Yes I have noticed 4 strokes tend to stumble less on transition. however most beginners cant or wont spring for a 4S engine on their trainer. Even used 4S engines command a premium price. These days I get all my used engines from my friend Bruce at brucercengines.com , he tests each used engine and gives them a rating so you know what you are getting. I help him do it from time to time, goes a lot quicker with two people.

It doesn't help when those OS plugs are shorter reach which makes it even harder to get a better combustion since the plugs element is recessed into the head by a few threads. For optimum performance, the bottom of the glow plug should be flush with the inside of the combustion chamber. Too long of a plug like the 4-stroke plugs with the protruding end in an engine designed for a standard long plug can sometimes cause problems by changing flame propagation since combustion would be taking place below the top of the combustion chamber. It also raises compression slightly. I've used an F plug in some 2-strokes and it's worked fine but the engines ran better and faster on a standard long hot plug.

The MC14 and MC4C are for 4-cycles and helicopters (the MC4C is a standard long reach plug, no protruding "nose"), MC55 and MC59 are for low and low-med nitro fuels. I venture to guess the 14 and 4C are the hottest followed by the 55 and then the 59. I liked the Enya 3 better than the MC4C in my Enya 4-strokes. Lower idle and better top rpm. The 59 didn't fair as well in the 4-strokes but did work okay. Idle rpm needed to be a little faster.

You should try some of the Merlin offerings you may like them. The 2003HD is my goto hot plug for 2 strokes while the 2009 is my 4 stroke plug. And he has such a wonderful selection of plugs that you would be hard pressed not to find exactly what you need. He even has Cox/Norvel and Nelson.

What's the pricing like? I get those McCoys 12 at a time for $62 shipped. If the pricing is comparable then I may try a few. I have the red Merlin that came in my Jett .35 that I haven't used yet. It has run so good on a 59 that I haven't bothered to change it. But I'm open to other options.

Give Aldon a call and see

Website says $6-$8/ea for the plugs you listed. When I get low I may give them a try.

If you are looking for any info on plugs when you call Aldon clear your schedule for an hour, if you ask about fly fishing clear another hour.

Are there any markings on the plugs now?
I bought a few Merlin plugs to try, but without any markings on them it is near impossible to find a suitable plugs once they have come out of their packing....

Last I knew he was working on it but the tooling for it was expensive and he is trying to control costs. I have taken to making them with an engraver.

The Merlin plugs are color coded, but if you use more than one kind of plug in a single heat range then it might be tough.

Marking them with an engraver not "making" them.

I used a standard long reach "Merlin" hot plug with success on an old Taipan Mk 4 2.5 cc glow two stroke. Nice plug. It has a 'red' insulator seal.

I rarely use O.S. plugs now. Nice enough but the intermediate length fits very few engines perfectly. Pricing is also high.

A few glow plugs are priced like the stock market. You buy a lot of your favorites when the price is low, and you can be sure the price will go up and down over a few years time.

You are not wrong, the reactive element in glow plugs is platinum/rhodium and its price fluctuates with the market. Often quite a bit.