Here a few thoughts on the subject. Having ample experience with both OEM (TEC) as well as aftermarket coils (Dyna), I might be able give some recommendations.
Coils almost never fail and under normal circumstances will survive your motor.
Problems can develop with sparkplugs, plugcaps, HT wires and their connections.
Sparkplugs are the first that will need replacement. Next are the plugcaps. The CB500/550 back then was a benchmark for reliability. Known troublemakers were the plugcaps however and Honda has experimented with several ones. Some aftermarket 10.000km service packs - I believe the brand was Dino - included, besides oil- and airfilter, plugs, also new NGK caps. For a good reason. It is my gut feeling, there's a lot of replacing of coils going on, where the issue actually lies elsewhere, either in the sparkplugs, the caps and/or the connections. The HT wires also age.
Let me confess that in the 90s I have fallen victim to Dyna's bragging about 36.000 V or so. They looked robust, but, being a generic product, the weakness lies in the - not tailormade - connections. Moreover, for a CB500/550 I find them too bulky. That high voltage is not needed as these coils will spark at the same voltage (~ 7-9 kV) as the stock coils do and as long as you're not drag racing a highly tuned bike, you can ignore Dyna's bla. Let's have a closer look at the connections. You yourself have to crimp the connectors that go into the towers and it depends on how good you are at it, to make it succesfull. On these bikes, there's not much room and it can be quite a hassle. Little rubber caps come with the Dynatek wireset, so everything is nicely shielded against moisture. However... these caps become rock hard after some time and will be extremely hard to remove and put on again. At first I was enthusiastic about the silicone plugcaps. Not now anymore. Nothing wrong with silicone - on the contrary - but these caps do not have a resistor inside, so R plugs are required to be streetlegal. I'll come to them later. Furthermore Dyna has chosen connectors that require a ferrule on top of the plug. Why? I don't know. NGK caps connect way more solid. More than once forsaid ferrules have come loose somewhat. The silicone Dynatek HT wires are extremely supple and although I like that, they can move about quite a bit in the riding wind. This is not good for robust connecting.
In Europe professionals that work on our CB Fours on a daily base, advise not to use R plugs and although I first ignored this as nonsense, I have come to the conclusion that R plugs and our bikes is not a good match. I have too many R plugs lying around now that gave up prematurely. BTW, I have noticed that Denso R plugs, increase their resistance over time and NGK R plugs seem to decrease. Why this is? I don't know. Maybe it's just my experience.
Just the other day I have replaced the Dynatek plugcaps by new NGK resistor caps and also replaced the dreaded R plugs by conventional NGKs. I noticed how well the NGK caps connected to the plugs and after I had started the engine, a miracle seemed to have happened! I realised I should have done this long before. I had been stubborn not to accept the knowledge of professional mecs, not without any further substantiation to their findings.
To do my further testing correctly, I will - for the time being - run the Dyna coils, but as soon as another issue occurs, I'll abandon them and the old OEM coils will go on again. After all these years they still give the right spark and they are a direct fit. They would be perfect, if only one could screw on new HT wires, the same way you screw on new NGK caps to the HT wires. I remember I have seen aftermarket coils that have this, but not recently. On the other hand, repro's like 4-1 has on offer, are relative cheap to replace.
Here some tips. When you pull the plugcaps, make sure your hand cannot travel too far in a brisk movement, compromising the connection!
For the sparkplugs, only use the Honda tool that came with the bike. This is what works best for me. Before I mount the sparkplug, I screw a ferrule like Dynatek caps requires, on top, but just a few turns. Now press the plug in the Honda tool, so the rubber inside that tool will have a firm grip. The sparkplugs hexagon is now exactly where it should be and the whole combination is stiff and very easy to work with as the plug will not fall out. Screw the sparkplug down and tighten. Withdraw the plugtool and then remove that ferrule again with a few turns. On removing the sparkplugs, do the opposite: withdraw the plugcap, then screw the little ferrule on top of the plug - a few turns is enough - and firmly press the Honda tool on it. No risk of damaging the porcelain and guaranteed no more prematurely falling sideways of the #2 and #3 plug, which would result in a hassle to get them out.
