Pipe thread question
- Thread starter paker
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SGKent
Banned
weld a bung or if there is room, weld a nut on the inside. Most gearboxes have vents in them. Can't you use that vent?
sberry
Banned
Something I have done is tap/thread a 3/8 x 1/4 bushing, jb weld it after threading. If you can live with 1/8 can do that too.
What’s the wall thickness? Should need around 3/8 or so for 1/8 NPT
https://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html
Also, don’t forget pipe threads are on a taper. So the more you tighten, the more ‘wedge’ you’re driving into the part. Think of it like a wood splitting wedge. As you go in, the force goes out. You could damage the aluminum housing if you go too tight.
So, yeah use the 1/8NPT for max threads. How much oil and how often does it get changed? 1/8NPT isn’t very big and will take time to drain. If it’s a lot of oil or a lot of changes, I’d go the weld a bung route and go as big as I could stand to make oil changes faster.
https://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html
Also, don’t forget pipe threads are on a taper. So the more you tighten, the more ‘wedge’ you’re driving into the part. Think of it like a wood splitting wedge. As you go in, the force goes out. You could damage the aluminum housing if you go too tight.
So, yeah use the 1/8NPT for max threads. How much oil and how often does it get changed? 1/8NPT isn’t very big and will take time to drain. If it’s a lot of oil or a lot of changes, I’d go the weld a bung route and go as big as I could stand to make oil changes faster.
bwringer
Well-known member
I would ABSOLUTELY NOT use tapered threads here. Great way to crack the casting.
There are skrillions of oil drain plugs hanging on the wall at your nearest auto parts emporium. Pick one with reasonably fine threads.
There are skrillions of oil drain plugs hanging on the wall at your nearest auto parts emporium. Pick one with reasonably fine threads.
matt_i
Well-known member
A 1/8-NPT is 27tpi, so the threads are fairly fine imo. Classic drain port for so many industrial gearboxes.
The judgement call is on whether the "wall" can handle the wedge/hoop-stress of the tapered pipe thread.
There is the route of going to an annealed copper or other sealing washer, design an o-ring face seal. The annealed copper is sold at my local Ace.
Welding is OK but if this is an assembly, some percentage of the oil that wets the innards, is going to "coke" into hard destructive particles due to the mega heat. Welding an oiled casting is not a good recipe for success especially if you've run into oil contamination with tig welding and the solution is usually to use oxyacet to burn-off the oil so the casting can be cleaned, also leading to more "coke". Also very bad if grease-lubed bearings get enough heat to "coke"....
Tapping isn't perfect either as metal chips can get involved into places you don't want them but there's always the route of washing them out. Coked oil is a time bomb imo, no telling when its going to let go.
The judgement call is on whether the "wall" can handle the wedge/hoop-stress of the tapered pipe thread.
There is the route of going to an annealed copper or other sealing washer, design an o-ring face seal. The annealed copper is sold at my local Ace.
Welding is OK but if this is an assembly, some percentage of the oil that wets the innards, is going to "coke" into hard destructive particles due to the mega heat. Welding an oiled casting is not a good recipe for success especially if you've run into oil contamination with tig welding and the solution is usually to use oxyacet to burn-off the oil so the casting can be cleaned, also leading to more "coke". Also very bad if grease-lubed bearings get enough heat to "coke"....
Tapping isn't perfect either as metal chips can get involved into places you don't want them but there's always the route of washing them out. Coked oil is a time bomb imo, no telling when its going to let go.
That would work well if the OP is able to spot face the casting. That'll require a rigid setup and drill press at minimum but preferably a mill. Not going to work if he's planning to use a drill motor.
Keithinsc
Well-known member
I will second the opinion on NOT using a tapered thread. Wedge action of the taper is too easy to crack something. Cast iron, I might be tempted though.
Tap it to fit a motorcycle type drain bolt, and use a crush washer.
Tap it to fit a motorcycle type drain bolt, and use a crush washer.
Falcon67
Well-known member
FWIW - Early C4 trans cases (Ford - 1970s) use a 1/8-27 NPS thread with an o-ring. The case has a slight boss, with the threaded port and a taper where the o-ring seats. Hand tight they will seal 30+ PSI air. .250 inch wall probably not enough for threads and a taper for an o-ring. For a drain plug, NPS plus a sealing washer may do it.
