Had another crack at this bolt today....here's how I went.





Went down the heat up and crack bolt path - also liberal sparys of penetrant, tighten then loosen...sadly nothing worked - temporarily I'll admit defeat as my get Blue Coupe running deadline approaches.

So can you REALLY fill transmission fluid through where the gearstick is usually mounted - I've had a look and there doesn't seem to be much room for the oil to get into the extension housing...

A mate of mine (dealership mechanic) was mentioning that to me this week - if it doesn't undo he then tries to tighten it more, then untighten again.

Looks like I've got a few strategies to try, if only I'd thought of them earlier...ahh the situation is still salvagable!

Heat from an oxy does work. Its all about breaking the seal like boof said, not wether its going to expand or not casue thats the whole idea. It will shrink back anyhow.

this might sound stupid but i've had similar issues and one of these 2 options may work ??
try to tighten the plug .... it may break the seal and allow you to unscrew it ...
or ..
if you can get a hammer in there with a bit of swing, hit the plug 'into' the gear box .... can have the same effect as tightening it by shocking the seal.

might be worth a try but just thinking of it, the plugs are tapered aren't they ?? give it a go anyway ... doesn't sound like you've got anything to loose

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Straight to the poolroom wall with that little beauty. Well done!
Theres not to much "dancing around" going on but you have a handle on what happens with heat. The expanding of the alloy happens along its structure (like slivers or fault lines in rock) depending on the grade and the manufacturing processes used.
I say it again - the co efficient of expension of alloy is greater then that of steel - therefore the alloy will be bigger than the steel for any given temp(up to a point).
The structures of most metals can be changed with the use of both heat and cold. Keep in mind that you will only be using low heat which will grow the alloy and it will return to its original "room temp" state once it cools.

I do see what you mean - there is a bunch of stress around that diameter - heating it up relaxes the stress a little, so the diameter should grow once it is allowed to relax a bit.

I'll have another crack at that bolt this weekend...thanks for the extra explanation - helps my brain grow bigger...plenty of room in there for more!

I don't know if i can explain it very well but the reason that the hole will grow larger rather than smaller is that when you heat it up, the metal expands. If you think about the inside of the hole, if the diameter gets smaller, you are actually shrinking the metal.
When heated the molecules will have more kinetic energy and need more room to resonate. or whatever Al alloy molecules like to do with their energy. If you think of the molecules that make up the filler plug hole as a bunch of people all crowded together holding hands in a circle, if they all move in towards the centre of the circle they get more squashed and uncomfortable, which would happen if you cooled the gearbox. If the people in the circle start dancing around and waving their arms and doing star jumps, they will have to increase the size of their circle, which is pretty much what happens when you heat the gearbox housing around the filler plug.
Sorry for a pretty crappy analogy and if you don't get my drift I do understand. Just TRUST me and others who know that it works!

Maybe the heat just breaks up the rust - my contention was that if Aluminum expands quicker than steel - wouldn't the hole get smaller around that bolt?

So it's proabably more about the threads moving, the rust breaking up - that sort of stuff.

Too much physics in my head I guess.

Your right about me not having much to lose - or I can go down the breather pipe/shifter hole filling option.

Unfortunately some of you dont have a good handle on what happens to what when "a little heat" is applied.
I would not have posted "the use of heat as a last resort" unless it worked.
Your choice - pull the box out and screw around or get under the car with a flat file and safety glasses and file two flats on the rounded head then wack your wrench on it and give the alloy case around the bolt head "a little heat" A small portable LP gas torch will do fine, even worth a try with you hot air gun if you can get the heat close enough because of the low temp of the heat gun. If you use an LP torch you can keep the flame small and only heat what is needed.
this will be heapes quicker than taking the box out and travelling all over the country to get it welded, then putting it back together...........
If your still soo desperate and want to get it welded, then drive it to an exhaust shop and get them to do it "in car" whilst on a hoist.

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hmmmm..... you might want to have another look at that photo...
the stilson doesn't really look like it is gripping on the flats of the head of the bolt though, gripping on the corners will round it but it is a bit hard to tell in that pic.

your argument about heat affecting the bearings becomes null and void if you decide to weld a nut onto the bolt. you don't need much heat to make a big difference and it won't affect bearings or anything else, maybe if you really overdid it the oil MIGHT suffer but you are changing that anyway.

it is totally your decision of course but if you have a gas set or heat gun handy you don't have much to lose aye?