I wonder about the bolt torqe on a alloy cylinderhead BSA M20-21. That for cast iron is avalibel in "technical sektion" but fore the alloy head? I don´t want to drag it to hard.
Get the biggest thickest ( usually sold as high tensile ) washers that you can to go under the head.
I regularly make up my own .
This is to spread the load and prevent the bolt collapsing the hole onto the bolt