Generally, the more pure the alloy, the better the thermal conductivity. As noted, the more pure the alloy, the weaker it is.
Bog std copper (unalloyed) has a thermal conductivity of ~380 W/m/K
8% Aluminium bronze - 70 W/m/K
5% Tin Phospher bronze - 80 W/m/K
Al/Ni/Fe bronze - 38 W/m/K
357 Al casting alloy (bit of a guess at a typical cast alloy) - 152 W/m/K
1100 (99% pure Al) billet (strong as mashed spud) - 221 W/m/K
If you want to improve the heat transfer over the boundary, use an insert with a greater thermal expansion than the body of the head - the additional pressure at the interface improves thermal conductivity. There may be an issue with thermal cracking in the head however...
FM
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