FWB metal pistol rings v. synthetic washers

[familytales]

The other day, in a thread started by Hsing-ee, I posted a quote from the Feinwerkbau 65 pistol manual:

"The piston seal of the Model 65 does not wear. A comparison may demonstrate its strength: A piston ring made of steel serves as a seal between piston and cylinder. If it would reciprocate 5000 times a minute, this would correspond to the operating conditions in the engine of a motorcycle. Even with a daily training program (sic) of 100 shots, only after 100 years a piston-ring running time would be reached that would correspond to 60 hours on the engine."

Fullmetaljacket ended the thread by asking why if engine type metal rings are so good then why bother using synthetic piston seals at all?

Seems like a very fair question to me. Would it just be down to machining costs could there be other reasons, like the maximum power obtainable using the less flexible metal seal?

[familytales]

Nobody interested in adding their 2p worth to this thread? Apparently not!

[Born Again]

In an engine the metal against metal (ring vs cylinder wall) is lubricated with a thin layer of oil (from below in a four stroke and above and below in a two stroke). Without lube they would quickly seize up. Synthetic seals can be self lubricating and can run dry if neccessary, they also have a certain amount of give that means your rifle wont get jammed solid by a grain of sand.

[Geordie]

In an engine the metal against metal (ring vs cylinder wall) is lubricated with a thin layer of oil (from below in a four stroke and above and below in a two stroke). Without lube they would quickly seize up. Synthetic seals can be self lubricating and can run dry if neccessary, they also have a certain amount of give that means your rifle wont get jammed solid by a grain of sand.

Yip - as said, the air allowed into an engine is carefully filtered, although this would be possible with an airgun too (in theory)

The other point about the oil, is that the piston setup in an engine is designed to combust a large amount of fuel, so the oil presence doesn't matter - in an airgun we're trying to avoid dieseling to a great extent.

JHMO,

Dan

[fullmetaljacket]

in an airgun we're trying to avoid dieseling to a great extent.

Fair points, but doesn't a spring piston airgun derive a substantial chunk of its energy from the limited amount of dieseling?

[familytales]

Agreed chaps, but, and this is the bit that I find confusing, FWB specify in the 65/80 manual that: "The piston mechanism of FWB air pistol (sic) requires no service and should normally not be lubricated. In addition all sliding surfaces are provided with a permanent lubricant."

The 300s rifle manual says the same but adds that the permanent lubricant is "synthetic". Could this be some sort of Slik50 that bonds to the metal surfaces? And would they have had this in the 60s when the guns were first made?

In the early 65 manual it also says: "There is no maintenance on the Model 65. It needs neither oiling nor new seals." (Of course this is not true since it does require a new breech seal every 15 or 20 years and the seals claim was dropped in the later manual.)

Still, the proven longevity of these pistols, with negligible loss of power or apparent wear over decades of use would seem to bear out the thrust of FWB's claims. It would be interesting to hear from anyone who has serviced these pistols (and the 300 series rifles) whether there is noticeable piston ring/cylinder wear in a well-used example.

Dan

[Geordie]

Fair points, but doesn't a spring piston airgun derive a substantial chunk of its energy from the limited amount of dieseling?

Yup - hence the "to a great extent" caveat

[familytales]

Many thanks for your posts folks.

Isn't there anyone out there who can explain why, if Feinwerkbau can get around the lubrication/dieseling issues by using metal piston rings, that the other leading springer manufacturers don't just copy them on this? (Or maybe they have already )

Let's face it, it would make life a lot easier for us if they did, at least maintenance-wise.

Sorry to bang on about this but I'm genuinely mystified

[Hsing-ee]

I think it also might depend on what the piston rings were made of .. I've heard of rings with copper in them containing some kind of 'impregnated' lubricant but this could possibly be complete marketing nonsense.

The Webley Osprey had piston rings, as did the Webley Service Mk II. The Osprey was apparently a very consistent rifle but I don't remember anyone saying not to lubricate the piston.

I have a feeling that this sort of design cannot be used on a powerful design, I don't think it would be robust enough or the vibration might cause the rings to dig into the cylinder wall and result in wear and scoring.

It might be down to a question of engineering finesse. When I re-lubed my Feinwerkbau Sport, the compression chamber walls were almost perfectly cut and honed, unlike any other gun I've worked on. I used Dri-Slide to lubricate the chamber, and just the tiniest smear of SM50 on the washer to get it down the cylinder and it worked fine effectively dry-lubricated. If the 65/80 series were as well made, which they would have been, and the rings properly made, bedded and fitted, then there may not be much in the way of friction going on anyway, especially if the surfaces have been treated with molybdenum that bonds to the steel. I remember seeing heat-pumps at the atomic energy research place at Harwell which were designed to work without lubrication on board satellites for years, so it can be done.

Have you had your pistol chrono'd to check it's power and consistency? Why not buy that Webley Hawk MkIII that is kicking around the for sale section and do some experiments?

atb

Alistair

[familytales]

Thanks Alistair. Very interesting. I think you're probably right about the engineering finesse bit, with very careful attention paid to honing and use of impregnated materials.

Don't know about you but after this thread now I'm terrified of getting one of those grains of sand in my FWBs....... looks like I'll have to cancel that beachfront shoot I had lined up

Dan

[u.k.neil]

I have been servicing/ renovating the 300 series of FWB target rifles for over 20 years and have never had to replace a piston ring (I do tell a lie, I broke one once and had to put a new on in..... ).

When you measure the ring gap in the piston housing the largest gap I have measured was .002" above a new one which IS negligable on a 25 year old rifle that has been used in competion three night a week for eight months of the year, every year of it's life.

Breach seals, springs and piston head buffers are another matter of course......

Neil

[familytales]

Amazing stuff Neil

[Hsing-ee]

There you are; in the end it's just a matter of craftsmanship.
I still want to know if those Chinese chappies have managed to do the same with their £150 gun! They could probably do with a bonus if they have!