We worked with ARP to select a set of super-duty 10mm head studs, rated at 190,000 psi pulling strength. These are bulletproof, use ground washers, rolled threads, and compact 12-point 12 mm nuts.
These are very similar to the head studs used on the 2.0-liter Toyota MR2 Turbo, and if they are good enough for a 250 HP Toyota, they should be OK on a 919T!
We installed these for testing by drilling and tapping the upper 919 cases from 9mm x 1.25 threads to 10mm x 1.25 threads. We screwed the stud all the way into the case, and put the head on after enlarging the head bolt holes to 10.5 mm. We lubed the stud threads and washer with ARP moly lube and started torquing.
We got to 100 lb/ft and nothing bad had happened yet! The threads didn't pull out of the aluminum case, the stud didn't stretch or break, and the nut didn't break. The stock bolts are 9mm torqued to 35 lb/ft, and these are 10mm torqued to 50 lb/ft. No problems with blown head gaskets.
Also, since the studs are threaded all the way down into the aluminum case, there's good thread engagement there, and no twisting motion of the threads when torquing, that is all done on the hardened steel stud. So, the threads won't pull out of the cases from overtorquing and sheering action.
