Engine/Transmission (1998.5 - 2002) taping exhaust manifold

[AHuber]

Hi out there,

For those of you who have tapped your exhaust manifold,well I just completed mine. I used the 5/16 drill and 1/8 n. p. t. I found it hard to start the tap as it seemed like it wanted to wallow out the hole. I finally got it started and it seemed like i had to tap almost the length of the tap to get treads good enough to start the probe nipple. The nipple screwed in o. k. and I was careful to make sure it was flush with the inside of the manifold. I used never sieze and it seems like the probe nipple is still a little loose. It cant turn out of there after the probe is installed. Just wondering if it will tighten up as the manifold gets hot. I guess I will have to keep an eye on it for a while. Did this ever happen to anyone else ?

Thanks

Al

 

[Mschevey]

I don't like being the one to tell you this, but if you ran the NPT tap in almost the whole way the threads in the port will be over-sized. NPT taps should be run in about 5/8 the threaded portion. At most 3/4 the length of the treaded part.



NPT threads develop their seal from compression between the mating parts. They get the compression from the taper used. If a 1/8 NPT port is correct you should get ~2. 5 turns by hand. Then wrench tighten ~2 to 3 turns more and that is the design fit. ( I don't have my machinist handbook with me to tell you exact numbers but these are close. )



As for what to do now I am not sure. You will have problems with the fitting coming lose and exhaust gases blowing around the threads. Any ideas from others??

 

[Mschevey]

I just saw you address. I went to collage at Valpo. for Mechanical Engineering. Mike

 

[RustyJC]

I guess you could Helicoil it back to 1/8" NPT if you can find a solid (i. e. , not wire-type) insert. Alternately, you could drill and tap it for 1/4" NPT and use a 1/4" NPT to 1/8" NPT bushing.



Rusty

 

[Loren]

Al,



Okay, I'll 'fes up where everyone can read about it.



When I tapped my manifold, it was late and I was tired. (That's my excuse, and I'm sticking to it. ) I knew that I wanted to drill the hole a little undersize, then enlarge it with successively larger bits until the tap would fit. This is how I do almost all holes I drill and tap.



Rather than measure, or drill a hole in some scrap material, I grab a drill bit that I'M SURE is too small. It wasn't. It was at least one size too large. After tapping the hole, the pyrometer probe fitting would wobble in the hole. I finally learned my lesson for that day and went to bed.



The next day, I figured I had nothing to lose by trying some radical solutions, since it looked like I was about to buy a new manifold. I thought of welding the hole closed and drilling and tapping a new hole. The solution I used instead was to braze the fitting into the hole.



I know I will never get that fitting out again, but if I need to, I'll buy a new manifold then. In the meantime, this works well. If you don't have the equipment to braze the fitting, I'm sure you can get it done for a very reasonable price at any welding shop. It was a simple job. Even removing the manifold wasn't difficult.



Good luck.



Loren

 

[Gary - K7GLD]

Well - ya really DIDN'T have to do all that...



All ya needed do, was drill THAT hole out for a 1/4 inch plug, and PLUG the sucker! THEN, PROPERLY drill and tap the OTHER side of the manifold for the probe...

 

[BillGotthelf]

When I read your first post I realized you used the wrong size bit for an 1/8" tap. The correct drill bit size in on the side of the tap. I believe its 19/64th's.

 

[Smokin Joes 2500]

Originally posted by Mschevey

I don't like being the one to tell you this, but if you ran the NPT tap in almost the whole way the threads in the port will be over-sized. NPT taps should be run in about 5/8 the threaded portion. At most 3/4 the length of the treaded part.



NPT threads develop their seal from compression between the mating parts. They get the compression from the taper used. If a 1/8 NPT port is correct you should get ~2. 5 turns by hand. Then wrench tighten ~2 to 3 turns more and that is the design fit. ( I don't have my machinist handbook with me to tell you exact numbers but these are close. )



As for what to do now I am not sure. You will have problems with the fitting coming lose and exhaust gases blowing around the threads. Any ideas from others??

MACHINERY handbook!!!!