Oil can O ring replacement

Some time ago I used an oil can to inject woodworm treatment, this was fatal to the can - lesson learnt! The can has been sat around for some time, today I took it apart. The O ring has gone hard and nolonger fits tightly in its grove on the plastic piston.I tried replacing it but it comes off as soon as I try to push it past the threaded entrance to its plastic chamber. I plastic cap screws in to seal it. Assuming that I can get a replacement ring that fits tightly how do I persuade it to go in without it pulling off. Is there a trick? I assume that when made it must have gone in easily. It was only a cheap can, but if possible I would like to repair it, or am I wasting my time? Has anyone elsedone it?
The handle pushes a rod that has a plastic piston on the end, the O ring fits in a groove on the plastic piston. The piston pushes against a spring to return it. This is held in place by a plastic screwin cap.

Seems like you have the incorrect O-ring for this application?

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

Thanks Howard.

As O rings are cheap I will give it a go, little to lose. Does the ring want to be a tight fit, i.e. need to stretch to fit or as exact as possible. My gut instinct is for it to be tight to give it a chance of staying in place. looking at sizes online I can't see an exactly identical size, assuming I have measured it correctly.

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).

The o-ring cross section should be such that it is compressed slightly between the bore and the bottom of the groove. It should be slightly loose in the width of the groove. The diameter of the o-ring should be such that its ID is close to the bottom diameter of the groove.

Good luck finding the correct size.

Russell

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.