HW40 Piston Seal Replacement

[TonyL]

So, after many years of Weihrauch ownership, this will be my first HW that has needed anything doing (other than a couple of cheapies bought as projects or optional fettling).

I bought this new in 2010, so 15 years old. And, just now, on the odd occasion when priming, you can tell it's not building enough pressure. If you back the piston back up and then re-compress, the pressure builds.

Still blindingly accurate, though.

So, I have a new piston seal. It looks amazingly simple to do.....back the retaining grub screw off and push the pin out securing the pump rod and simply pull the piston out? Then swab the cylinder to ensure it's clean, remove piston seal and thoroughly clean the piston. Maybe dress the air intake on the inside a little to ensure no sharp edges?

But what lube? I had been thinking of just wiping a drop of Pellgun Oil on the seal. But would silicone grease be better? And anything to do to the piston body, like buffing some moly paste in or leave alone?

Cheers peeps.

[evert]

You should stay away from silicone for the obvious reason, it makes metal surfaces gall.

I'm using Mac1 secret sause on both single and multi stroke pumpers, but have no idea what it contains. I'd use 30w lawnmover engine oil if I didn't have the bottle from Mac1.

[TonyL]

Ah, thank you for that, evert.

I was going to ask if 30w lawnmower oil might be suitable, as I have some and guessed that it might fit the bill. That's what I'll use then.

Thanks again!

[Shed tuner]

pretty well anything in this low pressure, low dynamic force situation. I often use a smear of moly when doing pumpers, just because it's handy... and they all last for decades...

As per Evert, I'm cautious with most pumps that whilst the o-ring is the primary contact point, there is some metal-to metal contact on the skirt of the piston too, so I also keep away from silicone. Obviously it's fine in a 1377/22 with the plastic piston

[TonyL]

Excellent further information Jon, thank you.

So, I might go for a tiny smear of the oil on the seal and rub some moly paste into the piston, then.

Great that the very cheap 'O' ring is the first fix needed in fifteen years.

I did feed a couple of drops of Pellgun Oil down it and held it vertically for some time when I first detected the priming pressure drop and it has helped. But it's such a ridiculously easy fix and I now have the 'O' ring, so it'll be nice to have it at its best again.

[evert]

Come to think of it, I rebuilt dads new HW40 around 2009 and THINK I used Superlube, a PTFE containing clear grease.
The gun is is still working.

One thing to remember is to NOT use spray oils/greases, as they have solvents that can diesel/ignite very easily on compression.
Or at least wait for the solvent to evaporate properly before assembling the parts. (I blew up the filter and hose connected to an FX pump many years ago).

[TonyL]

Good point re avoiding those lubes containing volatile carriers, evert.

I think that for this purpose, if I had already opened my tube of Molyslip Gearbox Additive, I might have used a tiny bit of that. I bet the Slick 50 types of additive would also be useful.

Come to think of it, if I combine two of the previously mentioned lubes, so mix up a little 30w oil with a bit of moly paste, that would make a very useful lube for this application. I seem to remember Charlie da Tuna making up a similar mix all those years ago and feel it might be quite appropriate here?