Working towards getting a handle on the “stock” clamp load, using the bolt measurements Midget provided, the effective spring constant of the stock bolt is 171,333 lb/in or 6,745 lb/mm. Applying a half-turn of 1.25mm pitch thread would then produce a bolt load of 4,216 lb with a stress in the smaller diameter section of 53,000 psi. This, assuming a stiff gasket and column of aluminum in the stack. In reality, the numbers would be somewhat smaller. Assuming a peak cylinder pressure of 150 bar or 2200 psi, the endload on a 75mm bore would be 15,070 lb. Then, assuming about a 180 degree F temperature rise, the stress and load would likely increase about 20-25%. The question then being, how stiff is the gasket? Does the temperature increase induced load increase further compress the gasket, leading to reduced clamping force upon cooling?
Given these numbers, it would seem that there isn’t a lot of margin provided on the clamp load. The stress level seems reasonable and the need for a stiff gasket is apparent. However, the half-turn of the bolt seems too simplistic. Is that turn predicated on a preliminary draw-up? Something like “Torque it to XX to stack things together, THEN apply the half-turn”. That would gain some margin in clamp force. Midget--is there anything like this in the assembly manual? Also, do they say anything about re-torqueing?
In light of this, the ARP 50 lb-ft spec, resulting in almost 12,000 lb of bolt load seems quite excessive. It is not clear how much consideration they may have given the system into which their part is fitted. It looks like they just specified a torque that gave their preferred 75% of yield on the stud. That much load might wreak havoc on the engine parts. Using a stud does have the advantage of possibly being able to pull the head without undoing the rest of the engine.
As regards the Rover edict to change the ladder along with the gasket type--this just doesn’t seem to make any sense. The materials are essentially the same and the minimal added stiffness borders on inconsequential. Sounds to me like “As long as you have it apart to change the gasket, might as well put in the “new” ladder”. The only other thing would be the possibility of their having made some oiling changes in the ladder, but this seems unlikely.