When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
It could be crankcase compression (worn rings) but as you have a new engine try a new cap the rubber seal go's in the inside try the new type with the grub screw
Strip and clean out your breather system before worrying about serious issues. And buy a NEW oil cap as the rubber seals in them go shitty after a time.
Could also be ring blow by pressurising the crankcase, but do the above first before worrying about that.
mine kept blowing off. since ive changed the head gasket and put a new cap on it doesnt happen. but theres still pressure quite abit of pressure in the head. not sure if thats normal or not?
have a look at the breater pipe at the back of the block, its usually to do with a bit of crank case pressure, or maybe think about a breather system, tend to work better then the stock ford system