TargetTalk

I'm working on rebuilding a bunch of Benelli Kite air pistols. The college team I help coach has 16 of them, and over half have sprung leaks. I have information on the size of the various seals. Some are nitrile rubber, which I can get in metric sizes from McMaster Carr. Others are polyurethane, specifically a Parker "Ultrathan V1" material, for use in dynamic sealing applications where wear is an issue. McMaster doesn't carry any polyurethane metric O-rings.

I tried to check the materials McMaster does sell in metric sizes for wear/abrasion resistance, and (this being the Internet) I get a lot of conflicting info. One source says Viton has good wear resistance and is fine for dynamic seals. Another says you should NEVER use Viton in a dynamic seal application. One O-ring vendor has a way to compare the properties of various materials. Every one I checked (including polyurethane) is rated "poor" for wear resistance, which I know is nonsense.

I will contact Parker and see if they know of a distributor that I can buy from. In the meantime, does anyone have a favorite metric polyurethane O-ring vendor they can recommend?

One issue with polyurethane is that it has a shorter shelf life than many other materials. Viton (for example) is supposed to be more shelf stable than most materials. It will be a question of whether they would wear out before the polyurethane ones fossilize...

Thanks!

ebay...China

eBay lists lots of polyurethane O-rings, but if you add 2mm to the search, it comes up empty. Most of the ones they sell are by application, and aren't listed by size. Those that are listed are almost all English dimensions.

The eternal "O" ring size conundrum, O rings are flexible and there is some latitude to crossing over to Imperial from metric and visa versa just get one that is the closest in size but be clever about your choice and make an educated choice. In the uk even smallish stockists have the facility to make the size you want in the material you want for not silly money. Have you walked into a bearing and seal stockist and asked yet to see what they can do for you. Good luck in your mission to sort the Kites out. As to material stick with the manufactures choice of material to start with but Viton can be tried for dynamic stuff but don't forget about choosing the appropriate hardness, which is a whole new ball game. The poly O rings are often in the harder band of rings especially in the dynamic positions. We use the Shore hardness scale in the UK the higher the number the harder it is and if it if not stated they will be Shore 70, the common harder ring is Shore 90.

I've been dealing with O-rings off & on for about 50 years, starting when I worked on oceanographic instrumentation in college. Between material, hardness & size, there are a lot of variables.

I contacted Parker, who makes all the O-rings & seals for Benellis. There's a major distributor the next town over, and I'm going to check with them about availability of the size I need for the older Kites. I've collected enough parts to service the pistols for the college, but I'd like be able to recommend readily available O-rings for other Kite owners. I think the vast majority of pistols can be fixed by replacing just two O-rings.

O-ring #1: There is a seal between the regulator valve assembly that appears to be a standard Nitrile O-ring (part #358Z). If it lets go, you will hear a hissing from the area where the cylinder mounts. If the sound varies when you place your fingers just behind the black plate the cylinder hits (part #356Z), that O-ring has died. It is specified as 1.47mm X15.49mm ID, which is really odd... It's used as a gasket, and I can't image the dimensions are terribly critical. McMaster Carr sells 1.5mm x 15.5 mm Nitrile O-rings that should work fine for less than $10/100.

O-ring #2: This is the seal for the pressure regulator piston. It it lets go, you will feel a breeze inside the front of the trigger guard from air flowing out the regulator adjusting nut (part # 369Z). The O-ring is 2mm x 13 mm on the older Kites, and 2 mm x 11 mm on the newer ones (part #366Z). Benelli uses a hardness 93 (Shore A) green urethane, which is pretty hard. Both sizes are readily available from McMaster Carr, just not in Urethane. The piston will move back & forth to open the regulator valve, hence the need for wear resistance. I've been researching more, and it appears that the O-ring hardness may be as much the key to wear resistance as the material. My current theory is that a hard (Shore A 90) nitrile O-ring will probably hold up better than a Viton O-ring that has a hardness of only 75.

A few years ago a club member had that second problem of feeling air coming from the front off the trigger guard on his Kite, he said he would have a look at it himself before the next session a week later, as there is no backup for them here. He turned up to the next session with it fixed and it never gave any more problems. I don't think he did more than find a O ring and just put it in and you bet it was a 70 nitrile. Sometimes we over think stuff. I did the same for the seals in my K12 cylinders, they were blue and felt like poly but could not get any at once but needed it for a competition at the w/e so just put in what I could get at the local engineering store (nitrile 90) and it got through the w/e ok and has not given any problem for over two years now. Later the seal in the other cylinder also went the same way, it is a dynamic seal as the quill on the regulator goes through it and it also seals the cylinder when you take it off, it had delaminated radially due to the quill movement dragging on it. By then I had got a seal set from Pardini and I'm sure that seal is poly, but that nitrile 90 is still in use in one cylinder. If you use a hard nitrile make it shine using silicone grease the same does for the shaft that goes though it just make it shine and no more, polyurethane rubber is self lubricating. Sometimes you just need to suck it and see if it is any good, best of luck in your mission.

With 16 Kites to keep running, I believe a little over-thinking is in order. If it was my pistol, I'd just grab my best guess from McMaster and be prepared to replace it occasionally. I asked Parker's tech support for the best trade-off between material & hardness for readily available O-rings, and I'm going to ask them how easy it is to get the urethane version from them.

Other than this one regulator piston O-ring, I should be all set for spares for my team. Part of this effort is to see if I can help other Kite owners keep theirs up & shooting.

In addition to several of the polyurethane O-rings, Benelli uses a couple special polyurethane "U" seals. There is one in the cylinder for the mounting pin, and another for the firing seal. The cylinder seal appears to be a standard Parker part, but the firing seal only has a Benelli factory part number and is probably custom. Fortunately, I don't think they fail that often.

I use either buna n or viton in my Steyr(s) regulator piston. No issues. I tried silicone and it lasted about 2 weeks. The other two last years without issue, but I change them just because I don't want to wait to find out when they might fail at a match. I use 70 on the low pressure side and 90 on the high pressure. I would consider the regulator piston a dynamic sealing area.

The seal on the inlet where the cylinder mounts is the one a change the most. That is dynamic imo. Shore 90 for certain in that location.

Grainger also has some sizes that I could not find from McM Carr.

I don't know if they sell to the general public, but I've had good luck getting o-rings from Engineered Seal Products (www.espint.com) for work.

marky-d

Seeing as you really do want to do the right thing and get it right here is a link to the people I used to use when I lived in Dartford (now live a long way from them) Look at the technical support bar it helps with material properties and nominal sizing and explains how to size from first principals. https://www.barnwell.co.uk/
This mob always sorted me out when I needed specialist O rings eg copies of the O rings for the K12 and many other engineering projects I've become involved with over the years, I'm looking locally for a similar firm near here now, but I now live more in a centre for agriculture rather than engineering. Good luck you will get to the bottom of it.