dig this: you're working with aluminum.. that is the material of the intake.. you're dealing with steel (most often), which is the material of the heads.. you've sandwiched a gasket between them.. the tty bolts stretch and conform to the chase, which is in effect what all bolts do, it's just they do it quicker and more completely.. we're talking 12lbs of torque- not much..
if you want to crack your manifold, by all means- crank a grade 5 or 8 to 80% tension of the bolts design..
if you want to skimp on the tty's (which, personally, I wouldn't, but that's me.. others do it all the time and with good results), use either 5's or 8's, but for the sake of the manifold, don't torque it anymore than 15lbs.. that thing needs to be held tight enough to maintain seal, but loose enough to float through heating cycles..
the 5's and 8's aren't softer metals with almost pliable tension strengths like the tty's, they are much harder, and therefor the threads won't conform and 'bite' like the tty's.. so, you gotta hedge your bet by using thread lock of some sort..
will the 5's or 8's suffice? yes.. are they easier to use than the tty's? yes, because you can re-use them at least once or twice more.. are they better? no.. they don't conform to the chase they are in, and they will have a tendency to want to loosen over several heating cycles.. loosen more without a thread lock, loosen less with..
there is more in play in this particular fasteners topic than the torquing ability of the fastener alone.
edited to add- the tty's made by design for this purpose are shanked before the heads.. and, about the thickness of the manifold where it is tapped.
Last edited by drewactual; 10-26-2011 at 04:51 PM.