Many years ago, when we were running the Veco 19, K&B 6.5, and OPS 40 and 60 marine engines, the choices for glow plugs were limited. I ran the K&B 1-L in just about everything. If my engine was taking plugs, I'd add a head shim.
Then there were the McCoy glow plugs, OPS glow plugs, K&B HP glow plugs and several others. Again, we'd add or take away head shims to get the best engine performance and reliability.
We were adjusting the engine to fit the glow plug we used.
Since then, engines have become more powerful. This was due to improved design, better materials, and better parts fit. Larger marine engines are now available.
At the same time, some glow plugs were no longer available. Other glow plugs became available.
Things change over time.
Glow plugs not only ignite the fuel, they establish the ignition timing in our model engines. That is why there are cold glow plugs, colder glow plugs and much colder glow plugs. An extremely cold glow plug will work in a 21 size engine, it just won't work well. Plugs that work well in a 21 size engine will not last long in a large engine.
Most model boaters are not serious racers. They like to go out and run their boats with friends on the weekends, but they don't travel to big races. They want a dependable glow plug they can run in their different engines. One glow plug for a few engines. They are getting convenience, not peak pervformance.
The serious racers, the people that travel, sometimes hundreds of miles, to compete in a prestigious event, want peak engine performance. Head space, head volume, and port timing can be changed to improve power and rpm. Many people change their engines to work better with a certain glow plug. That is a lot of work and testing to adjust your engine for a certain glow plug.
With the number of glow plugs available today, it is much simpler, and cheaper, to test your engine with different glow plugs. Instead of chasing what engine mod helps the glow plug, you can test different glow plugs to see what glow plug gives the best performance out of each of your engines.
Glow plugs have become expensive. This is because the wire alloys and gone up in price, the other materials used in glow plug manufacture have gone up in price, and the technicians operating the machines are no longer working for $4.00 per hour.
It is cheaper to find the best glow plug for each of your engines, than to replace engine parts and/or burn out a glow plug every time you run your boat.