can anyone tell me the size of the o-ring?

It's not an o-ring; it's actually a thin metal washer with a rubber coating – rounded on the upper side, flat on the underside. The aperture it sits in has an ID of 24mm and on OD of 27.6mm.

I suppose it could be replaced with an o-ring. Something like 1/8th of an inch with a suitable ID.

It's purpose appears to be to mate with the seal of the BB cup and prevent the ingress of water/dirt into the bearings and around the LH crankarm splines.

Isn't there a thin rubber 'o' ring on the inside edge of the mating face?

Shouldn't have edited the last one!! Do you mean this…

PS: anyone got an arm they can pop an image up of the area I'm talking about?

EDIT: no matter, clarified by stuartie_c.

183mm front M4 and 160mm X2 rear is probably the most versatile brake set-up around, especially with the adjustability of the Tech lever.

Design and build quality complete serviceability; the ability to bleed easily and without spending another £40 on essential bleed kits; and, should you need it, customer service, are all superior with Hope. Plus, your consumer pounds will be staying within the UK economy and supporting a (relatively) small business.

The problem with using a socket/extension to press a bearing in is that while the outer edge of the socket fits the aperture, the end face, which is going to be hammered against the bearing, is typically chamfered and actually 1mm or so inside of it. The risk is that you're going to be applying the hammering force directly onto the seal. So it's easy to turn a socket into a decent drift, just file/machine the chamfered edge down so that the corners are sharp/square and can be seated against the metal casing of the bearing.

Marzocchi was then bought by a big US suspension manufacturer, returned production to Italy.

2007 was the last time Marzocchi manufactured forks in Italy. OEM stuff went out to Taiwan (Suntour) in 2005 or 2006, then the aftermarket fork production followed for the 2008 line-up. Tenneco bought Marzocchi in 2008, but all fork production is still done in Taiwan.