Head bolt torque? Theory/ history / legend/ fact?
 

OK guys, I know that the 7MG-E calls for 58 ft lbs in a 3 step progression. I know many insist that number is wron g, that a mistake was made in the original literature, and the bolts should be torques to 76 or so foot lbs. (working off memory, but I think those numbers are correct, or at least close).

I have a pretty savy well respected engine builder with all the equipment and 30 years of experience, wins in evey kind of racing you can imagine in his engines, and did alot of work for toyota for 10 years.

He rebuilt my 7-MG-E. when I asked about the torque, he said he had never heard of such a thing, and that torque was typically based on the diameter of the bolts, and the head bolts on the 7MG-E are ment to be torques right around 55 - 60 lbs, which puts it in the original number. Too much torque will pull on threads in block and could pull it out.

so - what is fact? what is fiction? what is the history, and when did this amping up the head torque come about?

It's based on a lot more than just the fastener diameter. The bolts' elasticity, whether the fastener is meant to remain elastic or be torqued to yield, the size of the head's contact area, thread pitch and actual surface area of contact of said threads (used with head size to calculate torque needed to overcome striction to achieve the REAL torque level), how clean the threads are and what lubricant is being used and a few other factors. The threads are not the weak point, if you look at a cross section of the head you'll see there is a lot of room for squish in the head... I'd be a fraction more concerned about warping the head. The torque value will be provided by the fastener's manufacturer and may vary depending on the factors above. Studs installed with clean, chased threads in the block, installed with moly grease generally are spec'd for mid 90's ft lbs. Head bolts are usually around 75ft lbs (although I think newer ARP bolts are designed for around 90 now).

The ISO spec for that fastener, just based on diameter and thread pitch and static hardness, is 115 to 120ft lbs lubed. Dry is over 150ft lbs.

I know a lot more people who have had trouble going lower than using the specs provided by the fastener manufacturer. If you're using a composite HG and the block and head are NOT finished to the point of being anywhere near smooth as glass it'll be more forgiving of a lower torque level on a stock build... the factory spec did get *most* people to at least the 100K mile mark before the HG gave up.

I believe the torque was originally decided based on the torque used in other Toyota engines using fasteners of the same or a very similar size.