A question for 1/2a historians.
 

In looking at some of the older half A engines that I have sitting around I've noticed that they all seem to have a propeller shaft that has a 5-40 thread. My sample is a bunch of Cox engines, a couple of OK Cubs and a Baby Spitfire. I'm wondering if there was a specific reason for using the 5-40 thread. Was it a more common thread at the time? It seems like using a more common thread like the 4-40 would reduce production cost.Any engine historians out there?

I am not a Cox historian but have a question

What make you believe a 4-40 was more common in 1940s?

And do remember......if a customer needs a special part that you sell....modest price increase for the convenience is translated to profit...something every good business desires and needs

I don't know that 4-40 was more common for a hard fact but do have the observation that most of my old junk, old radios, cars, tools etc. tends to use the No 2, 4 and 6 screws. The only place I've ever run into No 3 or No 5 screws is in regard to model airplanes. It is just kind of a random interest in industrial history. Old guns often seem to have screw threads that you don't often see elsewhere and if you play with little British cars you should plan on investing in fine thread or even whitworth screws and bolts. I'm just wondering if early on there was a specific reason for the 5-40 screw or if one of the early 1/2 a engines had that thread and everybody else just followed that as a matter of convenience.

A 5-40 was a standard size. It still is. It is a 1/8" stove bolt thread, meaning an 1/8" hole drilled for the prop would be right. Later on the 1/4" 28 UNF (fine) became standard for the larger motors even for the Japanese who had been using metric for a while.

Yes,I would bet that the convenience of a common size hole for mounting the prop was probably the main factor. After I started this thread I found an article on the front page of the Model Engine Collectors Association website. It discusses the early days of half A glow engines in the late 40's to early 50's. Early on it was a pretty competitive market among several manufacturers but they tended to use the same props so I would think that the 1/8 inch hole and as a result the No 5 screw just ended up as the de facto standard. It's probably not that important. I'm just entertained by finding out the origins of things.