depends on the tolerances in the engines. I've seen TTPros that would almost have no head clearance at all without the shim.
Dave Shadel (www.pspec.com
) used to sell an assortment of shims for the thunder tiger, you might try his website, see if he still has them. He had a range of thicknesses from .001 to .005 I believe. You could use them to build up what you need.
I've not run one on 80/20 fuel, so couldn't provide much recommendation for you.