I checked four of the carbs I have (71-73 vintage) and they are all 1/4-20. The are studs screwed in to the threaded top of the carb flange with nothing protruding below the flange (no nuts). I have seen repairs where bad hole threads have been "fixed" with a nutted stud. The studs usually do not turn freely in the carb flange and I do not know how they do this, maybe locktite, glue, deformed stud threads, or?
Thanks very much for the reply and taking the time to check on that! I assume then that the upper part of the stud which the nuts would fit onto is also 1/4-20? Its hard to tell from the photos in the Chilton's book wether they neck down to a smaller size or not. It looks from the photos like the studs are not fully threaded - that is, there is a section without threads at the carb surface and the threads start maybe 1/4" or so above that? If so, maybe the studs are just screwed down until that unthreaded portion comes into contact with the face of the carb and that is what is holding it in place.
BTW, the reason I'm trying to determine the stock setup is I have created an adapter which allows a standard 5 1/8" flange (4bbl style) air cleaner to be mounted on the stock 2bbl 5200 series carb. I built one for my own vehicle and figured I would make it available in case anybody else would like to do the same thing. I don't imagine there will be much demand for something like this, but parts options for this vehicle seem to be a bit limited, so I figure every little bit helps.
When I do something like this, I generally like to include all of the mounting hardware so that it is as much of a bolt-on as possible, but since the only engine I have access to is my own (very molested by previous owners)
this stuff isn't exactly easy to determine. Sounds like I will either include 1/4-20 machine screws which would require folks to remove the studs from their carbs to install the adapter, or I could provide 1/4-20 nuts (I think spacers would probably also be required to compensate for the unthreaded portion of the stud, since the base of my adapter is much thinner than the base of the stock air hat).
If there is anyone here who has a stock carb and who would be interested in being a beta tester for these, please let me know... I'd basically just need some measurements to give me enough info to be able to put something together and send it out to you, then you could try to install it and let me know how it goes; some pics of it installed in the car would be nice, but not an absolute requirement. Note that what I am putting together is just an adapter; it wouldn't include the air cleaner itself, so that'd be up to you to purchase that. You can get a generic 4bbl air filter assembly with a paper filter for about $10 at Summit, or $20 at Autozone, etc. Since these engines are a very popular kit car swap and people's needs and tastes vary significantly, the purpose of my adapter is simply to allow a 4bbl style air cleaner assembly to be installed, of which there are hundreds of different styles; so folks can install whatever kind they like. So anyway, you'd need to purchase an air cleaner assembly, but once the carb adapter is sorted out, its yours to keep for your trouble.
As to the reason for the SAE threads, I speculate that the carbs were added after the engines were shipped over. The carbs I have American (I believe) names cast in to them, i.e. Motorcraft, Autolite, Holley.
That would make sense. I hadn't considered the possibility that they would be using different carbs depending on which side of the pond the engines ended up on. I suppose I could pull out a couple of the other mounting screws and such and measure the threads, if they are US that would pretty much confirm your theory.
