Gearbox Drain Plug Repair

[Colin Morgan]

When I came to change the oil in my gearbox recently, the 3/4" aluminium thread in the bottom of the box started to strip.  I have sealed the plug okay for now using the remaining strength of the thread and PTFE tape - the plug is in place, but will not properly tighten.  What have others done in this circumstance, please?  Is there an obvious next size up to tap? Or other solution?  

Thanks,
Colin

[Chris KC]

It's a bit of a long shot Colin but have you tried another plug?

Otherwise, I'm not sure I 'd attempt this in situ unless I had no choice. Once the case is on the bench it should be obvious how much meat is left around the thread. You could go one size up but I'd consider welding up and starting again. Swapping for another case might be cheaper.

[Bob Culver]

The thread in the alloy is quite short. i vaguely recalled recutting one in place 55 years ago so had a look. Seems to be 1/2 inch BSP parallel. (Assuming it the one and that this thread not the original!) Some other car plugs seem to have used this and it looks the same as original. May not be easy to acquire now. A BSPT thread and and taper pipe plug tempting but likley to leak without thread tape. Must not intrude too far.

[Stuart Giles]

Wasn't the gearbox plug 3/4" x 14 TPI from the factory, the same as the sump plug?

If so, a 1/2" BSP thread  is also 14 TPI, but is a bit bigger at .82" OD, so could well be a possibility to re-tap if there's some vestige of the Austin thread still in place.

[Colin Morgan]

Thanks for the replies.

I have not taken the plug out yet - when it started to behave strangely I tried to tighten it again - the general idea being to keep the car on the road, especially at this time of year.

Had a look at a spare box - there are only about 4 threads - the casting is thin - and yes, the gears aren't far above.

That 1/2" BSP sounds a possibility - there is no 22 metric fine, apparently, which might have been another option (if it had existed!).

[Chris KC]

I had a very similar experience when tightening mine a year or two back and found that a better plug - surprisingly - turned it from being a bit iffy to being completely OK. Worth a try.

[Colin Morgan]

Thanks - I will try this first!

Colin

[Colin Morgan]

Having been precariously sealed for a few weeks with PTFE tape, the old 3/4 14 tpi gearbox plug was successfully replaced today by a larger one - 1/2" BSP - with the car over a pit.  The original threads were very damaged and the old plug would only come out when pulled downwards whilst being rotated.  Then, once the old oil had stopped dripping, an 18.7mm drill (with added grease to collect the swarf) was used to clean up the hole. Finally, a high quality tap (also greased) on a half inch rachet drive was used to create the new thread.  This delicate and skillful operation was carried out by my friend Ken, in whom I have considerable confidence.  A newly made brass plug with a nylon washer then sealed well.  The oil level is back at 19mm below the top of the filler hole.

[Ruairidh Dunford]

Great news!

[Chris KC]

Always good to hear some 'closure' on these queries Colin, all the more so when the result is successful!