tgranthamfd 28 #1 Posted June 18, 2009 Just wondering how hard it is to tap the 1/2" NPT threads into the exhaust port on a K91? Apparently the one I bought isn't threaded, heard the best option is to thread the port vs. finding an adapter that will work well on a Suburban. Guess the one I bought is a later 60's instead of an early 60's, but it is almost the right color red. Oh yeah sounds good too. Share this post Link to post Share on other sites
oldandred 15 #2 Posted June 18, 2009 its no problem just dont run the tap to far into the block Ive done a bunch Share this post Link to post Share on other sites
tgranthamfd 28 #3 Posted June 19, 2009 Is there an easy way to tell, or just an inch or so? Share this post Link to post Share on other sites
Roundhoods Rule 0 #4 Posted June 19, 2009 Just talked to a machinist friend of mine about doing the same thing. He said if you can, stuff something into the exhaust port that will stop anything from going into the valve area. Something like a piece of shop rag or cloth that will stay intact. Make sure you use a tacky grease on the drill and tap so any metal chips will stick to it. He said they do it all the time for people who strip out spark plug threads and don't want to pull the heads off. Use a piece off electrical tape and wrap it around the tap and drill to mark the depth you want to go to. He says you really only need 3 or 4 full threads for it to be good but go as deep as you feel comfortable doing. Also in case you need to now, the drill for a 1/2" NPT tap is 23/32" (.719"). Good Luck Share this post Link to post Share on other sites
formariz 11,849 #5 Posted June 19, 2009 I have tapped 6 K91s so far and it is extremely easy. Since the existing hole on the k91 is slightly undersized for that size tap it is not that easy to go too far. When it becomes hard to turn the tap (as it reaches the highest part of taper) stop. You have at that moment about 5 to 6 threads. If your port has a slight "nib" on one side of its wall from the casting process grind it off with a Dremel before tapping it. Protect the valve area as recommended by the other replies. In addition to that every time I back up the tap to clear chips, I use a high suction vacuum to remove all the chips prior to continuing. Good luck Share this post Link to post Share on other sites
oldandred 15 #6 Posted June 19, 2009 use what roundhoods says and I use a vacum cleaner to get the cutting out and my heads are off when i do the taping with ex valve up Share this post Link to post Share on other sites
tgranthamfd 28 #7 Posted June 20, 2009 Cool !! Great information. You people are the best. Share this post Link to post Share on other sites
tgranthamfd 28 #8 Posted June 21, 2009 Next dumb question. I have been looking at taps, I see 2 sizes a 1/2" 14 and a 1/2" 11. Which one do I need to use? :thumbs2: Share this post Link to post Share on other sites
Duvall 0 #9 Posted June 21, 2009 Next dumb question. I have been looking at taps, I see 2 sizes a 1/2" 14 and a 1/2" 11. Which one do I need to use? NPT is 1/2-14 It should say NPT on the tap. I have never seen anything 1/2-11. Share this post Link to post Share on other sites
pfrederi 17,147 #10 Posted June 21, 2009 Are you sure the 11 tpi is a 1/2" tap??? 1/2 NC is 13 and NF is 20 5/8" is 11 TPI Share this post Link to post Share on other sites
Duvall 0 #11 Posted June 21, 2009 Are you sure the 11 tpi is a 1/2" tap??? 1/2 NC is 13 and NF is 20 5/8" is 11 TPI He will be using a pipe tap. Here is a chart for NPT. http://www.carbidedepot.com/formulas-tap-pipe.htm Share this post Link to post Share on other sites
tgranthamfd 28 #12 Posted June 21, 2009 Ok, the 1/2"-14 npt is the one I need then? What I did was went on e-bay and did a search for 1/2" Npt tap, that is where th 1/2"-11 came in to play. They both said they were Npt, I didn't know, I don't have a lot of experience with this area, didn't know the difference. There is a TSC close by, they might have the tap I need. Once again for the information. Share this post Link to post Share on other sites
Duvall 0 #13 Posted June 21, 2009 I think what you are seeing on flea-bay is 1 1/2 - 11 1/2 NPT (inch and a half on 11 and a half pitch) Let the tapping begin.... Share this post Link to post Share on other sites
tgranthamfd 28 #14 Posted June 22, 2009 Duvall you are right. Guess I should open my eyes sometimes. Thanks for clearing up my sleep deprvation, weekends just kill me. Think I need a nice relaxing hobby like a restore. Thanks again. went all over town today and couldn't find one, at least I do know what I am looking for now. Good thing I am not even close to putting the engine on yet, lots of striping and painting left to do. Once again thank you. Share this post Link to post Share on other sites
VinsRJ 721 #15 Posted June 25, 2009 I purchase one of the $5 1/2-14 NPT tapes from e-bay to re-cut the threads in my Clinton exhaust port. It was a good quality tool and worked perfectly. I like to use tranny fluid for cutting oil as well as the grease to catch the chips. Share this post Link to post Share on other sites
tgranthamfd 28 #16 Posted July 28, 2009 Finally got off my butt and took care of the job at hand. It was a pretty simple job and came out nicely. Did as you guys suggested, only didn't have air or electricity for a vaccum. Did roll it around to get the valve in the up position and put a piece of rag up all around the valve shelf, greased the flutes on the tap, went until it was getting hard to turn, whiped and rinced out thoroughly with penitrating oil, and had about 5 to 6 threads cut in when finished. Cleaned and regreased twice during the cutting. Looks good, but I haven't gotten a chance to try a pipe in it as of yet. for your informatoin and sugestions. Thanks Terry Share this post Link to post Share on other sites