Ian Can you tell me the purpose of the screw inside the filler cap of your Primus 71 marked AB 1937 in the ref gallery? I have just got one of these and a brass disc has been soldered on the outside of the cap maybe to cover a hole? Any ideas? [Before I remove it to see what is underneath!] Mine also has that double row of knurling on the filler cap which appears to be a bit unusual. cheers, Nick
G,,day . the screw would be part of the pressure relief valve in case the fuel tank gets too hot. It is a spring loaded valve. It was probably not sealing correctly on yours and someone has plugged it by soldering the disc over it. Not a safe idea!!!!!!!!!!!!!!!!!!! cheers, kerry
No - The screw was to retain the seal & not part of the SRV if the cap is original . SRV's came in later as seen in image. The 'safety' pin was the earlier feature. Image from Fettlebox: Your's sounds like a B.
From the double row of knurling, yes - but why would there then be a need to solder on a plate? There's no through-hole to block. Very odd...
Nick As Ross said on the topic concerned, the screw may have been to stop the seal falling out. I was having a chat to a stovvie in the US some time ago, he speculated that Primus had shrinkage problems with their seals. I have got another 71 in the SRG that has two rows of nurlings, and no screw, presumably Primus would have sorted their shrinkage problem out. I have since picked up another 71, three in all now, with two rows of nurlings, and I have seen many since. I am happy to say that the two rows of nurling is another variant. Can you add a picture of your cobbled affair.
Interesting theory. Given these are petrol the demands on the seal are greater but I thought a company of Primus's experience with petrol blowlamps would have got the seal type correct from the outset. From experience I'd suggest the problem could possibly be due to swelling & the screw was to help prevent the washer doming down into the filler oriface. Anyone know the design of contemporary & earlier Primus petrol blowlamp caps?
of the petrol primus blowtorches I have , none have a screw in the cap . one I believe to be 1927 model 857 . cheers, kerry
Thank you all for your input the lid is indeed a type 'B' thanks for the excellent photo. I will remove the screw and see if there are any signs of damage visible from the inside before de soldering the cap, I cannot imagine anyone doing this for no reason! The owner was ham fisted! the burner bell has had its threaded tube twisted off and the square spindle is twisted to bu..ery!The frying pan has also been panel beaten to make it a looser fit on the large pan. But it will rise from the ashes!!!!!! I will add some picture when time allows later today probably. Thanks all Nick
Looks like the brass disc is to cover a hole that goes through the filler cap as Ross suggested! I can live with the repair; its part of the history The knackered burner bell is another matter though! Nick
Is there a known time period for the different caps? I'm thinking good quality rubber stock for the gaskets maybe got a bit thin on the ground when the worlds armies geared up for war in the thirties?
I can see the way that you are thinking Pysen,it does seem strange that the internal screw only lasted for a couple of models maybe there were problems getting the washer to cap to seal properly and this was an easy fix? Or were the washers made of leather or something that needed to be compressed? Nick
I just read up on the rubber industry. Apparently the work on synthetic rubber commenced during WW1 due to shortages but didn't reach very far. After the war rubber from the colonies flowed again so research was put on hold. Later during the thirties it took of again partly due to demand for oil resistant rubber and American and German companies registered several new formulas for synthetic rubbers. Maybe the teething troubles of those new compounds are to blame for the shrinkage? The evolution of the cap seems straight forward enough. A-original design. B-screw put in to secure the gasket. C-domed cap to allow for the screw since previous design was too thin and prone to failure.
British fixed three legged paraffin stoves, also suffered from ballooning of the filler cap seal. Their solution was quite simple, punch a hole out of the filler cap seal. You will never find ballooned seals on travel caps, combined air release valve filler caps, and filler caps that have an SRV. The most likely cause of ballooning is a soft rubber compound in the first instance. When the stove got up to working temperature, the rubber would become even softer, air trapped behind the seal of the filler cap would expand, along with the pressure in the tank would pull it downwards
Ian and Pysen Excellent investigation by you both may well have given us the answer and a bit more of the jigsaw that is the history of the 71 may well have fallen into place! Another mystery solved, the panel beaten frying pan is not off of the outfit, its from something else as there is no bracket for the handle to fit in or the strap to go through! It amazes me how long it takes me to notice something fairly obvious! Nick
The pressure in the tank would force the seal further into the cap, surely. The rubber may well have sagged when the tank pressure was released but the metal (and seal) was still hot...
Hi, I believe the reason these "rubber" seals bulge is because they absorb fuel and swell. The expansion is apparent in both thickness and diameter. There is nowhere else for the swelling material to go so it bulges downwards. As Ian says, the UK manufacturers punched a hole in the centre of the cap washers and this allowed any swelling to be accommodated without bulging. Regards, Kerophile.
