ORIGINAL: earlwb

Well once upon a time there used to be quite a few different sizes of glow plugs.
At present we have two basic thread types, the regular 1/4x32 threaded glow plug and the Nelson plugs.
After that you have the short, intermediate (OS), long glow plugs and 4 stroke type (OS F plug style) plugs , along with the Turbo plugs too.
My old vintage glow plug thread shows some of them http://www.rcuniverse.com/forum/m_10545790/tm.htm
At first they had big glow plugs designed to replace the spark plugs people were using at the time.
Champion even made glow plugs for a number of years too. Some of the early glow plugs looked like spark plugs.