Clearance From The Top Of The Heat Riser to Steel boiler or Masonry?

Still working on my final design for a 5" or 6" batch box rocket water heater/bell and was wondering what is the recommended clearance between the top of the heat riser and a masonry bell top or water boiler?

Would a mild steel boiler cope with the exit temps from a heat riser?

Many thanks in advance for any help.

John, Peter says a good practice is to make the top gap a foot. On a 5 incher, i've gone down to 3 inches gap, between heat riser and cast iron top.

I'd say, use the same gap as your diameter, if it's refractory masonry. If it's the boiler, it could most certainly cope, if cooled by water. The only problem i envision placing it over the heat riser. Condensation of the gases against the cooled metal. And steam flashing.

Thanks for your quick and helpful response Satamax. Would you suggest the same gap for the boiler as for the masonry?