The Stovebolt Site Forums The Shop Area Engine Shop Drilling and Tapping a 3/4” hole

Working on a 270 that has a 3/4” drain hole installed in the block located forward of the fuel pump hole.

This was probably used for an auxiliary pump mounted to the motor.

I want to thread it, and install a plug. I am on a steep learning curve learning about taps, thread types, and how to fit a threaded pipe plug into this hole. For example: a 1/2” square end plug almost fits into this hole but it has a slightly larger diameter than 3/4”. I came to the conclusion that I need to enlarge the hole and tap it so the plug will fit.

1. What size of drill bit do I use? According to the link do I use the 23/32” or 57/64” drill bit?

http://www.carbidedepot.com/formulas-tap-pipe.htm

2.Do I use a tapered reamer before i use a tap to thread this hole? If so, what size? 3/4”?

3. What size and type of tap do I use?

4. Do I use several progressively larger drill bits to get to the final diameter from the 3/4” hole that is present?

Forgive my ignorance, but this is new to me, and I need to accomplish this task correctly.

Thanks. Tom

You can use a 23/32 inch drill per the chart you linked without using a taper reamer. Cast iron cuts pretty easily with a tap, so you shouldn't have any issues.
You should use a 1/2 inch National Pipe Thread tap for your 1/2 inch plug.
If it's easier, use intermediate size drills. Not essential, but might be easier for you. Again, cast iron cuts pretty easily.

If the engine is not torn down so you can clean the chips out, you can fill the drill and tap flutes with grease to catch the cuttings.

When tapping a pipe plug hole, it is important to stop and try the plug in the hole often. You can always go deeper with the tap if needed, putting metal back in the hole is the hard part !

Cast iron will cut very easy without any cutting fluid. Drill to the smaller side of what the chart says incase your bit wobbles or wonders. Tap slow and steady. The plug should almost be flush when finished, if the cast iron is thin where the hole is, finish up with the plug sticking out of both sides. Just don't go to deep.

Here are a couple photos of where I am, and what I want to accomplish by tapping this hole. Ignore the threads in the 3/4 hole - those were surface cut with a 3/4-14 NS tap (not pipe tap) ...before I knew that the hole was undersize for a 1/2 NPT plug. They barely scratch the interior diameter.

First photo shows location of the drain hole in the 270 block.

Second photo show the 228 oil filter canister and drain line that was stock on this 228 (41 GMC project.).

I want the option of using this type of stock filter by either tapping and installing a reducer to fit a 3/8" brake line nut...or plug this 3/4" hole, and drill and tap for a full flow filter system on the other side of the block.

The drill chart specifies a 23/32" bit for a 1/2 NPT plug. Yet, 23/32" = .718...still less than the .75" hole. When I place a 1/2 plug next to, or try to thread it into the hole - it is oversize to the 3/4" hole. So... it might be best to just try using the 1/2 NPT tap, tap it as is, and see how the plug will fit.

Seems like my best option...other than enlarging the hole to fit a 3/4 plug (last resort) (57/64" drill = .89 hole) or, try to fit a threaded bolt into the hole and use a nut, fender washer, and gasket to seal the hole without tapping. However, there is no room inside the block for the hex head of a bolt to fit due to the location of the hole near the interior front of the block.

Seems like a simple thing - but I'll tell ya, with my lack of experience, these simple things require a steep climb on the learning curve...

If you're just wanting to plug the hole, you could use a well plug, which is just a rubber stopper with a bolt thru it that expands the rubber to be tight in the hole (similar to the transom drain plug for a boat). But that wouldn't work with your oil filter drain inlet.
I didn't do the math initially to see that the 23/32 drill is actually a bit smaller than 3/4", but just tapping the existing hole with a 1/2 NPT tap would likely seal since the major diameter of a 1/2" pipe plug is 0.84 inches. Since this isn't a pressure application, a little more sealer than usual on the threads should help it seal.

X2 on a plug. I'd look for a small "freeze" plug and tap it in for a more permanent fix.

SimS

Thanks to all for the tips and how-to's. I'm always learning. . Never considered a 3/4" freeze plug (never even thought about searching for one). I recently mentioned to HRL, that sometimes, the obvious is not readily seen by me. This to me is the least invasive and lowest risk way to deal with this hole in a block. May mean I then convert to full flow oil system.

Thanks again.

Tom

I suggested using a couple of 3/8" fender washers and a bolt with a piece of rubber gasket material under the outside washer. Then cross-drill the nut and bolt for a roll pin or a cotter pin to make sure the nut stays tight. A freeze plug and some brush-on Permatex sealer would be simpler.

If you want to incorporate an oil filter drain into that hole, just drill and tap the freeze plug for a 1/8" NPT thread and insert a 1/4" tubing compression fitting into the plug before you drive it into the block.
Jerry

Your answer about drilling and tapping the plug, beat me to my next question. Thanks.

Since you've got a slight spiral engraved into the hole already, I'd suggest using red LocTite on the perimeter of the plug instead of some sort of flexible sealer. That will assure that the plug stays snug, and doesn't leak.
Jerry

The freeze out plug sounds like a great idea, however the size of the hole needs to be matched the freeze out plug. Judging by the provided picture, the fact that a 3/4 tap scored the hole is suggesting that the hole is a bit smaller than 3/4”. A 3/4 tap would pass right thru the hole without scoring the surface.

You need to get an accurate dimension of the hole, then you can determine if a freeze out plug is going to work.

If it was a straight thread 3/4" tap and it scratched shallow threads in the hole, then yes, the hole would be smaller than 3/4". As I said above, tapping with a 1/2" NPT tap would probably work just fine. Then you could install a 1/2" NPT x 3/8 or 1/4 NPT bushing to connect your filter drain line, or even a 1/2" NPT x compression fitting.
A freeze plug would need a nice smooth hole to seal well, unless you gooped it up with sealant. Even then it might be liable to work loose if you tap a fitting in it. I think that freeze plug holes are cleaned up with a reamer or machined for a smooth finish rather than just drilled with a twist drill.

Measure the hole with calipers and and take the calipers with you to the parts store to measure what they have available. Don't be shy of metric ones. Engine blocks don't know the difference.

Cup type freeze plugs are pretty flexible- - - -they will fit a range of several thousandths without getting loose or leaking, and there's virtually no pressure or vacuum in a crankcase- - - -measured in inches of water either way! If it's a little tight, a drill press and a file or sandpaper makes it easy to tinker with the diameter- - - -especially with a hole in the middle for a bolt and nut. Loose- - - -a socket, a hammer, and a flat steel plate or a vise base makes for a good way to expand the diameter a little.
Jerry