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

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.

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.

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.

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.


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.


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


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.


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!

 

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.

 

Hi, Aktopp produced a great sketch in 2007:

 

Here is a direct link to the great sketch:

 

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

 

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

 

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

 

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

 

gee it is easy when you know how .

thankyou !!!!!!!

kerry

 

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

 

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

 

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.

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/

 

A good drawing showing how the system works, here.

 

Thank you very good technology

 

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

 

I’ve been a bit slow to respond to this older thread, but I was looking for something else and stumbled across it and felt a need to question the original poster’s assertion that the SRV seal should be of square section.

His example of the seal had evidently distorted and hardened with pressure from the spring and heat from the stove to retain a flat cross-section when removed from the cap.

It’s actually a conventional O-ring when new, but as @snwcmpr too pointed out, is often seen flattened and retaining that shape when extracted from a cap that’s been in use for a significant length of time.

The OP constructed a test rig to establish the release pressure, which was a good move, but the flat mating surface of the cap …




… combined with spring pressure, heat and fuel vapour from the stove could - in my opinion - result in sticking of the seal to the brass, making it non-functional as an SRV in time.

My contention is that a conventional O-ring’s circular cross-section would be less likely to do this, particularly if the material it’s made of corresponds in terms of its elasticity and resistance to heat and fuel to the manufacturer’s specification.

Anyone not sure of whether their SRV O-ring has bonded itself to the brass cap’s mating surface could either build a test rig as the OP did or dismantle the SRV to inspect it, replacing the O-ring if necessary and re-setting the SRV with the same number of turns of the penta-cap screw used to remove it.

A final point, the SRV piston seat for the O-ring is concave, suitable for a circular cross-section O-ring but not one of square cross-section.





John

 

I agree, @presscall.


Cheers

Tony

 

Thank you John.

 

wish this site had 'like' buttons so i could easily let you all know how much i learn from it

now to find a suitable allen key to grind into penta screw remover

as the 8R that i bought recently has had the tank resoldered back together, it's possible the over-pressure valve didn't work and it got so hot it meted the solder and popped the tank apart

someone has butchered the 5-sided internal nut?, presumably with a too small 6-side allen key

before the cap gets used again it seems it should be thoroughly disassembled, inspected and repaired as necessary

 

Among the many things I really like about CCS is that it has no 'like' buttons....