I've changed both driveshaft boots on both sides as they were tired and leaking a touch.

I couldn't get the filler plug out the box so I removed the speedo drive to get 1.1ltr of oil in.

I've had a look and there's a minor leak from the area dropping a couple of drips. Now I could remove the drive and fit a new gasket but it was in good condition and the drive to box is a well machined fit.

It looks to be coming from the cable fitting area which you tighten up to the drive thread.

So I ask is there an O ring or seal that has failed? I cannot see how the drive can not be oil tight without a seal....

It may be that the box is now overfilled, as the filler also acts as the levelling "device". The increased splash will then cause or accentuate leaks. Although there is a measure of how much oil goes in the box, it isn't always exactly what is needed as there can be some residual oil left in.

I can't remember if the speedo drive has a seal but there may be meant to be an "O" ring inside the machined hole that it slides into. i have vague but maybe inaccurate memories of a small gasketwhich goes between the drive and the box.

Machined surfaces can be so good that a gasket isn't needed; an example is the joint which separates the two halves of the box.

Anyway, my best guess is too much oil. Additionally I think you will have to accept a certain amount of "weepage" of oil from the box over time, possibly from various locations.:bang:

Thank you for your reply.

On the Gerstl site the part is listed as one piece for Euro55. It doesn't come apart however I'm curious to know if there's a small O ring in the drive. Whilst I can't confirm, I think it wasn't wet before I disturbed it.

I agree re weepage. I've done the driveshaft boots and they are both dry. There's a small weep from the gear selector rod but not a drip.

I suppose the best course of action is to try to get the filler plug out to see if it's running a touch high. Better to have more than less! The swine's stuck and broke my favourite tool!!

I'll think I'll whip it off and assemble with a touch of sealant and go from there. Not the end of the world in the grand scheme of things!

Good advice on getting that square plug out in recent posts here:
https://www.fiatforum.com/500-classic/355964-newbie-1970-fiat-500-owner-4.html
There is definitely an "O" ring as I describe in the hole that the gear selector slides in.
A tiny bit of quality sealant on the speedo mating surface won't do any harm.