Having previously dissected the mini-pump cap from my 8R, I now set about putting it back together. Here's where I left off... https://classiccampstoves.com/threa...-8r-mini-pump-cap-revealed.33084/#post-338011 I needed a new SRV seal. I punched out some 8mm discs from 1.5mm viton sheet. I used the closest size wad punch I had on hand that was close to the original seal which is about 7mm in diameter. I trimmed one down by first punching a very small hole in the centre with a plier type hole punch. I then clamped the seal using the mini grinding disc spindle in the Dremel tool, and gently ground it on a piece of medium emery paper until the right diameter to just fit the cap opening was achieved. I started with 1.5mm sheet which was slightly thinner than the original seal (at around 2mm thick). Aftet further consideration I thought this would have needed the cap screw to be set too deep to achieve the right seal pressure. I made another from some 3mm sheet which was slightly thicker than the original. After grinding it to the correct diameter, I punched the centre hole to approximately 3mm by selecting the appropriate size on the plier type hole punch. I was happy with this as my replacement seal. This is it fitted to the piston. There seems to still be enough of the tip protruding through the inside of the cap to provide a good seal for the NRV. The next job was to set the penta screw to provide the correct pressure relief. I decided to make a rig to achieve a known test pressure. Nothing too technical mind! I dug around in my fittings tin and found a threaded steel bush that just happened to be the same thread size as the cap fitting on the stove tank. I recycled the brass valve stem from an old car inner tube and soft soldered this to the bush. (Both pieces having been first cleaned and tinned to ensure they would allow the solder to take and ensure the joint would be gas tight.) I was feeling very smug at this point and went looking for my static bicycle pump that has a built in pressure gauge and a screw on Schrader valve inflator. Having read somewhere here that these caps were set to a crack pressure of around 40 psi, I figured I could just pump up the the rig and adjust the screw until it released at the 40psi mark except...the NRV tube on the inside of the cap was to long too allow the cap to screw onto my test fitting. D'oh! In the picture below I hadn't refitted the NRV but you need this before it will hold pressure! I unsoldered the fittings and then resoldered a piece of brass tube between the Schrader valve stem and the bush to provide plenty of clearance. With the screw set to about the original depth and finger tight this gave a crack pressure of about 25psi. Pumping up to 60 psi saw pressure blead away via the SRV opening at 25 psi each time. You can see why the thread lock would be needed. Happy with that result I now just need to refit the screw with some thread locking compound and give it a try. I'll do this tomorrow as I was summoned for dinner which ended my afternoon's shed session! I understand these stoves operate at around 15 psi and the tanks are factory tested to 70psi - if I recall correctly! The vapour pressure of hexane, which is a good match for Gasoline, is about 36 psi at 100 degrees C. I figure somewhere in the 30-40 psi range should be a good setting for the SRV in the cap to allow the stove to operate safely. I'll post on the outcome once I've finished. Steve
By way of a postscript the rebuilt cap holds pressure and the stove burns equally well with either the original or the mini-pump caps!
Hi Steve, I like your test rig and would like to make one for myself. But being in the US metric parts are hard to find, do you know the diameter and thread pitch of the steel bush you used? It would make things a bit easier if I know what I was looking for. Thanks Chris
Hi Chris The bush I used is very close or may even be 3/4 UNF (16TPI). You only need a few threads to get a seal on the cap especially with a new cap washer. If you have an engineering supply store nearby I'd take the cap a try it on some fittings. Remember you need enough length inside the fitting to allow the NRV to fit inside. Brass would be a better choice than steel If you plan to soft solder your Schrader (car tube) valve stem on. Go well! Steve
