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MG MGB Technical - Rear axle drain bolt

Wanted to change the diff oil today and something really really annoying (close to the point of tears!) happened! I've managed to shear/split the top part of the square bolt that stickes out from the casing of the diff where one puts the square shaped tool to screw it out. It has split and twisted on two corners! I was sloppy thinking it would not take much effort so I did not put all the tool all way in. Bit of effort required as it turned out and as I pushed it went a bit round and then the two corners split...

Any suggestions for what to do next - these will be VERY MUCH appreciated


Can you weld something to it to grab?
Art Pearse

You might wind up having to drill the remnants of the bolt out. See if you can locate some left handed drill bits. Drill a pilot hole, with the smallest bit, and proceed to enlarge the hole in graduated steps. When you reach the same size as the bolt, the remaining part of the bolt should spin right out and re-threading the diff won't be necessary. When you replace the bolt, use a brass pipe plug to prevent a repeat of this happening again. RAY
rjm RAY

Weld a piece of square bar in whats left of the drain plug. When out convert the plug by welding on a hex nut.
Allan Reeling

Mine did the same. There was enough left sticking out to use a drift and hammer first on one side then the other.

It now has the stub of a 3/4" AF bolt welded to the end.

It's a tapered thread, doesn't need to be done up 'tight' as it only gradually tightens unlike a standard bolt that tightens rapidly once it starts.

PaulH Solihull

Thanks for advice - welding not possible so will try the other methods. The next hindrence will be getting the axle high enough of the ground to get proper access (it's the drain bolt underneath not the filler) - also one of the reasons it went wrong. I could not see the bolt properly, was trying at arms length - so a lesson learned - make sure I have good access next time and in a secure safe environment...!


There is some taper to the sides of the hole, one source of the trouble with these things. Many tools do not go all the way to the bottom, especially if there is dirt/grease in the square hole, hence all the pressure is exerted at the top. Further down the hole the sides are supported by the threaded-in part, so it does not split or spread, and so does not slip.

If you make a tool by filing/grinding an appropriate sized bolt to the correct size and taper, you can usually get them out (and prevent the problem to begin with). I have never had to resort to welding etc, though I have any equipment you can imagine, and have met many damaged plugs.

Note that the bolt/blank for the tool can be metric or whatever is to hand. Extensions for cheap socket sets are a good starting point too.

If you do have to weld, just take it somewhere and have them do it; your axle will survive another while without an oil change - most never get one. The simple act of welding on the plug and letting it cool usually leaves it loose.

FR Millmore

I mean of course 'off the ground', I've started to make a square 'peg' out of an old extension from a socket set using a grinder...actually FR you've hit the mark because on closer inspection the splitting has not gone all the way down in the 'hole', so hopefully I'm saved.
More later....


You have a problem getting it out - plenty of advice here - many years ago, I replaced mine with one that needs a key - supplied by Moss Europe - ask Martin Smith in London.

R Walker

To get the greatest access to the rear of the axle support the body at the rear spring front hangers and let the axle hang down - with due consideration of whether the rebound straps are up to taking the full weight while you are working in the vicinity!

Previously I had made an adapter from a large bolt, and tried a 1/2" 'wobble' socket extension, but both still needed to be pressed into the plug very hard to stop them 'turning out', if they worked at all. There's plenty of room for a hex headed plug, if they can put one on the side, let alone the bottom of the gearbox, I don't see why one couldn't have been used for the axle level plug. I did try a gearbox level plug, but although it is the same thread it's at a wider part of the taper so wouldn't go in the axle.
PaulH Solihull

This thread was discussed between 27/06/2012 and 29/06/2012

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