Drilling and tapping oil passges for NPT plugs

[fairlaniac]

I have my '66 427 block I'm prepping and thought I change the welch plugs to NPT plugs. I might be able to set the block up on my drill press to get the front and rear plugs drilled. However the ones in the galley may need to be hand drilled. Never doing this before, can it be done with a hand drill? I have a junk '59 block I plan to try first to get the feel. I'm just picturing the cast iron wanting to chip or break away using the hand drill. Any tips from anyone who has actually performed this?

Thanks,

[blykins]

Doug, all you need is a hand drill.  The drilling is easy, tapping is a little more difficult, but only because you have to pay attention to depth with a pipe plug.  Make sure the one behind the distributor is deep enough to not snag a distributor.

[fairlaniac]

I've seen where some people say to drill dry? I've always used cutting oil when cutting most metals. I would plan to do so unless convinced otherwise.

So not to create a huge debate on the plug behind the distributor. Due to the "trickiness" I plan to keep it a plug.

thanks,

[chris401]

I've seen where some people say to drill dry? I've always used cutting oil when cutting most metals. I would plan to do so unless convinced otherwise.

So not to create a huge debate on the plug behind the distributor. Due to the "trickiness" I plan to keep it a plug.

thanks,

A little more info than what you asked for: ACE Hardware has the plugs, long tap and long drill bit to do a full oil galley modification. Personally I have used an old 18 volt Snap-On drill and WD40. My corded drills tend to grab. On that valley plug right behind the distributor I tap it about 1/8th turn at a time. The FPP procedure cautioned against distorting that area. I have only done 4 but the last two did not get the main to cam galley drilled or beveled out.. A few guys have said that is a weak area and they prefer to open up the bearing if they do anything.

[Chrisss31]

I'd buy a new tap for the job and definitely use oil.  Tapered NPT taps can be a bugger if they're not sharp.

[fairlaniac]

I shop at www.mcmaster.com and get a work discount :-) I have 6 new 7/16" drill bits and 2 new NPT 1/4 taps. I agree, never use old dull bits or taps on "critical" work.

thanks!

[Ranch]

Doug , if your concern is the drill grabbing and pulling you to deep or worse breaking, just put a small flat on the cutting edge in the flute.  Tapping should be no big deal, a tap handle gives you better control, but sometimes a Tee handle gives you better room to work.  If you mark your tap so that you’ll leave about 7 threads (5 threads on a worn tap) above the hole that should get you in the ball park.
I have a 62 solid lifter block and my concerns were in the valley going down to #5 main with that dog leg so I just left mine alone with welch plugs and a couple of stake marks on them

[Falcon67]

I hand tap in cast iron blocks using Tap Magic.  If a bit of drill work is required, same fluid.  Just work the tap easy and test the plug for depth periodically. 

[gregb]

Ranch, please tell more about the small flat method for keeping the drill from grabbing.  What do you use to make it a file?  Sounds like a very useful piece of information. 

[Chrisss31]

Drill bits are too hard to file, best to do it on a bench grinder with a wheel that has good square corners.  Basically it reduces the effort of the drill bit pulling itself into the metal, works great on cast iron and is a must do for brass and other like metals.

[CaptCobrajet]

It makes a much nicer job to use piloted core drills.  The pilot centers the bit in the oil passage, and when using a hand drill, it will follow the drilled hole.  Your larger hole will be centered and straight.  You do need to check your depth as you are tapping.  Be sure to clean really good and put some anti seize on your pipe plug for the depth check.  If you are dry and get debris in the threads, it will lock up and make real problems trying to remove it.  It takes a while to do it right.  Don't attempt to run a pipe tap into the 9/16 straight thread in a 427.  Those plugs are the same as the front lifter plugs in a 460....... easy to get.

[blykins]

Be sure to clean really good and put some anti seize on your pipe plug for the depth check.  If you are dry and get debris in the threads, it will lock up and make real problems trying to remove it.

Best tip right there.  You do this wrong one time and you'll never do it again.

[C6AE]

A suggestion; or rant even 
Typically the "hole" for a pressurized oil gallery in an engine block would be tapped with a NPSF (straight,fuel) tap and the plug would be a NPTF (tapered, fuel) This to automatically set the plugs at a specific depth, including flush if desired.
 
The "plugs" are special "fuel, dry seal type" and available in several profiles (regular, short, and SAE extra short) and two different tapers 3/4" per foot or 7/8" per foot depending on the desired protrusion.
The correct taps and plugs are easily sourced, but they will never be in a "hardware store". The tap drill sizes can also vary from plumbing stuff depending on the finished tap size, best to check this carefully.


The pilot drill suggestion from CaptCobrajet is great...

This is not water pipe threads... the thread form is completely different.

more here https://www.huyett.com/getmedia/3dedc73c-15c5-4828-9a13-73316622d6ba/GF-ThreadDesignGuide.pdf.aspx?ext=.pdf
Although it can be made to work, in my opinion a good way to screw-up a good block is to use hardware store water pipe taps and plugs...

(Cast iron is machined dry, there is enough graphite in the alloy to work as a lubricant. Using lubricants to tap cast iron can lead to interference or jamming of the tap).

Edit for clarity;

These are all considered "pipe" threads and there are many variations.
An explaination of the difference between "water pipe" and "fuel" thread forms is well explained here.
https://www.ring-plug-thread-gages.com/ti-NPT-vs-NPTF.htm

The difference being the very tip of the thread form having a different interference than the taper itself and a difference in precision, or class fit.


(In production work it can be impractical to form a tapered thread to a specific depth. A 1/4 revolution of the tap will be a much greater difference in plug depth. Expensive to control that when you are doing hundreds in a day. Hence a uniform dia straight thread female, and a tapered male plug.)

[chris401]

This is not water pipe threads... the thread form is completely different.

more here https://www.huyett.com/getmedia/3dedc73c-15c5-4828-9a13-73316622d6ba/GF-ThreadDesignGuide.pdf.aspx?ext=.pdf
Although it can be made to work, in my opinion a good way to screw-up a good block is to use hardware store water pipe taps and plugs...

(Cast iron is machined dry, there is enough graphite in the alloy to work as a lubricant. Using lubricants to tap cast iron can lead to interference or jamming of the tap).

That is actually good advice assuming someone would walk in and pickup the first thing they see. Best to have a machinist pick it out for you if your not sure. Survival used to have the plug kits. I wouldn't remember the correct names of the Allen head plugs I used, my last one was 5 years and 23,000 miles ago.

[338Raptor]

C6AE,
Thanks for the excellent info. I didn’t know any of that about the type of threads and plugs used in engine blocks.  That’s what I love about this forum.  There are a bunch of members with vast knowledge of relatively obscure but totally relevant topics.