From needing some extra heat on the Arrow GTR .21 motor to first crank it over, I thought of making these.
Custom Machined from Steel 2.5 inch OD and bored to size for ID with about .015 clearance, also is milled out for exhuast port and around carb body, to concentrate heat to where its needed down thru the piston casing
http://www.youtube.com/watch?v=HnM_eHtNCxs
I have a few already to size for the Arrow GTR .21
$21.50 includes shipping,
http://pages.suddenlink.net/hotheadsrc/