Socket for filler plug on the steering box? Iirc, it's an 8mm square (8mm = 5/16 in.). I've never had an 8mm size square socket. Some alternatives:-
You could use 2 x 8mm open end wrenches positioned opposite each other to unscrew the plug.
I've used to use a 5/16 in. square brake adjusting wrench as used on old British cars.
Afaik, 5/16 in. square is a size used in refrigerator work?
I once made my own 8mm sq. wrench - drilled an 8mm hole in a length of 1/4 in. flat bar then filed the round hole into a square.
There may even be a bi-hex (12 point) socket of a size larger than 8mm that will fit an 8mm square sufficiently well to loosen the plug.
(I recall that e.g. a 15mm bi-hex socket will fit the 13mm square head filler plug used on the smaller old Fiats e.g. 500, 600, 850.)
The idler box was usually ignored in servicing. Checking/topping-up the steering box oil is mentioned in the Owner's Handbook - it stated SAE 90 gear oil.
As regards topping-up? why? unless it was leaking? I vaguely remember that the oil was to be topped up to 8mm
below the filler plug opening, can't remember if this was for the st. box or the idler box.
I do believe in leaving space for liquids to expand either due to increase in surrounding air temperature (idler box is close to the exhaust system on lhd cars; steering box is close to the exh. on rhd cars) or being heated in operation - fluids forced through a restriction will heat up and therefore expand -try feeling a cars 'shock absorbers' (suspension dampers!) after a run - be careful you don't burn your fingers!
If the oil has no space to expand into, it will find a weak spot, likely the oil seal in this case. Some units containing oil that will heat up, incorporate a breather, the 124 trans. has one on the bellhousing near the main casing, the rear axle casing has one also - these breathers should be checked if an oil seal fails.
People on various forums recommend using grease instead of gear oil in the steering box and idler box - I suspect this is to try to prevent oil leaks. Would you install grease in your shocks as a solution if they were leaking oil? I don't think so.
The correct solution for oil leaks is to strip and rebuild the units, replacing any worn bushes/bearings (so shaft's don't wobble around and impair the seals operation) and then fit new seals.
Steering box rebuilds can be very costly (if you can source the parts), so-called rebuilt/overhauled boxes supplied on an exchange basis depend on the ability and integrity of the person doing the rebuild...
I've seen mention that internal parts are not available for the idler box but that the oil seal is still available.
Some resort to fitting the other type of idler box that was used on various Fiats (+Ladas), i.e. the non-hydraulically damped unit that has simple bushes.
This I regard as a retrogressive change - Magazine Road-testers, back in the day, commented on how much nicer the 124 Sport Coupe and Spider steering was due to having needle roller bearings on the st. box output shaft and a hydraulically damped idler box.
There's another thread currently running on here by S854 ("Don't hate me..." Apr 16, 2026) who has bought an early restored 124 Spider on an auction site, he supplied a link to the site and opening this site and clicking on 'Comments', revealed various Q's and A's about this car - someone from Fiat Plus (supplier of genuine Fiat parts) / C.Obert & Co. provided current info re the idler box/availability of parts/ changing to bush type idler box etc.
Here's a page from FiatPlus concerning a 'rebuilt' Idler box, it mentions the box being filled with grease and the internal wings being cut back.... :-
View attachment 485173
TLDR?
Q. Should I just leave it be?
A. Yes.
