Drilling & tapping a "too big" vent hole on a steam radiator

branimal

I have a radiator with a vent hole that's bigger than 1/8" npt. I push a 3/8" drill bit thru the hole with no resistance. I did some googling and it seems I need 1/4" npt bushing. I ordered a bushing, a 1/4" npt tap and 7/16" drill bit. (Drill America brand).

The 7/16" drill bit keeps slipping in my cordless drills chuck. I was about to grab my 9amp corded drill but I decided to ask here first before I screw the rad up.

1. Does this rad look like it can handle a 1/4 NPT bushing? In other words, is there enough meat there to make a 7/16" hole.
2. How can I ensure I'm keeping the drill square to the hole.
3. Is there an easier solution?

Thanks.

Larry Weingarten

Hello, I can help with the question about the bit slipping in the chuck. Smarter people need to deal with the other questions! I grind a small flat on the side of the bit, where it goes into the chuck. This lets one of the three jaws grab it, to prevent slipping. It does slightly off-center the bit, which I've never found to be a problem.

Yours, Larry

KC_Jones

The OD of 1/8” pipe is .405, so a 3/8” (.375”) bit should go through.

1/4” pipe is .540” OD.

JUGHNE

Did you try an 1/8" pipe nipple in the hole first?

branimal

JUGHNE said:

Did you try an 1/8" pipe nipple in the hole first?

I tried a 1/8 npt  vent wrapped 15x with tape in the hole.  Zero grip.

Hap_Hazzard

You should use a tap handle on the tap. They aren't meant to be used in drills. You should also use some cutting oil. Tap magic is good for hand tapping.

EBEBRATT-Ed

@branimal

You don't have any other choice. Hold the drill firmly and don't let it wobble

WMno57

branimal said:

Does this rad look like it can handle a 1/4 NPT bushing? In other words, is there enough meat there to make a 7/16" hole.

No, I don't think there is enough meat there. For that reason I suggest a Heli-Coil. The hole you drill for the Heli-Coil will be smaller than the hole you would drill for 1/4 NPT.
The Heli-Coil kit is not cheap. But it is cheaper than screwing up your radiator, and then having to replace it.

delcrossv

WMno57 said:

branimal said:

Does this rad look like it can handle a 1/4 NPT bushing? In other words, is there enough meat there to make a 7/16" hole.

No, I don't think there is enough meat there. For that reason I suggest a Heli-Coil. The hole you drill for the Heli-Coil will be smaller than the hole you would drill for 1/4 NPT.
The Heli-Coil kit is not cheap. But it is cheaper than screwing up your radiator, and then having to replace it.

+1 . Use a Heli-coil and set the Helicoil insert with red lock-tite. Done.
IIRC you don't need to buy a kit, you can buy the drills and inserts individually.

archibald tuttle

can't tell from the picture, not enough light in critical areas. is that a drill cutting in the hole from early attempt to drill out to 7/16. If so I'd say it looks like there is enough metal there. I've done that before when the 1/8 threads are worn or stripped for whatever reason.

The helicoils i see take a 3/8" hole (don't know the depth of the thread, it us also probalby a lower thread. but exterior of the 1/8" thread at the wide point of the taper is almost more than that factory. Me, I'd just drill it carefully.

The exterior diameter of the tap isn't going to take more than than an additional 3/32 on each side at its widest (which admittedly is where the casting has the smallest diameter but you don't have to tap to the aboslute fullest extent of the tap.

you also might need a bottoming tap depending on the amount of backthrow in the interior of the radiator tubing (i.e. I start with a standard tap and then take a harbor freight tap and cut it off to finish).

Pumpguy

I wouldn't recommend grinding a flat on the shank of a twist drill. That will cause the drill to rotate eccentric from its centerline, probably resulting in an oversize hole.

I suggest you try using a drill motor with a keyed chuck, not the keyless type.

1/8" NPT Helicoils are available, but I wouldn't recommend their use for anything other than a permanent installation. Keep in mind that a Helicoil is a diamond cross section wire coil that fits into the special oversized tapped hole and your male fitting threads into the ID of that diamond cross section wire coil. At 27 threads per inch, that Helicoil wire is really quite fine.

