; LET THE BOMBING BEGIN!

 

[illflem]

Mike, I always thought that grease fittings where just 1/8" NPT and 1/4" NC bolt thread. 1/4" NC tap has always worked for me on zerks. As I was saying before about metric pipe being rare, metric zerks come in 1/4" and 1/8" NPT, there is nothing metric about them. The smaller metric zerks are metric bolt thread though.

 

['956Wheel]

Having been a machinist for 25+ years, (life sentence) Bill is right, it is the i. d. that determines pipe size. Before all the foreign made stuff, pipe was very close to the designated size, 1/16", 1/8", 1/4", etc. Yes there is a 1/16" pipe, thats the thread on grease fittings, 27 tpi. Then you have straight threads, this is conduit.

When tapping pipe threads keep in mind that the length of the tap that is in the material is all cutting due to it being a tapered thread, what this means is that lubrication is critical, anti seize works very well, CLEAR THE CHIPS OFTEN!

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9535hundred