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The circ in the center looks like a larger motor, is it a 26 series high head? What model is that one. Maybe it is just a different vintage as the others, same size?

Two fingers disassembles a Discal for service. The cap and valve mechanism is replaceable if the needle or seat is damaged.

We see a Caleffi fix on that type air purger from time to time :)

Funny, I just mentioned the flame rod in another thread and how difficult it is to reinsert correctly in A and B style heads. However, as @HVACNUT has pointed out, the condition you report is often the result of a combustion problem and in your case with no sooting it is reasonalbe to suspect it is excess air although similar flame rod results can arise with too little air. (exception being you talk about sputtering during operation. I don't know if you had been carefully familiar with the sound of operation before you noticed these restarts or if you have acquired that first hand knowledge during these drop outs such that you might not realize there can be a mildly wave like form to the sound of operation which isn't really sputtering. If this is really sputtering you've got the kind of problem where you ought to shut it down. I don't know if the gas valve could be fluttering. In some installations you might be able to observe the flame if there was an observation port. I sometimes make an observation port by drilling the door or plate where the unit is installed and adding a piece of quartz glass or equivalent but it is very unusual or would mean a crapload of excess air that the process is blowing itself out or marginally igniting and these are intermittent ignition so you wouldn't get a full 10 minute cycle as you describe if the flame were actually going out.)

and if the relay picks up within its retry limit, it won't lock out and it sounds like that is what is happening.

do you have the old 60200FR relay or the newer 60200FRS that has a display. that display allows you to push the up arrow (which if you can read the fine print on it has a microamp symbol) and that will display the microamps generated by the flame rod throughout the light and running process. (I wish it did not automatically return to regularly scheduled programming as quickly as it does as sometimes you are watching through a pure or repurge and just as the boiler fires it disappears and if you aren't quick you won't hit it again in time to see the reading the resulted in a drop out, if one occurs.). these days carlin tech doesn't really even want to talk to you about maginal flame rod pickup if you have the old relay. And, to be fair, despit the $150 you gotta throw for a new relay, it is so much easier to diagnose flame rod output compare to trying to meter it that I keep these relays on hand and replace them anytime I service a unit that has the old realy.

you are looking for something around 2.2 microamps but the control will run with maybe 1 (carlin doesn't tell me that number, only that that 1 is too low even though it may prove flame at that level).

slightly complicating to your post is that this cannot be an atmospheric boiler. the ezgas is used to convert a boiler that took an oil power burner, i.e. forced draft from the burner end, with a gas alternative. but the setup was never atmospheric. (at least I can't imagine how it could have been-maybe some clever macguyver has an atmospheric boiler they could eliminate the draft collar and find enough room to low mount the power burner and kind of seal the base of the combustion area. now you've got me thinking about trying this sometime in the shop for the fun of it, but I don't this you have atmospheric boiler.

IIRC the desired combustion set points are 3-6% oxygen and 8 to 11% CO2 which are fairly typical so I try to get within the range and then monitor CO and flame rod microamps with an eye towards less of the former and more of the later but balancing them to hit the desired 2 and change microamps and usually the range to accomplish that doesn't allow me a lot of luxury to play with efficiency numbers, they just end of being whatever they are.

so the question I posted (Aside from asking for tips on inserting the assembly back into the head), was also how you really assess if the microamps produced reflects any degradation in the rod itself and I haven't had that question answered yet. I will be putting it to my carlin connections when I see them at AHREpxo later this heating season. Because literally every flame rod issue I have encountered with an EXGas I have solved by checking the combustion set, but maybe there is some kind of degradation in the rod that means I am compromising either CO emissions (still well within nominal 400ppm limit although I would prefer below 100ppm and can't always quite make that) or efficiency in order to get the flame rod response I need to satisfy the primary safety control. it may be that it isn't a rod problem and I could consider downfiring the given burner. the setups are not easy for changing the firing rate, which is too bad. I have taken in later installations to adding a union on the downstream side of the gas valve to make it easier to change the orifice (one of the carlin techs told me when I mentioned that that that is how they set up for testing so they can change easily. I think they ought to ship 'em to be set up like that, bit I digress).