Me, I'd tap the hole out to 1/4" NPT and then use a 1/4 X 1/8 reducer bushing to get down to the needed 1/8" NPT tapped hole.

To tap a 1/4" NPT hole, you need to drill out to 27/64", but 7/16" will work just fine.

Since you're drilling into a previously drilled hole, there may be a tendency of the drill bit to grab, screwing its way into the hole and taking you along for the ride.

To prevent the possibility of the drill bit grabbing, take a small sharpening stone along the center line of the drill bit and stone a minute flat on each of the drill's cutting edges. Doesn't have to be much of a flat, a 1/64" on each cutting edge will do.

When tapping cast iron, a little spray of your favorite aerosol penetrating oil on the tap is all the lubricant you need.

branimal

Looked into helicoils - interesting idea - but they are expensive for a 1/8 x 27 NPT.

I took some more pictures of the hole. @archibald tuttle - It definitely looks like someone tried to drill this out.

Did my best to take some measurements as well.

Current Radiator vent hole ID: 0.3975
Air vent threads with tape: 0.406
Boss (I'm referring to the OD of the metal circle around the vent hole): 0.7560

1/8 NPT TAP: ~0.410
1/4 NPT TAP: ~0.539
7/16 DRILL BIT: 0.4375
1/4 Bushing: 0.525

I'm going to use my corded drill (keyed chuck) and give this a shot. @Pumpguy I don't have a sharpening stone? Can i use corner profile metal file?

Pumpguy

branimal said:

Looked into helicoils - interesting idea - but they are expensive for a 1/8 x 27 NPT.

I took some more pictures of the hole. @archibald tuttle - It definitely looks like someone tried to drill this out.

Did my best to take some measurements as well.

Current Radiator vent hole ID: 0.3975
Air vent threads with tape: 0.406
Boss (I'm referring to the OD of the metal circle around the vent hole): 0.7560

1/8 NPT TAP: ~0.410
1/4 NPT TAP: ~0.539
7/16 DRILL BIT: 0.4375
1/4 Bushing: 0.525

I'm going to use my corded drill (keyed chuck) and give this a shot. @Pumpguy I don't have a sharpening stone? Can i use corner profile metal file?

No, you really need something harder than the drill bit.

Do you have anything you would use to sharpen a kitchen or pocket knife?

You could even use some 320 grit or similar wet-or-dry sand paper backed up with a flat surface like a ruler from a combination square or similar.

branimal

The keyed drill quickly made a hole thru the vent hole. Literally a few seconds going slowly. There wasn't much material to cut. I tapped it and installed the 1/4" x 1/8" bushing. Tested it with a spare vent and it works fine. (I know I need a right angle vent).

Upon closer inspection of the boss area, you can see a small hole. See pic. A very tiny amount of condensate is coming thru it. Maybe a small drop during a boiler call. If you look at the previous pictures I posted you can see the small hole as well. I'll fix that with epoxy after I strip the old paint off the radiator.

Thanks for the help guys!!! Always fun fixing these old rads.

Pumpguy

From the picture, it looks like you could run that bushing in a little deeper, of course after running the tap in a little deeper first.

Do you thing going in deeper with the bushing would close up that hole? Might be worth a try.

branimal

Pumpguy said:

From the picture, it looks like you could run that bushing in a little deeper, of course after running the tap in a little deeper first.

Do you thing going in deeper with the bushing would close up that hole? Might be worth a try.

interesting point. the hole is tapped all the way thru. I was a little hesitant to keep cranking on the bushing for fear of cracking the radiator.

Maybe I'll try with some pipe dope on the bushing.

WMno57

branimal said:

the hole is tapped all the way thru.

Yes, but because this is NPT, the hole is a tapered (cone) shaped hole. If you run the tap in a little deeper, you will be able to screw the bushing in a little further.
Use good tapping practices. Back the tap off every turn (or less) to break the chips and keep from breaking the tap or cracking the radiator. You are correct that you don't need much torque on the bushing to get it to seal.

branimal

Update on this old thread....

I stripped the radiator of the old paint and I was about to run electrolysis to remove the rust. I decided to test the radiator at 2psi to see if it could hold air. I plugged the vent hole with a 1/4 x 1/8 bushing and it was leaking. So I tapped a little deeper (with oil) and the boss area started to crumble away. I then tried the psi test and it's losing air.

