Replacement Pistons For Weihrauch HW55

[T 20]

As there was a thread on here a while ago by someone looking for a replacement piston for a HW55 I thought I'd pass on a little discovery I made this afternoon.

These Weihrauch pistons are now obsolete, but by chance I realised that there is a readily available alternative if you can accept losing 2mm of stroke ?

The leather sealed SMK b2 piston body will fit with minimum modifications needed and is spot on in dimensions except for reducing the stroke a little and the need to use a TX200 seal and adaptor.

B2 pistons are available for £18.50 from Knibbs --- I bought one last week.


Hope this helps someone ?



All the best Mick

[Mach 1.5]

Thanks Mick. I know the old HW50 piston will also fit but again next to impossible to find. Mach 1.5

[TonyL]

Great information, gents.

Will the B2 piston rod work fine with the HW trigger then, Mick?

I wonder if anyone has looked into the potential suitability of the HW30 piston? Or would that be too short in the body / rod not long enough / cocking slot to incorrect dimensions?

[Mach 1.5]

Isn't the HW30 piston 26mm and the 50 / 55 25mm. Mach 1.5

[Shed tuner]

Isn't the HW30 piston 26mm and the 50 / 55 25mm. Mach 1.5

just a haircut on the lathe

[landymick]

HW30 seal is 25mm

[T 20]

Will the B2 piston rod work fine with the HW trigger then, Mick?

In a word --- no.

The cutout in the b2 piston rod is exactly 1mm too shallow for the Rekord top sear to hook onto and set the trigger.

image.jpg

But this cutout can be deepened with a dremel or grinding wheel as long as you're carefull not to touch the hardened face where the hook latches --- I did this three years ago on my long stroke HW50 build.

Something else that will need a fettle is the piston OD, front and rear --- these areas need reducing in OD a touch, and when I say a touch I mean thousands of an inch.

Vortek do an adaptor to fit a Diana/TX piston seal to the front of the piston so that's quite simple.

In the b2 piston the spring diameter will have to be 18.8mm OD which doesn't leave much choice but a Titan number 11.

With the Titan 11 spring you'll need a custom Spring guide or the spring guide out of a b2.


This may sound a lot of trouble to go to but if the alloy piston breaks on a HW55 it could be the cheapest way to get the gun back up and running.

Now to prove the above concept, I have a spare HW50 action (same bore and stroke as a HW55) and brand new b2 piston and TX seal to hand, so tomorrow I'll do all the above mods and report back.
I only have a well used b2 Spring but that should get us the 6ftlbs we're looking for from a HW55.



All the best Mick

[Neil54]

I knew a B2 would be good for something.

[TonyL]

I knew a B2 would be good for something.

Lol!

[TonyL]

Isn't the HW30 piston 26mm and the 50 / 55 25mm. Mach 1.5

HW30 seal is 25mm

Yep, the '30 has a 25mm bore. Later '50 / '99 is 26mm.

[TonyL]

just a haircut on the lathe

I wonder if that might make the '99 piston a viable option (for those with the lathe and prowess)?

[TonyL]

In a word --- no.

The cutout in the b2 piston rod is exactly 1mm too shallow for the Rekord top sear to hook onto and set the trigger.

image.jpg

But this cutout can be deepened with a dremel or grinding wheel as long as you're carefull not to touch the hardened face where the hook latches --- I did this three years ago on my long stroke HW50 build.

Something else that will need a fettle is the piston OD, front and rear --- these areas need reducing in OD a touch, and when I say a touch I mean thousands of an inch.

Vortek do an adaptor to fit a Diana/TX piston seal to the front of the piston so that's quite simple.

In the b2 piston the spring diameter will have to be 18.8mm OD which doesn't leave much choice but a Titan number 11.

With the Titan 11 spring you'll need a custom Spring guide or the spring guide out of a b2.


This may sound a lot of trouble to go to but if the alloy piston breaks on a HW55 it could be the cheapest way to get the gun back up and running.

Now to prove the above concept, I have a spare HW50 action (same bore and stroke as a HW55) and brand new b2 piston and TX seal to hand, so tomorrow I'll do all the above mods and report back.
I only have a well used b2 Spring but that should get us the 6ftlbs we're looking for from a HW55.



All the best Mick

Will look forward to your next instalment, Mick.

[Weasle]

As there was a thread on here a while ago by someone looking for a replacement piston for a HW55 I thought I'd pass on a little discovery I made this afternoon.

These Weihrauch pistons are now obsolete, but by chance I realised that there is a readily available alternative if you can accept losing 2mm of stroke ?

The leather sealed SMK b2 piston body will fit with minimum modifications needed and is spot on in dimensions except for reducing the stroke a little and the need to use a TX200 seal and adaptor.

B2 pistons are available for £18.50 from Knibbs --- I bought one last week.


Hope this helps someone ?


All the best Mick

Many thanks for this. I ended up taking my 55 piston apart and dressing out the damage that had be caused by the 'Ox' spring to the ali head and bonding it back using green loctite, so far, touch wood its still holding up. I will, however follow your lead and get one.
BTW, I used a vortek adapter and seal that I had originally bought for a knackered G80, and again that seems to be working well.

Cheers Steve

[T 20]

These Weihrauch pistons are now obsolete, but by chance I realised that there is a readily available alternative if you can accept losing 2mm of stroke ?

My mistake, you lose 3mm of stroke (giving 62mm stroke) due to the TX seal being 6mm thick not the 5mm as I used for my calculations, this should be no big deal on a 6ftlb target gun --- I'm not chasing power here, just trying to find a way to keep these lovely old guns going.

Oh, and yes, the piston is in the action and is latching nicely on the Rekord trigger which allowed me to accurately measure the final stroke.

I've just got to fit the seal, find a suitable spring guide and assemble now for a bit of testing.

[TonyL]

Sounding all good there, Maestro. a Thank you for the chat and I'll look forward to hearing how this one goes.