Re-drilling/Re-tapping hole for larger size.

[JCyrbok]

I have to replace my Thermo-couple for my EGT gauage. The orginal from SPA was a 1/8npt fitting with a 1/16in diameter thermo. The new one they use is a 1/4npt fitting and 1/8in thermo.



Any pointers or tips on drilling out the 1/8npt to the 1/4 npt? I have had bad experiences in the past trying to drill out holes, where the bid either bits weird and makes the hole oval or it bounces around once through and chews up the edges of the hole. Maybe it just me, LOL.



Any way any tips would be appreciated.



Thanks

J-

 

[Ron Bissett]

Drill Drill

If you have a drill stop use it ,If not tape around the drill can be used to keep from going to deep, I run the drill up to speed then very slowly insert in to the hole with little force to keep from grabbing the old threads (light touch is Best) Hi speed light push keep the drill straight up stop on brake through reverse straight up and out . Ron Bissett in Metro Louisvlle KY

 

[Firehunter]

what type of material are you drilling into? if it is cast iron be real careful that stuff is very brital. You then need to find out the if tread is tapered or straight. If it is tapered you should drill first with a size "R" drill bit, then used an 1/4npt tapered reamer. go all the way till you hit the chamfer on top, this will give a nice start for your treads. This is very important because if you try to tap the hole that is straight you will break you tap inside the hole and then you really screwed or if is straight and you use tapered you will not have a proper fit. . also use a shop vac as your drilling to collect the chips so they won't fall back into the hole when you done.

 

[JCyrbok]

The hole is in the exhaust manifold for the fitting for my thermo-couple, the threads that are there already are 1/8 npt threads.



Thanks

J-

 

[Firehunter]

so your still going to have to make it bigger and use a tapered reamer

 

[Bob Wagner]

chris, why not buy an 1/8" npt x 1/8" diameter compression fitting? I have a ss one I can mail you

 

[JCyrbok]

Thanks BOB, that one hit me like a brick when I woke up today ( i work nights) Got one on the way, DUH!!!! It was like 8. 00 bucks so well worth it.



Thanks

J-

 

[RonA]

Grind a small flat on the cutting edge of the 7/16" drill and it won't grab. Cast iron and cast steel cut fine with neutral or slightly negative rake cutters.

Ron