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?

[On-the-target]

I use Pelgun oil to occasionally lube the piston seal on my HW40 with no problems.

[TonyL]

Is that the odd drop occasionally, applied to the piston and allowed to migrate down with the pump tube held vertically, or pull the piston out and apply the oil directly to the seal? I've done the former once or twice over the years and also recently, where it did seem to help restore sealing.

[PhatMan]

Hello to All,

I had it direct from the old git that is Mr. Tim McMurray (of Secret Sauce fame), that 30 weight motorcycle fork oil was just as good as his Secret Sauce.

Have fun

Best regards

Russ

[evert]

Hello to All,

I had it direct from the old git that is Mr. Tim McMurray (of Secret Sauce fame), that 30 weight motorcycle fork oil was just as good as his Secret Sauce.

Have fun

Best regards

Russ

Thats some good info! Fork/dampener oil should be good for seals.

[On-the-target]

Is that the odd drop occasionally, applied to the piston and allowed to migrate down with the pump tube held vertically, or pull the piston out and apply the oil directly to the seal? I've done the former once or twice over the years and also recently, where it did seem to help restore sealing.

Exactly as you suggest "the odd drop occasionally, applied to the piston and allowed to migrate down with the pump tube held vertically" My own HW40 is showing signs of needing the treatment, will be giving a few drops of pellgun shortly.

[TonyL]

Hello to All,

I had it direct from the old git that is Mr. Tim McMurray (of Secret Sauce fame), that 30 weight motorcycle fork oil was just as good as his Secret Sauce.

Have fun

Best regards

Russ

Thank you, Russ. Yes, come to think of it, 30wt. fork oil has been mentioned by some in the past.

[TonyL]

So, yesterday I was just about to remove the piston and replace the seal. Before I did so, I thought I'd just try it out to see how it was performing. The answer was "very well". I had about 15-20 shots with it and all was good. So, obviously, working the Pellgun Oil through had worked nicely. Apart from one shot at the end where compression was low.

So, before I replaced that seal I thought, "why not give it a go with the 30wt. oil and a little moly". Applied a couple of drops of oil to the piston and held vertically and then applied a small amount of moly paste to the piston body and applied another drop of oil to make a runny black "moly oil". Whilst working it through the pump, I noticed that at certain positions of the pump stroke, with the pistol held upside down you can see and gain access to the air inlet hole on the pump tube. I thought this would be a better way of getting the oil where it was most needed, ahead of the piston. Applied one drop of oil and held pistol upside down for a while to allow it to get into the tube properly. Then worked it a few times and wiped any bits of excess oil off.

Wow! Smoother AND compression seems much fuller and priming effort now much higher and more consistent. Had about another 25-30 shots and seems perfect. I'll give it another go very soon and see if this restored performance remains but, for now, looking good. And I still have the seal ready to fit as and when needed.

Thanks again for your input chaps.