Small Gearbox Problem 2 Years In The Making

[JimX]

This is mostly just a warning to anyone that has bought a Skyline and never changed the gearbox/diff oils or let the workshop's apprentices do it. Make sure all your gearbox and diff drain and fill plugs aren't done up too tightly or it can split the casings!

My story:

A few months ago I noticed a small but growing patch of oil under the car in the garage. At first I thought it was just spillage from my general servicing but as it grew I realised it was transmission fluid by the smell. So I decided to have a better look under the car while I was upgrading my turbos and noticed a very thin film of oil over the whole gearbox. The leak seemed to be coming from the top of the gearbox.

There were 2 leaks in the gearbox!

The first one was the rubber boot on top of the gearstick had split over time, so it was no longer sealing and there was oil leaking out around the top and splashing down all over the sides of the box. This was a $20 part from Nissan I think (2 days for it to arrive) and very easy to fix.

I thought that was that, but after a quick test drive I noticed it was getting another film of oil starting from the drain plug. Long story short, the compliance yard had done up the filler plug too tightly and it had created a hairline split next to it over time. However it had only just started to crack it enough to start leaking oil after about 20000km of driving, so it was only done up a little too tightly. Over around 3-4 months I had lost around 300cc of gear oil, which may well be too much to lose even with normal driving. Fortunately gear changes were still very slick and I didn't have too many gear shavings on the magnet and the oil itself is very clean, but I still feel there are more than there would be had I lost no oil.

There are few ways to fix the split casing (as I found from searching in here), from dropping the box and welding it properly from the inside, or using some sort of putty or welding it in place from the outside. Since my leak was very small and I got several months warning about it, I went the cheap and nasty fix to see if it works. I cleaned the whole area around the split, applied some thread lock into the fracture, smoothed it down and pushed the thread lock into the crack as best I could. Then cleaned the surface again and smeared on a thick top coat of JB Weld and let set for 24 hours before re-filling the gearbox (I'm about to do the last stage now). I will check it every few weeks to see how it holds up over time and report in here.

The funny thing about the split is it doesn't start right at the threaded part of the filler hole. There's a 1cm gap of flat unbroken metal before the split starts and goes for about 2cm. I did plenty of overlap with the JB Weld to make sure I covered any splits that I couldn't see yet.

I do have one question which I couldn't find an answer to after a lot of searching and will be useful to anyone changing their gearbox/diff fluids and not wanting to kung-fu chop their gearbox/diff housings like a bonus stage in Mortal Kombat. Does anyone know what the torque settings are for the drain and filler plugs are? If I can't find any information I'm going to treat them like a taper-fit spark plug (just a pinch of a turn with the socket after finger tight or so)

One last hint for anyone unable to crack those over-tightened drain and filler plugs. Heat them up with a little butane blowtorch. I used a $30 one I got from Jaycar but a cigarette jet lighter might be enough. I was unable to budge the drain or filler plugs at all, so I tried heating the plugs uniformly around the edges for about half a minute and then they both undid very easily after this.

Many thanks to wrxhoon who helped me find the sources of these leaks and their likely causes.

[JimX]

I found the torque specs I used for the Suzuki Swift GTI that I changed the gearbox on recently. A completely different make/model, but it seems to have the same sort of large tapered fit plug into an alloy casing so I imagine these should be fine. I'm going to use them anyway.

Oil filler/level plug: 36 to 54 Nm

Oil drain plug: 25 to 30 Nm