Diff drain plug

[aggie]

There isn't one on early Mk 1 Spitfires which makes oil changes quite a drag.  I've heard that it's feasible to drill and tap the diff casing and install a drain plug.  Anyone out there ever done this or knows anyone who has ?

[rogerguzzi]

I have done on several Triumph differentials usually when overhauling them but is possible to do from under the car.

Just drill in the center of the casting area that was intended for it, as for what thead to use I did mine 1/4 bsp but any thead would do as long as you have a bolt of the right size.

You can swill any swarf out by just pouring some paraffin into the filler hole.

I fit a magnet into my drain plug so it you are unlucky and shead a tooth it would be caught so hopefully you could get home.

Roger

[Clive]

And to add, I am pretty certain none of the cars had drain plugs. The only possible exception may be very early Heralds?? but that is only a possibility.

As Roger says, almost any thread will do, but something with a taper thread isn't a bad idea, so it matches the other drain/filler plugs on the car. Maybe 3/8bsp?

[JohnD]

Agree, and to remove any doubt, its the plug that should have a taper, the drilling in the diff. case can be straight.

A sump plug is ideal.   3/8" x 18 NTPM

John

[Colin Lindsay]

1 hour ago, clive said:

And to add, I am pretty certain none of the cars had drain plugs. The only possible exception may be very early Heralds??

Yes, they had drain plugs fitted at the factory; the practice stopped fairly early on in production, probably to save money.

As the guys have posted, drill through the centre of the casting rib and plug with a sump plug, John's right in that it should be tapered to prevent it falling out.

[DerekS]

I did mine ( Spit 1500 ) a few months ago. On mine, there was a dimple in the casting that turned out to be the ideal spot. I've tapped it to 5/16" UNF and used a short set-screw with a sealing washer. It won't come loose but I will keep an eye on it.  I used the cap of a spray-can around the drill to stop oil being flung around as the drill broke through and had a container ready to catch the oil.  The size was big enough, the oil drained easily. Drilling cast iron is straightforward,  keeping the drill speed low and not putting a lot of weight on the drill helps.

[aggie]

Well, that about seals it ( excuse the pun ).  I was under the impression that it was only early cars ( see note on rimmerbros web ) but that's obviously not the case.  Thankyou all, I feel fairly confident about doing this now.

[DerekS]

The casting was just over 5/8" thick at that point, so plenty of meat for a good thread and well clear of any rotating internal parts, too. To be honest,  I was slightly apprehensive about drill-snatch on breakthrough, that's why I took it nice and easy. Well worth doing, good luck!