It might be posihble to fit the plunger and O ring by pushing htrough a suitabnle internall tapered dolly.

But as NDIY says, ensure that you have the correct sized O ring.

If the oil can is elderly, chances are that the O ring was Imperial, so you may have to do some sums to find the nearest Metric equivalent, which will either be a gnat's too small, or so too big that it does not self return easily!

You may get lucky and find someone supplying an assortment of Imperial sized rings, but don't hold your breath.

Hopefully nthe original ring was standard BS size, which might make things easier.

Howard

The pump O ring on my trolley jack started leaking. It too had gone hard and flattened, and I did not have the correct size in my box of assorted new seals. One comedian at a hydraulic company wanted £10 + postage for a single new O ring !

So I went back to my kit of seals and chose the one that fitted the groove, but had too big an OD for the bore. I mounted the O ring on a suitable shaft in an electric drill, (I do not have a lathe), and reduced the OD of the O ring using a file, until I could get it into the bore, where it made a nice tight fit. Pump and jack now work again.

(In case anyone is worried, this application is non critical, since if the modified O ring fails, all that will happen is the pump will stop working. If that happens the jack will simply refuse to pump up any further; the jack itself will stay where it was; held up by the non return valve, and I supplement the jack with axle stands).

O rings come in many materials, metric and imperials sizes and in various sections, The choice is VAST so you should find one that will do your job. Good luck. Noel.