Hidden secrets of the Optimus 8R mini-pump cap revealed!

Discussion in 'Fettling Forum' started by Funfundfunfzig, Jun 18, 2016.

  1. Funfundfunfzig

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    I finally solved a mystery (for me at least!) of what was going on inside the mini-pump cap that allowed it to act as both a non-return valve (NRV) and safety relief valve (SRV) at the same time.

    In case you were wondering here is, from left to right; the mini-pump, the mini-pump cap and a plain cap for the 8R stove shown in this picture.
    image.jpeg
    My interest was more than academic as the two mini-pump caps on both of my 8Rs are leaking and I would like at least one working to avoid having to shuttle my one good plain cap between the two stoves.
    image.jpeg
    The cap has an NRV that is like the NRV on most stoves - a rubber pip sitting on the end of a spring held in a cylinder that presses onto a small orifice in the cap. This is on the right in the above picture.
    image.jpeg
    In the pump cap that orifice runs through the middle of a little piston that is inserted from the top or outside of the cap.

    A rubber seal (like a square profiled O ring) fitted into the cap should stop leaking when the shoulder of the piston is pressed against it by a strong steel spring. This is held in the top of the cap by the hollow penta screw. The darker coloured steel spring and small piston are to the left of the cap body in the picture above.

    Looking from the outside (or the top of the cap) you can just see the seal in the bottom of the threaded hole below.
    image.jpeg

    If you flipped the cap over and looked from the inside you would see the hollow end of the piston right in the middle of the cap. In the image below, the piston is fitted. It provides a sealing seat for the NRV pip.
    image.jpeg

    And below it is sitting beside the cap so you can see where it lived when fitted.
    image.jpeg

    Here's the piston below. It's hollow so air can be pumped in and pass down the middle of the piston. Normal pressure can't escape as the NRV seals against the end of the piston. The stronger top spring holds the piston itself down securely against the square seal from the outside.
    image.jpeg

    If the pressure became too high inside the tank, gas would be forced out past the outside of the piston through the opening in the centre of the cap and it should lift the piston itself off of the rubber seal, providing the required pressure or safety relief feature.

    Genius!
     
    Last edited: Jun 18, 2016
  2. Funfundfunfzig

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    Here's an additional picture of the seal the goes in the cap for the SRV. It is around 7mm wide and 2mm thick. The inside hole is approximately 3mm. I'll need to buy or make one of these before I can reassemble the cap.
    image.jpeg
     
  3. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Hi, Aktopp produced a great sketch in 2007:

     
  4. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Here is a direct link to the great sketch:

     
  5. kerry460

    kerry460 Australia R.I.P.

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    G,,day very interesting .
    please help this computer useless person .
    how do i save threads like this ??????????????????????????????????????????
    so i can find them later .

    cheers
    kerry
     
  6. Jeopardy

    Jeopardy Subscriber

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    Hi Kerry
    You got me thinking the same. So I had a quick play. You use the bookmark button at the bottom of each post put a meaningful note in the dialog box that pops up and accept. At the right top of the screen you will find your user name where you can manage your account. Click on that and your bookmark list is at the bottom of the menu.
    Regards
    John
     
  7. snwcmpr

    snwcmpr SotM Winner Subscriber

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    I have found that @kerophile always has the right topic and post in his reply and is the most accurate with the information. I would like to see if we can add him to the website as the best source. :) Better than a bookmark, I think.

    A search does come up with a wealth of information. This site has a pretty good search function. Not like a lot of sites I frequent. Patience is another virtue, not only in penetrating oil, but spending some time reading about what others have already learned.

    Ken in NC
     
    Last edited by a moderator: Jun 26, 2016
  8. Giri

    Giri Subscriber

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    Thanks for the great post.
    I'll be fixing one of my pump caps in the future.
     
  9. kerry460

    kerry460 Australia R.I.P.

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    gee it is easy when you know how .

    thankyou !!!!!!!

    kerry
     
  10. sa3spd

    sa3spd United States Subscriber

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    Hi,

    The job of that little SRV seal can be done quite nicely with a Viton o-ring.

    The ones I use are an AS568-006 size as suggested by former member BD (Gary) before I found CCS. The dimensions in inches are 1/8" ID x 1/4" OD x 1/16" CS or W where CS is cross section, W is width and both refer to the same measurement.

    The metric size listed on my supplier's website is 2.90 +/- 0.13mm ID x 1.78 +/- 0.08mm W.

    When I bought them, I needed three. The package contained 167! So if you can't find something suitable locally (and aren't in a hurry, as I don't know how fast mail gets from the States to you fellows Down Under), please let me know and I'll send you a few.

    Rick C
     
  11. Rickybob United Kingdom Banned

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    clever stuff! i got one of these caps with the pump a while back and now i understand how it should work
     
  12. snwcmpr

    snwcmpr SotM Winner Subscriber

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    I just looked again.
    I believe you have your 'bucket' in backwards. The bucket should have the 'pin' in the spring, and the pip IN the bucket. No need for a o-ring style of pip.
     
  13. Funfundfunfzig

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    Hi Ken

    There is a second SRV bucket or what I'm calling a "piston" that works differently to the one in the NRV.

    This one has a narrow tubular section that passes through the cap and actually presses against the NRV pip on the inside of the cap. It has a bucket that the SRV spring sits inside and the other end of this spring sits inside the penta screw that you can see from outside in the middle of the cap.

    You can see the arrangement of the SRV parts below. The pip or seal is different in that it is a ring rather than a disc, and it goes over the narrow section of the piston and not inside the bucket.
    image.jpeg
    I've reassembled the cap using a seal I made. I've also included details of a rig I made to test the relief pressure for when I was resetting the spring pressure.

    Details are shown here...

    https://classiccampstoves.com/threads/8r-mini-pump-cap-rebuild.33133/
     
    Last edited: Jun 27, 2016
  14. shagratork

    shagratork United Kingdom Moderator, R.I.P. Subscriber

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    A good drawing showing how the system works, here.
     
  15. Tantra

    Tantra Subscriber

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    Thank you very good technology
     
  16. snwcmpr

    snwcmpr SotM Winner Subscriber

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    The SRV seal in this pump cap is fairly thin. It is still soft.
    20200623_192615.jpg