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Pneumatic Speargun Parts

Thread Status: Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.

Steve Anderson

Active Member
Jul 8, 2017
Needing help identifying one particular part that being the U-Cup seal that goes on the piston of Pneumatic Spearguns. I know you can buy the already made up kits for each individual gun. I have all the different size o-rings but it’s the U-cup seal I can’t find other then in a complete kit. Can anyone tell me where I could find these seals by themselves?


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My suggestion would be to find an aftermarket piston that uses regular o-rings and replace your piston with it. Internal diameters of the barrels are usually 11 or 13mm. Mares Cyrano is 11 (for a 7mm shaft) , Sten and others are 13 (for 8mm shaft). The metal piston part can be adapted to your trigger and the front part where the shaft's butt goes is also usually standard. Otherwise take a look here: https://www1.mcmaster.com/u-cups/loader-type~o-ring/ or here: https://www.theoringstore.com/index.php?main_page=product_info&products_id=7459 or here: https://www.oringsandmore.com
Also be aware that your u-cup is in all probability metric and is somewhat deformed from new. The match has to be perfect to 0.1 - 0.2mm in sizes which is tough. That is why I suggest switching to the o-ring piston
Thanks for the information. I gave McMaster and Carr the measurements and the only response I received was they couldn’t help me. I will try the others you suggested. Thanks again!
Thanks for the information. I gave McMaster and Carr the measurements and the only response I received was they couldn’t help me. I will try the others you suggested. Thanks again!
When I was making a pump for my Mares Cyrano the original one had a u- cup. I realized I could not find a replacement so I machined the piston of my pump to accommodate two orings. That is why I suggested to abandon the direct replacement idea. Good luck!
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Because Mares have been using that cup seal for over 50 years they are probably made to their special order for the 13 mm size. The 11 mm equivalent will be from the early nineties, hence such long production runs make it worthwhile to have special seals made. A rubber goods manufacturer may make most of the seals for various brands as such items will usually be sub-contracted out. Compared with the cost of a gun the seals are relatively cheap and can be bought from a number of vendors, you often see them selling on eBay.
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