How many different size nuts are there for a 1970/72 wiper motor arm attachment nut?
I have a small for one motor arm but need a larger one for the other. I have no rule where to obtain this larger nut or figure out what size it is. Can somebody direct me please?

 

I have a new/replacement wiper motor here (for a '70 442) and the owner seemed to have a problem with the same stud on it. I just checked/compared it to an original used motor from a '70-2 and the original uses a smaller 1/4" fine thread hex flange nut while this new wiper unit uses something close to a 5/16" thread size nut (non-fine threads). I tried putting an actual 5/16" nut on there but it catches after a couple turns and stops.

Net result is the one here (and maybe yours) may use a metric nut...whatever thread size is closest to 5/16" and not fine threaded.

 

8mm is close to 5/16"

 

Wow metric nuts umm okay. Guess i will go looking for a 8 mm.

 

If the wiper motor with the larger stud that YOU HAVE is an old/original one....then maybe it's not metric on the stud and may use a 5/16"-18 (ie not fine thread) thread size. Again, the one I referred to is 100% new and was likely made overseas.....not a rebuild, etc.

 

Try to lay a bolt alongside the stud to see if they mesh perfectly. If they mesh perfectly, you have the same thread. If not, try the other (metric or American) to see if it meshes perfectly.
From there, try a nut on the bolt, then on the stud.

 

A metric thread is referred to by what believe is it's shank diameter, same as SAE. M6, M8, etc. Different thread pitches, too. There is a M8-1.0, a M8-1.25 and a M8 - 1.50, for example, to add confusion to the mix.