I see people are now selling replica tin-plated steel caps to fit the old 1 to 5 gallon fuel cans in 1-3/4" size. They look identical to the original. But try as I might, I have not been able to find a source for the smaller size caps, which are generally known as 1-1/2" size.
The same screw on caps were used on some mower fuel tanks in the old days, with the addition of a pinhole dead centre and a suitable lining which had another pinhole (or two) off-centred. Of course, the ones on the old fuel cans were identical apart from the fact they lacked a pinhole. It was not unusual for mower caps to either rust away or get lost, at which point the cap off a fuel or oil can could be used as a suitable replacement. All you had to do was put a tiny hole in the centre and add a suitable washer and "Bob's your uncle". The problem of course is we come to the present day and find lots of vintage fuel and oil cans that are missing their lids!
Now I have been told in good faith that the actual size and type of the thread on a 1-3/4" fuel can cap is technically called a 45/400 thread. The 45 is easy, as 1-3/4" equates to 45mm. There is also two different thread types, being a "continuous thread" and a "non-continuous thread", but I can't recall which of these was used on the old fuel cans. You can still purchase these with either "SolvSeal" lining or foil lining. Both are perfectly suitable for contact with fuel and oil.
Logically we can assume that the smaller 1-1/2" fuel can caps would have 38/400 threads, as 38mm equates to 1-1/2". I did some research and it seems most plastic caps used around the house these days have 38/400 threads. So I gathered up as many as I could find from around the house and shed and tried them on the old one gallon fuel and oil cans. Not one of them would fit. They are all just a smidgen too small for the thread to even begin to make contact. It is almost as though the old fuel and oil can caps have threads that are more like 38.5/400 or 39/400 in reality.
Another thing that is interesting is the old Briggs and Stratton caps are said to be 1-1/2". Well I tried one on an old fuel can and it too was slightly different. The fuel caps will not thread on to a Briggs fuel tank. The Briggs cap will just barely thread on to the old fuel cans, but are very tight and you are flat out getting half a turn. I am not sure if the difference is because modern threads are actually metric-based or if the angle of the thread is different. The only thing I do know is that I am still unable to find suitable replacements for vintage 1-1/2" fuel and oil caps.
Does anybody know if someone is making replicas of the originals in 1-1/2" size? I have several one gallon cans here that would be very happy to be re-united with their long lost lids.
Here is a photo of the tin-plated caps available in 1-3/4" size on places like ebay. Photo is from ebay.
