the epoxy resin is put into the crank to reshape the inlet as companys like JP etc bore out the insides of the crank to get more airflow down the middle! the epoxy tidys up the end nearest the carburettor & also directs air down the crank
some of the Modified On-road engines have had this for a while
apparently all the new 14mm cranked Novarossis etc have this but the Top421B i'm using doesn't[]
i think this is because mine was a very early engine, (i've been using it all year) but i've read somewhere that this filling is done on all the engines now