Head bolt torques? legends/ facts/ history?

[Carl Johansson]

So i run a stock 7MG-E in 3 different supras in the 24 hours of lemons series. stock because it has to hold up to 16 - 24 hours of racing at a time. So far we have had great luck, actually getting 225 racing hours on one engine, before it gave up. So in order to try and replicate that performance, i quit fooling around and took it to the best engine guy i could find! a pretty savy well respected engine builder with all the equipment and 30 years of experience, wins in every kind of racing you can imagine in his engines, and did alot of work for toyota for 10 years.

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


So dave rebuilt my 7-MG-E. when I asked about the torque, he said he had never heard of such a thing (numbers being "wrong") , and that torque was typically based on the diameter of the bolts, and the head bolts on the 7MG-E are meant to be torqued 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?

 

[3p141592654]

This is a portion of Reg Riemer's original post. The link to this document on the Suprasonic web site is broken, so I pulled it from Google cache. Otherwise I'd just link the doc.

ecently 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.

 

[CyFi6]

This is worth a read
http://www.suprasonic.org/public_html/sonictech/mk3/mk3_pmtn.html

EDIT I've been beat

 

[Carl Johansson]

Thanks guys, I really appereciate that. Now if I could, one more question: because I am an idiot and posted my original question in the wrong forum. so I should have put it in the 7MG-E section, the motors are non turbo. would that make a difference? or does the information stand for both versions of the engine?

 

[supraguy@aol]

Same bolts for 7mge and 7mgte. No metallurgical or dimensional differences between either engine's block or head as well.

 

[IJ.]

It was a change from Asbestos based HG's that caused all the issues, the Tq Value was never updated by Toyota...

 

[Nick M]

It was a change from Asbestos based HG's that caused all the issues, the Tq Value was never updated by Toyota...

I can never find anything from Toyota on it. Has this been word of mouth or something else?

And you are one of the few "internet people" I don't see as full of shit, so don't take my question personal.

 

[IJ.]

I can never find anything from Toyota on it. Has this been word of mouth or something else?

And you are one of the few "internet people" I don't see as full of shit, so don't take my question personal.

There was a big long discussion about it 10> 12 years back Nick and one of the people involved had actual "facts" to back up his claim.

 

[Nick M]

I have read it before. I can not picture how that turned into 58 lb/ft. Often the repair manuals get changes because there are mistakes. Yet Toyota does not acknowledge this one. Meaning the 58 lb/ft.

 

[hvyman]

If they acknowledged it they would have prolly had to make a recall.

 

[super51fan]

Talk about recall I see the dealer doing Subaru head gaskets 10 to 1 of what Toyota ever did on supras . I don't hear anyone in a uproar about that. In the subi world Yet people still complain about 25 year car. Most head gastket jobs fail due to poor workmanship and lack of knowledge.

BTW at stock boost level the factory head gaskets usually went 100k or so before failing. I bet if someone had access to warranty info the MKIII had some of the lowest warranty dollars ever spent.
Or not spent. Yes the warranty was 3/36 until 91` and 92' had 5/60 powertrain coverage. Never seen or heard or them failing at factory power levels under the OEM warranty.

I feel OEM head bolts torqued to 72-75 are perfect. Does not matter oem or mls design. IMO .

 

[hvyman]

Yes but the number of units sold is far less than every oter model toyota made. In 92 only a little over 1100 in the us. Compare that to a camry or corolla.

Subaru also have the same motors and designs in all there cars so of course your going to see it fail more.

 

[Nick M]

If they acknowledged it they would have prolly had to make a recall.

I and others changed so many on the V06 campaign, they might have reconsidered. Even though that is out of character for this company. The V06 recall was 3.0 V6 in the pickup and 4Runner.

 

[super51fan]

Sounds like you were around when the MKIII platform was in production. Did you really replace that many head gaskets On the 7m engine?

 

[Nick M]

Sounds like you were around when the MKIII platform was in production. Did you really replace that many head gaskets On the 7m engine?

What? The V06 campaign was in the late 90's for the 3.0 V6 truck motor. The MA70 first gen Supra came out in 1986. I was 13.