Glenn Herbert wrote:2" is a good minimum top gap between riser and barrel; for a 6.5" system, 1 1/2" may be enough, but 2" is safe.
Side gap would preferably be at least 1 1/2". If the riser is not centered in the barrel, the side with more space is likely to get hotter due to more airflow.
Did you build the riser with firebrick laid flat or on edge? Full or split (half thickness) brick? The less mass in the riser, the quicker it will come up to full operating temperature. Around a firebrick core, I think 1 1/2" of perlite-clay should be okay. More is better if you have the space for it.
Daniel Ray wrote:You never want the cross section of the exhaust to go below the riser dimensions. If you are building a 6.5" diameter heat riser, you want to maintain 6.5" inches throughout. Use material that is rated for high temperatures, cement will fail at relatively low temps. http://donkey32.proboards.com/