I doubt that there is any difference in the quantity of heat energy being created with either head configuration, iron or aluminium (assuming compression ratio and ignition timing are the same), but I think it's safe to say that the properties of the aluminium head allow it to shed combustion heat faster than the iron variant, no doubt why the factory installed them on the M21 sidecar "slogger".
The thermocouple ring is copper, so I did remove the spark plug crush washer to install; sealing has not been an issue.
A spark plug of a given heat range is designed to transfer enough heat away from the plug's tip to prevent it from becoming an incandescent "glow plug" during hard running, but at the same time keep it hot enough to prevent the plug from fouling,
By design, combustion heat is to be transferred through the body of the plug into the head to be dissipated. (One of the reasons a plug is to be properly torqued during installation) The wrong plug in an OHV engine can lead to major engine failures due to combustion chamber hot spotting and "holing a piston". Our SV engines are much more forgiving in that regard.
Just casting about here, but it makes me wonder if the plug body in the head is not one of the hottest points on the engine, right along with the exhaust valve.... (?)
Also, if you check for head temp operating ranges set for reciprocal aircraft engines, 450F is commonly considered a safe max head temp. If you consider that aircraft engine operating parameters are extremely conservative, there could be considerable latitude for safely pushing the limits on a motorcycle engine.
email (option): teladeluji@msn.com
Visions of crumbling aluminium cylinder heads....
email (option): teladelujo@msn.com