People that are lazy or maybe don't know better have used 1/8-27 NPT type fittings and have managed to crack the case at the boss. Some get away with it, some don't.
People that are lazy or maybe don't know better have used 1/8-27 NPT type fittings and have managed to crack the case at the boss. Some get away with it, some don't.
Wrench97
Well-known member
Biggest hole as possible to get the oil and debris out as quickly as possible nothing like having to stand around for 20 minutes waiting for 1 quart of oil to drain out 
fsae0607
Well-known member
Biggest hole as possible to get the oil and debris out as quickly as possible nothing like having to stand around for 20 minutes waiting for 1 quart of oil to drain out
That's what beer is for
To the OP's point, I'd likely use a reasonably-sized drain plug and sealing washer. I agree that the case could crack with NPT threads.
BillK
Well-known member
Paker,
How thick is the casting ?? Unlike the others I would have no problem using a NPT plug. Unless you have no self control when tightening the plug I cannot imagine breaking the casting. 1/8" pipe is awful small for a drain plug. I would use 1/4" pipe. All it needs is a few threads.
Thats my opinion,
How thick is the casting ?? Unlike the others I would have no problem using a NPT plug. Unless you have no self control when tightening the plug I cannot imagine breaking the casting. 1/8" pipe is awful small for a drain plug. I would use 1/4" pipe. All it needs is a few threads.
Thats my opinion,
My apology for a late reply. The gearbox is a small transfer case in a car without a drain hole. The unit is sitting next to a catalytic converter and exhaust pipe. Even with heat shields, the transfer case oil becomes a black goo. Premature failures at <100k miles have been reported. When opened, black oil with fine solid suspension reduced to 1/2 of the original volume.
The transfer case has a vent and a fill hole (1/4" npt). Draining through the fill port was a pain. I did it once and am determined to add a drain hole. The same car is allegedly sold in the Middle East with a drain hole in the gearbox. For your reference, the repair job costs $700 for the part + >$1000 labor.
I don't have the skill or tools to remove the transfer case. So welding a boss is not a practical solution. Some brave owners added a 1/4" npt plug. Based on their photos, not even 4 threads, 3 threads max. That's why I am considering 1/8" npt. Since I am the only person who will use the drain port, I am not too concerned about cracking from over-torquing.
Between 1/8-27 npt and 1/4-32 (or 28), which would you recommend? Thank you.
The transfer case has a vent and a fill hole (1/4" npt). Draining through the fill port was a pain. I did it once and am determined to add a drain hole. The same car is allegedly sold in the Middle East with a drain hole in the gearbox. For your reference, the repair job costs $700 for the part + >$1000 labor.
I don't have the skill or tools to remove the transfer case. So welding a boss is not a practical solution. Some brave owners added a 1/4" npt plug. Based on their photos, not even 4 threads, 3 threads max. That's why I am considering 1/8" npt. Since I am the only person who will use the drain port, I am not too concerned about cracking from over-torquing.
Between 1/8-27 npt and 1/4-32 (or 28), which would you recommend? Thank you.
Last edited:
SGKent
Banned
what kind of car?
LS6 Tommy
Well-known member
SGKent
Banned
seems to be a common issue. Found lots of threads where folks added a drain plug to the PTU. This is one. There are other threads online with exact spot to drill and tap.
https://www.fordedgeforum.com/topic/26795-ptu-drain-plug/?page=10&tab=comments#comment-148567
I would be inclined to use a bolt with fine threads & a sealing washer like used on oil pans . that way you don't have the worry of the taper putting outwards pressure on the casting.
even get a replacement common thread drain plug & tap the casting to that
even get a replacement common thread drain plug & tap the casting to that
claymont
Well-known member
You only need a couple of threads (3 or 4) in the casting to hold the plug in. Since its tapered, it'll lock up nicely. You're not going to need a lot of force to hold this plug in place. Loctite makes a thread sealant with a thread locker that works well.
Measure the bottom of the plug and mark that on the tap; go two turns more and check plug depth. Adjust tap depth to suite your required plug depth. Leave a few plug threads proud of the case to allow for expansion of case threads when you remove and reinstall the plug if you over tighten it.
Loctite 483629 Thread Sealant - High Performance, 1.5 Fluid_Ounces
eBay item number:
192414301522
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buzzworth
Well-known member
1/4" if you have room for it. Use a brass plug, the threads will seal better.