I guess understanding the mechanics of the 7M BHG has a lot to do with it.
I've run 22 to 25 psi on a factory head gaskets for years at a time. (This is not to say I don't run MHG's - anytime I get the head off a MKIII it gets a MHG - it's extra insurance.) However I have run on stock gaskets without trouble. The 7M actually doesn't have a weak headgasket. This is a proven fact. Where the BHG problem comes in is Toyota's torque spec for the factory head bolts.
The manuals say the stock torque value is 58 ft/lbs, this is the primary design flaw in the MKIII Supra. Myself and others have tested the head bolts and torque spec of 58 ft/lbs is way too low.
What this means that 7M head bolts when torqued to the factory specifications of 52 to 58 ft. lbs. is in very low tension related to the bolts actual yield curve.
The 7M head bolt is:
12mm-1.25mm thread pitch
Property Class: 10.9 grade 8
yield strength=147,353 PSI
tensile strength=160,550 PSI
By calculating the unit strain for the bolts based on the average yield strength, elongation numbers were calculated by Reg Reimer years ago. What he came up with was that the 7M bolt has a total elongation of .0134" {.3399mm}.
The test data shows that to get the proper bolt elongation, the torque values for the 7M head and bolts is in the 68 though 72 ft. lb range without putting the bolt into the plastic region.
What all if this mean is that that as the motor expands and contracts, the bolts (when torqued to the factory spec) actually work themselves loosen over time. Without the firm clamping force of head to block, the gasket inside "slides" around during this expansion & contraction. Take a look at 7M gasket. The thinnest points are between the compression chambers & the water jacket - this is why they fail here 1st under this scenario.
I know a lot of you guys are new to these cars, and others have been around for a couple of years, and a lot of the questions you guys come up with were asked and answered YEARS ago.
