What size pilot hole for tap??

[MacHaggis]

:confused



I don't guess I've ever understood pipe threads. I'm getting ready to put on EGT and boost gauges and, based on some advice, will use the 1/8 NPT fitting for the pyro. I tried googling around, and found a whole variety of info on what size pilot hole to drill. All over the board, from 11/32 to 15/32, none of which makes any sense to me in terms of machine threads, unless the 1/8 in "pipe talk" is an inside diameter???



Any useful advice/guidance will be most welcome.

 

[BHolm]

5/16" is what was reccomended by Geno's, where my gauges came from. For the 1/4" probe you need a 7/16" hole.

 

[Spooled-up]

unless the 1/8 in "pipe talk" is an inside diameter???

That is correct.

 

[MacHaggis]

Thanks, guys!



I've been scouting out the clearances and angles for this project and I suspect there'll be quite enough colorful language in the garage even without trashing my manifold.

 

[Gary - K7GLD]

All the taps I have, have the recommended pilot hole drill size embossed on the shank...

 

[SSage]

When you buy the NPT 1/8 tap it will have the drill bit size on the package. You can buy both at most auto parts stores. When tapping, I stop short of all the way through and try to get the probe tip to rest in the center of the port. Don't go too deep and bottom out the probe. Mine is screwed in four complete turns. If I tapped it all the probe that came with my pro-comps it would have touched bottom.

 

[MacHaggis]

ThanksSSage, strikes me as a good tip.



I've got another thread going about how thick the manifold is, just because I don't want to break through until I get to the largest bit while drilling the pilot hole. I don't hae they pyro in hand yetto eyeball and think it all through, but from what you've suggested, as long as I've got at least a 1/8 diameter at the bottom of the hole, and the tap can cut deep enough in the pilot hole, I can insert the pyro without it chafing and breaking off on the bottom edge of the hole.



I'll have to ponder,but the lights are coming on.



Thanks again!

 

[BHolm]

Thanks, guys!



I've been scouting out the clearances and angles for this project and I suspect there'll be quite enough colorful language in the garage even without trashing my manifold.

:-laf Isn't that the truth! I went with a 1/4" probe and post turbo location because I have an exhaust brake with a drilled and tapped hole already there. Should have been easy right? The plug was stuck, I twisted off the head, had to remove the brake, drill and tap anyway :{ If i had known THAT, I would have gone 1/8" and put in the manifold. I considered putting the bigger probe in but it looked like a REALLY big hole to be drilling in that manifold



Good Luck, Have Fun

 

[MacHaggis]

but it looked like a REALLY big hole to be drilling in that manifold



Good Luck, Have Fun

Lawd, yes! If that 1/8" takes a 5/16 hole, I figger if I went with the 1/4 I'll get a second use out that 4-inch hole saw I used to put the gooseneck in.



I'm sure I'll have a blast but . . . it'll be a good job to look back on.

 

[Cattletrkr]

... it'll be a good job to look back on.

I was a little freaked out when I did my first EGT drill/tap. I knew where to go with it, but had no real idea what I was doing. Got lucky and missed the webbing, which I didn't know existed until afterward. It's pretty daunting until it's done. You'll look back and laugh when you're done and wonder what you were so worried about. It's actually really easy.



Now the boost drill/tap, that'll really make ya sweat if you don't know for sure what an intake manifold is and you don't have a description or a picture. Got lucky on that one too. Man I've come a long ways since then...



Just keep tellin' yourself, "Git 'er done!!!!!!"

 

[rjones]

I ran a 21/64 drill thats what was stamped on the tap... ... . And only tapped far enough so the probe was centered in the middle of manifold... ...

 

[MacHaggis]

You'll look back and laugh when you're done and wonder what you were so worried about. It's actually really easy. . . Just keep tellin' yourself, "Git 'er done!!!!!!"

Thanks fer the engouragement. I could overthink these things real easy.



By the bye, I don't have the tap in hand yet, either, but none of the taps I do have show any dimesion but the thread and pitch.



NPT is a little further over than my machine-threaded mind has been.

 

[JimB1]

MacHaggis, NPT= National Pipe Thread, a tapered thread series and entirely different from machine screw threads which are considered straight. Whenever in doubt, practice on some piece of scrape metal stock about same thickness. Even scrap piece of wood, thought not ideal, would give some idea of how things would work. JimB.

 

[MacHaggis]

Whenever in doubt, practice on some piece of scrape metal stock about same thickness. Even scrap piece of wood, thought not ideal, would give some idea of how things would work. JimB.

I like that idea. I've got an old tractor manifold laying around. Probably iron instead of steel, but, yeah ... I think I'll do that.



Thanks!

 

[Diesel Manor]

1/8"NPT requires a size "R" drill bit. We now sell them for those who have a hard time locating them. Size R is equivillant to 0. 339". Here's the link if you need one: http://www.dieselmanor.com/dm_products/DM-TAP1418.asp

 

[tandersen]

This might help. http://www.engineersedge.com/tap_drill_chart.htm

 

[MacHaggis]

Holy smokes! :--)



I've bookmarked, printed and saved that. I'd googled all over for something like that and struck out.



Thanks!