Recently I purchased a pair of new head bolts from my local Toyota dealership. One bolt from the 1995 Supra 2JZ-GTE engines, as well as a new bolt from the 7M-GTE. I then contracted a local certified engineering metallurgical company to perform tensile strength tests on the head bolts to compare yield strengths and torque values.
I have lab data reports based on the ASTM A370 tensile test, giving tensile strength, yield strength, ultimate load, yield load, as well as deformation data and maximum tightening torque values for the head bolts from the 2JZ GTE and 7M-GTE engines. Some results of the test are given below.
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... ultimate load=70,198 N... % elongation=17... % reduction of area=66 2J head bolt is: 11mm-1.25mm thread pitch {Property Class 10.9 grade 8} yield strength=148,948 PSI... tensile strength=162,581 PSI... ultimate load=68,997 N... % elongation=19... % reduction of area=66
The metals used in the head bolts of the 7M & 2JZ engines are identical in metallurgy +/- manufacturing S.P.C. This is a good material; it stretches smoothly in the plastic region of the curve before it snaps.
By calculating the unit strain for each of the different areas of bolts based on the average yield strength, the following total elongation numbers were calculated. The 7M bolt has a total elongation of .0134" {.3399mm}, and the 2JZ bolt has a total elongation of .01093" {.2775mm}.
By comparing the elongation differences of the bolts, related to the corresponding different thickness of the aluminum in the engines cylinder heads, and allowing for the total length of the bolt shank plus 50% of the length of the threads, the only apparent difference is that the 2JZ bolt has 36 percent more thread than the 7M bolt does. The 7M & 2JZ bolts appear to be designed with the same steel to aluminum expansion stretch theory. I believe the bolt designs are different only because of the different ratio of the bolts metal area versus the thickness of the aluminum cylinder heads the bolt is designed to hold down. The torquing procedure for the two head bolts is also different, as is the head gasket
Toyota service manuals say that the 7M engines head bolt torque specification is 52 to 58 ft. lbs. According to my findings the 52 to 58 ft. lbs. specification for the 7M might be too low a torque value to keep the bolt in acceptable tension, not to mention the normal compression of the head gasket after time. As mentioned earlier many 7M engines that experience head gasket failures have many head bolts that can be removed from the engines failed cylinders by hand, or are very loose when removed.
My calculations show that the 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. Calculations based on my test data show torque values for the 7M head bolt could be as high as 68 ft. lbs. to 72 ft. lbs. without putting the bolt into the plastic region. On a cold engine this extra torque would allow more tension on the head bolts after the head gasket compresses to normal operating thickness.
Many Supra owners that can afford it are upgrading their 7M engine to the expensive HKS stopper type metal head gasket for the 7M engine. This gasket comes in a number of different thick nesses. My Company stocks the HKS gaskets and I ship them all over the world. The HKS gasket will hold well to over 20 PSI of boost over 400 hp as sea level with upgraded fuel and turbochargers. The factory head bolt torque may be satisfactory with a metal head gasket since metal head gaskets do not deform as much as the soft OEM gasket used in the 7M. I would still recommend torquing the bolts to 72-ft. lbs. When upgrading to Metal head gaskets much must be considered with regard to machine work and the finish of the engine block deck and cylinder head gasket surface. See the following link for full details on doing correct machine work to your engine