Is there anyway to salvage this radiator? Or is it a loss? I can buy used steam radiators fairly cheap around here in NYC.

ethicalpaul

Are you using tape? Or just metal on metal? Tape it up and you may be able to get a seal. This is (should be) a very low pressure you are trying to seal against.

mattmia2

There are probably ways you could fix it by epoxying the bushing in or brazing a bung over the hole although if you can get a used radiator for next to nothing that might be more cost effective.

ChrisJ

ethicalpaul said:

Are you using tape? Or just metal on metal? Tape it up and you may be able to get a seal. This is (should be) a very low pressure you are trying to seal against.

Look close.
It's split vertically on the bottom side of the hole.

I'd say that's the end of the radiator. Even if you can get that crack to seal it's going to spread.

branimal

yes - the test was done with pipe dope.

Brazing cast iron is beyond my skillset. I do have an old Lincoln stick welder. Not sure I want to epoxy it. It's going into a rental unit. Renters don't care about dripping water on my brand new floors.

Yeah once the multiple layers of paint were stripped off, the crack become evident. Or maybe it happened when I tried to re-thread deeper the second time.

Pumpguy

If the iron is sound, brazing (torch bronze welding) is not that difficult for anyone with a bit of experience with an oxy -acet torch. That said, that iron doesn't look too sound to me, kinda crumbly.

If it were me, I'd give it a go, but not have very high hopes of success.

Mad Dog_2

When I have one of those " wobbly " tapping in a radiator, I can often tighten it up with a short 1/8" Black nipple with lots of Quick Wick (Thin) and it will grab and usually hold. I've never tried it, but Propoxy claims you can drill and tap it...mad Dog 🐕 

AdmiralYoda

Since this is a rental unit... here is a cheap idea for you to try. First...this only applies if there is another boss for taping on the radiator. If there is...tap that for a 1/8 NPT and use that as the vent.

For the busted hole:
Install a high temp expandable plug and crank it tight. Leave remaining boss and the internal threads intact, they will help provide some bite for the rubber plug.

Since this is low pressure it might do the trick...or it might not. But if you have another boss to tap a vent into you could fix the broken hole for cheap. Not sure of the longevity but might be worth a try.

https://mcmaster.com/products/plugs/expandable-high-temperature-tapered-plugs/?s=rubber+plug+expanding

ChrisJ

AdmiralYoda said:

Since this is a rental unit... here is a cheap idea for you to try. First...this only applies if there is another boss for taping on the radiator. If there is...tap that for a 1/8 NPT and use that as the vent.

For the busted hole:
Install a high temp expandable plug and crank it tight. Leave remaining boss and the internal threads intact, they will help provide some bite for the rubber plug.

Since this is low pressure it might do the trick...or it might not. But if you have another boss to tap a vent into you could fix the broken hole for cheap. Not sure of the longevity but might be worth a try.

https://mcmaster.com/products/plugs/expandable-high-temperature-tapered-plugs/?s=rubber+plug+expanding

But that's not going to seal that crack, if anything it'll help it spread?

mattmia2

If you wanted to save it you would have to drill at the end of the crack, grind it wider, then fill it with brazing material, then braze a bung over the remnants of the tapping.

AdmiralYoda

@ChrisJ uh oh... didn't see the crack... Your time would probably be better spent finding and restoring another radiator unless you are handy with a torch!

https://youtube.com/watch?v=2k0NhkbKEcM

ChrisJ

AdmiralYoda said:

@ChrisJ uh oh... didn't see the crack... Your time would probably be better spent finding and restoring another radiator unless you are handy with a torch!

https://youtube.com/watch?v=2k0NhkbKEcM

I had a feeling that's what happened.
I tried to word it so it wasn't disrespectful as I felt it was a mistake.

ethicalpaul

Crack or no, I'd have to try that real good, long-set-time JB Weld before I trashed a radiator I just had cleaned. Again, the pressure we are talking about here is so low.

