I have a 99 CC LB 6spd 4x4
I've been looking at gauges for years and finally got the wife's blessing. I ordered Hewitt gauges because IMHO they look stock compared to the literally hundreds of gauges out there.

So I have a question about my thermocouple. I see everywhere says drill a 1/8 npt into the manifold with the greased bit/ magnet/ shop vac, but when I got my gauges in I found out they ship with 1/4" npt thermocouple "unless otherwise specified" which is not mentioned on their website...

So, is it ok to drill the course 1/4" threads in the manifold or do I need the fine threads of 1/8" npt? Just wondering if the exhuast manifold is thick enough. If i were drilling the headers in my Corvette hands down I'd go 1/8", the 360FE's cast manifold on my 71 f100 would go 1/4" without batting an eyelash.

Thanks for your help!

 

Id say the manifold is about 1/4 inch to 1/2 inch thick.

I ordered a drill and tap special for my gauges. Its the 1/8 npt. If you decide you need it Ill give you a deal on it Ill never need it again.

As far as which one you need to use I have no idea how it makes a difference. Im sure someone will know.

 

Might be off base here but when the instructions say to drill an 1/8th inch hole, do they follow it up with tracing a 1/4 tap?

Only way I know you can thread anything into a hole.

Denny

 

No the tap is 1/8. The bit is like an R or something like that.

 

A 1/8" NPT is about 1/4" in diameter. Don't get confused by the numbers. NPT is a whole bunch bigger than regular drill bits. You can't use a 1/8" drill bit and a 1/4" NPT tap. They are two different sizes and don't mix.

 

1/8" npt needs a 11/32" or R drill bit for the tap, 1/4" npt needs a 7/16" drill bit for the 1/4" npt tap. I have everything to drill either hole, just wondered if the manifold is thick enough for 1/4" npt thermocouple that was shipped with the gauge without needing to weld a bung vs all instructions and forums mention 1/8" npt

 

OOOOOOOh, that's the question.

These are very thick casts and use alot of oil on a slow sharp bit.


Center mass!!

 

The manifold is think enough that you'll probably get 2 threads worth of engagement in the 1/4" npt. I think that adapter will seize in the manifold long before you have any chance at mechanical failure. However their are a couple things I would consider before going with the 1/4 npt. 1 it will take up a larger cross section in the manifold; might not be enough to make any difference. But it will certainly be bigger than the 1/8. Second I would assume that bigger adapter probably means bigger probe, which could translate to slower response time. I say "could" because I have less than no desire to pull out a heat and mass transfer book and make "accurate" guesses. It is memorial day weekend and I have beer to drink. Though, through the beer goggles I'd hazard a guestimation that more mass equals slower response as it will take longer to change the temperature of the probe around the thermocouple. But again that's a beer guess.

 

The OP obviously has this part under control but...Pipe is measured by the inner diameter (the hole it in) and tube is measured at the outer diameter. Pipe usually has a thick wall and that is why the bit vs pipe size is so much different. Pipe usually gets threaded so a thicker wall is necessary. Taps have, "use drill size", stamped into them if you ever want to know exactly what they recommend. If, for some reason, it's illegible or not there I hold the tap and the bit against each other and eyeball up the bit that will drill the hole without getting into the threaded area.

NPT is also tapered and you don't want to run the tap all the way in or your fitting won't get tight. I run the tap in about 1/2 way and test for fit and go deeper if needed.

 

1/4 NPT is doable. The gauges I used in one of my no-longer-owned Powerstroke pickups had that size bung for the thermocouple.

There is plenty of meat, just be aware that starting the tap on a curved surface is tricky and the larger the tap or smaller the radius of what's being tapped, the trickier it is.

 

Cleatus did you notice any difference in sensor lag between the 1/4 and 1/8? Last night's beer brain was pretty sure the thermal mass difference would dampen the readings. Overhang brain isn't so sure it would be noticable. I wonder what properly hydrated brain will decide? Either way nothing beats experience!

 

Got er dun

 

Once I heard the manifold was as thick as it was i decided to do it; My guess is 5/16 ;-) drilled n slowly tapped for an hour, hooked up thermocouple. Only ran it for a few short seconds but the needle climbed at a nice pace. Gotta get some sleep b4 working another 12 tonight. Thanks for the help!

 

Theoretically it shouldn't make any difference because the probe sticks in the exhaust stream and that's what matters. I'm no expert and without seeing gauge needle movement side by side using both types mounted in exactly the same spot I feel that seeing any appreciable difference would be impossible.

 

So after a short 3 day trip with the 5th wheel the egt needle would usually move 2-400* within a second of mashsing on the right pedal in the foothills of the Ozarks (zf6) The highest I could get it was 1050. I wasn't easy on it on purpose. Now to put in the ts 6 pos and see what all changes next week. If the 1/8 npt does have a faster response time I doubt I'd notice enough to care at this point. Thanks again