seems like the 21tm is based of the 18tm
same stroke but bigger bore
looks like the 21 is replacing the 18
anyone wanna confirm?
think all os did was stuff a bigger piston in the 18tm

we all know that the manufacturer claimed power
is a bunch of bull
but it seems as though os thinks that the 21tm makes
the same power as the 18tz...thought that might interest you

if you looking to keep all the easy start and stuff
you might as well go with the 21tm
if you actually want big block power, then go with a true big block