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MG Midget and Sprite Technical - Leaking diff drain plug

Is a couple of wraps of PTFE on the diff drain plug a viable/long twerm solution to this small but annoying oil leak?

Les
l snowdon

I thought it was a taper thread so shouldn't need PTFE tape?
Daniel Thirteen-Twelve

Where did you get that info from Daniel?

All taper pipe threads require some kind of sealant, PTFE IMO is ideal and is industry standard.
The other types of fitting are parralell threads that have a copper of fibre sealing washer, taper thread rely totally on some kind of sealant.
Robert (Bob) Midget Turbo

Robert,

The design of a taper pipe thread coupling with a straight or taper pipe thread is intended to form a pressure tight seal without the aid of a sealant, Machinery's gives the relevant standard as BS21:1973 which may have been superceded by now. One point though is that the seal may be accomplished by local deformation of the thread material as the thread form will never be perfect and thus requiring a high torque to form a seal. While a sealant can be used to aid sealing the likes of PTFE also help as it is an extremely effective dry lubricant allowing the joint to be done up more tightly with less force and filling any minor voids.

Sort of reminds me of people using sealant on the gas bottle fittings such as oxygen, acetylene, propane etc, POL type fittings, if required something is wrong with the couplings.
David Billington

If it's a drain plug then why not replace it with new (and the oil if it's over about three years old as neither are expensive)

I was told that there are two thichnesses of PTFE tape(?) certainly the stuff I've seen recently is so thin you need a few wraps to build it up on the thread but I've still got some from years ago that's a lot thicker

I know there's a thread about drain plugs because that's where I learnt the new drain plug socket is 3/8" hex instead of 1/2" square and some suggest using plumbing fittings

My drain and fill plugs only go in a couple of threads so the PTFE tape wouldn't have much to go at
Nigel Atkins

Yep, PTFE is the way to go with taper threads. One and a half turns of tape, no more, no less is the standard RAF way for PTFE, thread sealing, for the use of. For my car though, I wind it round about twenty times till you can hardly see the thread!! (joking-about 2 or three turns is usually about right!)
John Payne

Hi Bob,

I have taper thread fittings on my car that haven't been fitted with PTFE tape or any other kind of sealant and have never leaked. I'm not wholly certain but they are probably all NPTF threads and the fittings will be alumnium alloy.

I also thought that taper threads have been around a lot longer than PTFE tape.

I'll have another look at the drain plug on my car's back axle to see if it drips or not as I know I didn't use tape on it and it's probably the only steel on steel fitting on the car.

I do have PTFE tape (several partly used rolls) but almost never use it.

I see that Carrol Smith agrees with you but he doesn't say tapered threads were designed for use with a sealant... he just says always use one.

Totally agree about parallel threads being sealed by copper or fibre washer though in the past I've also used bonded (Dowty) seals, usually to replace an O ring.
Daniel Thirteen-Twelve

Daniel,
I agree with you about taper threads being around long before PTFE and I have never used it on cars at all (old or new).

However like many things that have been invented long after the original design I guess there is a case for using it if it improves the sealing qualities of the joint. The downside is that many people with leaks may be tempted to use it when they really should be renewing the plug as it is probably damaged.

Trev

T Mason

This thread was discussed on 04/05/2011

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