Another day with some transmission fun, this time all "bottom" work.
It deserves a proper telling, but unfortunately I didn't take pictures. I got the replacement speedo gear, so ventured under the car to fit that, which was more difficult than it should have been and I still don't have it 100% complete(there's a retaining fork that needs about 4 hands, and with multiple elbows to get around the fixed cross member, but it's tight enough without it).
Basically, though, I've had a transmission leak that I think was coming from around the overdrive since I got it back. The overdrive has its own little mini oil sump, even though oil freely circulates between it and the main box, and there's a tiny screen and gasket that give some degree of filtration for the overdrive parts.
In any case, though, I started by draining the main oil out of the gearbox. MGB gearboxes, whether overdrive or standard, specify 20W-50 engine oil. Many folks want to put gear oil in them, but common GL-5 spec oil is a no-no because the GL-5 additives aren't friendly to the brass synchros. You really need to find a not-so-easy to find GL-4 spec oil if you insist, and really that's not great since 90 wt. on a cold morning is like moving the gear stick through a gearbox full of syrup. The overdrive is even more sensitive, both for not getting along with GL-5 additives and also it being way too thick for the hydraulics to operate correctly.
I pulled the main drain plug, and sure enough my nose got met with the stench of gear oil. If any of you have ever worked with gear oil, you know the smell, and there's really no mistaking it for engine oil. I let it run for about 20 minutes while I did other stuff, including actually getting the car off the ground(there's just enough clearance to get a drain pan under the plug) and getting the speedo pinion fitted. When I pulled the drain pan away, it was still dripping, but I finally just said "good enough."
The O/D unit has its own drain plug, which of course I pulled and only got a little bit of oil out of. Haynes advise
Note that even after removing the drain plug, a small amount of oil will remain in the area covered by the filter screen plate and gasket.
Taking that to heed, I spread newspaper under it, pulled the 6 bolts, and gave the cover a smack to get it to fall. Let's just say that "a small amount of oil" was probably 6-8 oz, or really it just kind of rained oil from everywhere. Thank goodness for the oil absorber. Of course this was dirty, nasty gear oil that was black and tarry aside from stinking.
Whatever was holding the gasket/screen in place, it really didn't want to let go. I spent a while trying to get it loose and got some pieces, but finally used a wire brush and just abraded the leftover gasket material away. I then cleaned a lot of nasty junk out of the pan, which BTW is about the size of my hand.
All buttoned back up, and I was set for the arduous task of refilling the gearbox. This is a "side fill" gearbox-basically there's a bolt in the side that you remove, and put oil in until it runs out the hole. My car originally came with a top fill gearbox(used 68-74, side fill came in 74.5), which has a tiny dipstick accessed by removing a plug behind the center console. It's certainly a lot of fun to try and get oil into that awkwardly placed hole.
With the side fill, however, the easy answer is a pump. The common way it's done is with a hand pump that screws on top of an oil bottle. The pump I have claims each stroke pumps 4cc. That's a lot of pumps to 6.5 imperial pints.
In any case, it was moot since I couldn't get the fill plug to cooperate. I was pretty well done for the day at that point anyway, and needed to clean up oil, so I hit it with some PB blaster and figured I'd come back tomorrow with a 6 point socket and a breaker bar.
Part of me wants to change the rear end while I'm in a fluid changing me, while another part of me doesn't want to touch any more gear oil for a while.