I'd put a short nipple in there with this stuff on it, then apply more liberally around it

https://www.amazon.com/J-B-Weld-8281-Professional-Reinforced/dp/B014OVHAOY

ChrisJ

ethicalpaul said:

Crack or no, I'd have to try that real good, long-set-time JB Weld before I trashed a radiator I just had cleaned. Again, the pressure we are talking about here is so low

JUGHNE

IIWM, I would go for another rad.

This time check all the threads.

I would remove the old inlet spud first, I had to cut one out on an newly power coated rad and over cut into the bushing , but was able to seal the new spud.

But you risk scratches on the new paint etc.

I would pressure test even before paint stripping .

AdmiralYoda

Here is a silly arts and crafts idea that shouldn't take specialized tools or skills. Might be worth a try to save the radiator.

Don't try tapping the 1/4" hole any deeper. Leave it as is. Drill a hole at the bottom of the crack, doesn't need to be big. Use a thin metal saw blade and cut along the crack from the threads down to the drilled hole.

Use a flexible, high temp epoxy to glue in the 1/4"-1/8" bushing with the 1/8 vent pre-installed into the bushing. Use plenty and don't overtighten. Snug but not tight tight. Then fill the crack with the same epoxy. Let it cure a couple days then pressure test to 2psi.

You'll want an epoxy that can handle 250F+ and has a high elongation percentage...that's its ability to stretch.

Here is a link to McMaster's toughened, flexible epoxies. Not exactly cheap and you'll need a cartridge gun.
https://mcmaster.com/products/epoxies/toughened-epoxy-structural-adhesives/

Here is a product from Permatex. More user friendly. 2 part epoxy in separate tubes.
https://www.permatex.com/products/adhesives-and-sealants/epoxies/permatex-steel-weld-epoxy-2-oz/

branimal

mattmia2 said:

If you wanted to save it you would have to drill at the end of the crack, grind it wider, then fill it with brazing material, then braze a bung over the remnants of the tapping.

I have another rad that has a similar issue. I don't need either of these rads right now. If I find a deal on fb marketplace for a brazing torch I might have a go at this. I can weld ok. And I know how to solder copper. Brazing looks a bit tricker. If i fail, I could sell the oxy-acet torch and trash the rads.

What do you mean by brazing a bung over the tapping?

mattmia2

https://www.mcmaster.com/12555K69/

Big Ed_4

Never use a bushing on cast , always a coupling . A bushing works like a wedge.....

mattmia2

Big Ed_4 said:

Never use a bushing on cast , always a coupling . A bushing works like a wedge.....

How is the taper on a nipple going to be less of a wedge than the taper on a bushing?

archibald tuttle

you could grind small flat on dri

mattmia2 said:

Big Ed_4 said:

Never use a bushing on cast , always a coupling . A bushing works like a wedge.....

How is the taper on a nipple going to be less of a wedge than the taper on a bushing?

I have the same question. its i guess if you have steel volume stacked as in the bushing format so the radius of steel vs. air/steam is changed, perhaps the expansion with heat would make greater expansion with more steel so it would push a little harder against the cast. although if we're talking a cast radiator, that doesn't seem to threaten integrity of the rad, although maybe folks are suggesting over time it could work the threads?

Big Ed_4

The bushing is the first wedge against the cast , the nipple would be the second wedge against the cast . Bushings are not perfectly machined . If you sock them up you can crack the cast . Ask me how I know that . Safer with a nipple and coupling.. I agree a bushing and a 1/8" vent is not as much of a concern . Just adding a little helpful info

retired1

I'm not a plumber but a landlord. I recommend you trash this radiator that is in need of a weld because of significant corrosion around the thread. Air valves always fail and stop releasing the air pressure they are supposed to release. In 2016 in the Bronx, two toddlers died when a valve burst and steam filled their bedroom. It was a 48 unit 50,000 sq. ft building and the landlord had the boiler cranked up to get heat to the entire building. The valve failed and the pressure caused it to blow.

Steamhead

retired1 said:

.... the landlord had the boiler cranked up to get heat to the entire building. The valve failed and the pressure caused it to blow.

If it takes more than 4-6 ounces to distribute steam thru a building, that's a PROBLEM. The 1-pipe steam system in the 32-unit building in our FAC ad drops to low fire at 5 ounces. By then all the radiators are substantially hot.