Heat rizer format and dimention

Hello Experts,
next weekend I want to build my first RMH. But with the Heat riser I have some questions.
I read an on line book there was written the top of the heat riser has to be higher at the border and deeper in the middle.
Ernie always suggest a cob top with the outside deeper than the middle. Some drawings I found it horizontal.
The distance is always 2 inch to the top
But also the distance on side between Heat riser and the barrel is between 1,5 and 5 inch.

So ? What are the right dimensions and the right shape ?

Tx a lot in advance
Tom from Latvia

Thomas Krueger : While not an expert let me see if I can help ! Do I understand that you have read Ianto Evans' Book 'Rocket Mass Heaters and have access to it ?!!

In a 6 inch Rocket Mass HeaterRMH system 1,5 inches is the minimum gap between the top of the Heat Riser and the inside Bottom of the Barrel ! In an 8 inch system
2,0 inches is the minimum gap between the Top of the Heat Riser and the inside bottom of the barrel, I think that I saw the 5 inch quote somewhere, and immediately
translated that to 5 cm which is 2.0 inches, or very close!

If the top of the Heat Riser is left flat, then the fine fly ash rising from the combustion chamber has a place to settle out, while I do not have proof that this Could clump
together and reduce the gap between the Top of the Heat Riser- and the inside bottom of the barrel, it is a potential problem ! Tapering outward from near the inside of the
top of the Heat Riser towards the outside will allow more of the fly ash to return to the inside of the Burn Tunnel where it can more easily be cleaned out !

The distance between the outside of your Well Insulated Heat Riser and the wall of the Barrel should be a minimum of 2,0 inches, it can be bigger, the constant cross
sectional area of the transitional area should start out as nearly three times ( 3 Xs ) the cross sectional area of the burn tunnel, and taper down smoothly to the point
that it meets and turns 90 degrees to flow into the bell shaped mouth of the Horizontal Chimney, allow for a clean out, and a deep ash pit at this point, and be generous,
you will be the one cleaning the fly ash out !

If you have any more questions come back here 24 / 7 and find someone to talk with you ! If There is a Rocket in your future we Will help you make it! Pyro-logically Big AL !