An engine shouldn't transmit much (if any) heat through to the crankshaft and especially all the way to a spinner. Given that your engine has a bushed crankshaft there are two possibilities I can think of. One is that your fuel doesn't have enough oil in it (and castor is preferred for bushed engines at about 20%). This might be because of the blend or a bad batch (it's happened!). The other possibility is that your engine has a particularly tight fit between the crankshaft and the bush or there's no free play in the crankshaft. You should be able to push the crankshaft in and out very slightly, sliding it in the bush. Whenever I start a new bushed engine I always put a finger under the crankcase below the carb to check that it stays cool.