Room seems to be stuck in a narrow range of 66 and short cycle.

How frequently does that cycle repeat? If it's only two or three times an hour, it's a bit too often — but the real bottom line is this: the job of the thermostat is to hold the space at or very close to what it's set at.

It's doing that.

I wouldn't complain.

Hot water coil needs the boiler to maintain a minimum temperature. the fact that the time it takes to reach that temperature is always 4 minutes just proves the point that it taks one BTU to change the temperature of one pound of water one degree. Every time the boiler drops to the cut in temperature on the water heater limit control it takes the same amount of time to do the same job. with the same temperatures and the same amount of water.

It's temperature … Not Time!

what’s the water temperature?

Why 66*?

Kind of hard to trick a thermostat into doing what it's not supposed to do. Most of us spend a good deal of time and effort on keeping the temperature in a space in as narrow a range around the set temperature as possible!

If the thermostat does have an anticipator — older ones do — try what @Mark N suggested: disable it completely or set it for as long a run as possible. If, like many newer thermostats, it has a cycles per hour setting, try setting it for as few cycles per hour as it will accept — typically one.

But I would ask: why would you want a huge overshoot? What is the reasoning behind that?

Are all the rads getting some steam every cycle? An 8 minute on time is kind of short for steam.

Hay Coach! are you talking about your steam boiler when you said "Wouldn't it be better if it ran longer and less frequently"? The answer is probably yes but maybe not. This is something you can test using the heat anticipator on your old Honeywell Chronotherm. Move the heat anticipator all the way to the longer setting and see if you use less fuel. If you do use less fuel and you can live with the temperature swings of 3° to 7° without the wife complaining, then you will know your answer. If you don't save on fuel, then put the heat anticipator back to the original setting and be more comfortable.

Let us know the results of your research

66-70 gives you more heat loss than 66-67.

No, that's not more efficient and it's far less comfortable.

It doesn't matter.

You're losing more heat 68-70 than you are 66-67 and that's that.

If you want to save energy and be uncomfortable then set the thermostat to 55.

This is a slide that I used in my heating systems classes. It is a simple and quick explanation of how a heat loss is calculated. In the example I go over the three ways that heat can transfer or travel from one space to another space. Convection, Conduction and Radiation, and how different building materials have a resistance to that heat moving from one side to the other side. 

That resistance is called the R-value, like 3-1/2" of fiberglass insulation has an R value of 11. To do a heat loss you need to convert the R value of a building component into a U value. The way to get a U value is to divide the number 1 by the R value. So the example building component block in the slide has an R value of 4 so the U value would be ¼, which equals 0.25 .

With that U value of the building component we then take the total Square footage of that building component and multiply it by the U Valve and that is how many BTUs of heat will transfer from one side of that wall to the other side of that wall when the difference is 1°F. With that number you then multiply that total by the temperature difference between the inside temperature and the outside temperature and you will get the total BTU heat loss for that building component. Do the same calculation for all the building components: doors, walls , windows, ceilings, floors, etc. that are exposed to the outdoors and you will have a total heat loss calculation for the building.  

The important takeaway from this is the part that states: to "multiply the total by the temperature difference between inside and outside".  Multiply the difference in temperature to get your total  

So when your indoor temperature goes to 70° your heat loss is greater than the desired 66° temperature by 4°. That means that you are losing heat from inside your home to the outside cold by 8% more when the outside temperature is 20°  So for those 15 minutes or so you are wasting fuel that you could be saving if the burner was off 10 minutes sooner at say 62° and the overshoot temperature got to 66°, then started to drop down to 65° before the oil burner started again.  

Only a learning thermostat can do that for you, but that will be an interesting learning curve because the algorithm in those thermostats take up to 4 days to learn that, and by then the outdoor climate may have changed from bitter cold to normal or even mild temperatures and the learning needs to start all over again.  

As I said earlier, this is an experiment that you need to conduct at your home and see if the fuel is higher or lower based on the settings you choose to get the job done.  You can put an hour meter on the motor wire on that burner to see how many hours the burner operates. That hour meter times the firing rate of the nozzle will give you the gallons used (or close enough for comparison)

Let us know your results