Anyone recognise this petrol tank thread or cap

John, It has the look of a Lister type cap.
Martin P
John wrote:

Looks like a David Brown 25D or Cropmaster radiator cap to me. Regards
Dan

So many different threads were used that the only way is to measure it.

Why not modify the tank rather than the cap.Fitted properly to vent overboard with a nice "U" bend the chance of water contamination is minimal.Or if you are really concerned fit a water trap between tank and engine. Mike.H. PS Just come back from an excellent day out at Bristol harbour festival.

Many thanks for the replies - I'd like to investigate the Lister and David Brown 25D or Cropmaster radiator cap suggestions - do you have any ideas how a person in the Warwickshire could do that without having to travel all over the country? I've no idea how to measure a thread of this size - a quick search on the internet found some expensive 'rods' but they were much too expensive. I know that the overboard vent suggestion is the BSS way but my boat is a vintage launch (I think around 1908) with a petrol / paraffin engine. The petrol tank is only a pint or so and I'm reluctant to have to drill holes in the side of the boat for vents. I can't think of a neat way to do it without spoling the appearance of the boat.
Thanks again John

John,
It MAY be a BSP thread, many were. You can buy a thread gauge for a couple of quid that will tell you the number of teeth per inch. Alternatively, carefully count the number of threads per inch, measure the diameter and let us know the results, we may be able to identify the thread for you. Once you know the thread, it's a relatively simple matter to buy the appropriate parts or get one made up.
Regards
Mark

Then you may not need a vent or flame arrester! There are a variety of exemptions for historic craft with 'original' installations. Also, do not necessarily interpret the *specifications* for vents with a need to have one.
Chris D

Thanks Mark,
I dug out an old thread gauge which said BSW and Metric on it. The closest I could find was 18G 5/16 which means nothing to me but it was very close in terms of pitch. It looked as if it was between 1.25 and 1.5 metric. I The diameter is 50.59mm or more probably 2". Do these figures help or do I need to source a BSP thread gauge and are they still available.
Regards John

Chris, Thanks - for this - I'd more or less ruled the exemptions route out for the time being because it takes so long - probably about six weeks. I'd hoped to be able to get my boat on the water before the summer's out, by complying with the rules, but Are you saying that if I get rid of vents then I don't need flame arrestors - if so that would simplify matters greatly? Thanks John
with the existing regs and try and maintain enough for the time being hoping to get my boat in the water

I am saying that I own a historic boat which has gone through a BSC every four years with three different surveyors since about 1983 and none have queried the lack of either. One was heard to remark, "Ah, no vent, therefore flame arrester not needed".
Other surveyors' interpretations may differ - but in the case of petrol you have a good case for it NOT venting inboard. However you need to be able to answer the question, "So, how does the fuel get out if there is no route for air to get in".
I would be inclined to ask the vent question on uk.rec.waterways where a couple of surveyoys hang out. Alternatively, speak directly to your intended surveyor about this question before the actual test. They are generally quite approachable for advice.
HTH
Chris D