Reason for BHG on 7M's a design flaw?

[Angry7M]

I always wondered why so many 7M's tend to blow Head Gaskets. I know most of the people that do blow HG's seem to not take care of basic maintainance on their supras, or something goes wrong during the rebuild process.

I've always wondered is maybe there was some kind of flaw in the design of the 7M head?

I've had my Supra for several Years now, and ive replaced the HG's when I first purchased it, and so far I have had Zero issues concerning the HG for Years now.

Read Post Number 6 and on, the Shit throwing on the 7M starts their.

http://forums.hybridz.org/showthread.php?t=141746

 

[IJ.]

Way back they used a different material HG when they changed they didn't update the spec.

 

[Supracentral]

The reason for the failures was determined long ago (around 1995) by Reg Reimer:

The factory original head bolt torque value of 52 lb-ft to 58 lb-ft is potentially too low to keep the bolts in tension and the head tightly sealed against the block. After many thermal cycling of the engine (starting, driving, and turning off the engine), the head expansion and contraction may cause the head bolts to slowly unscrew themselves, thus, reducing the tension on the bolts. Not enough tension simply spells imminent failure again. The physics of this phenomena is; when the engine heats up the aluminum head will expand and then contract when cold. If there is insufficient torque on the bolts, then there won't be sufficient force on the head against the gasket and the engine block. Therefore, at start up, coolant will begin to flow between the head, thus contaminating and saturating the gasket - it then will begin to become "mushy". Once the head gets warm and expands again, the gasket will then be easily crushed. After many cycles the problem is exacerbated, resulting in complete failure of the gasket. The ideal head bolt torque values should be between 65 lb-ft and 82 lb-ft with 72 lb-ft to 75 lb-ft being most ideal. This will provide sufficient tension on the bolts during the many start up cycles.​

So in other words, it's the gasket material and the torque spec for the bolts. Installing a modern head gasket and torquing to the proper values (72 - 75 ft/lbs) and this "design flaw" goes away for the most part.

With a metal head gasket, it adds even more protection. And as to "fixing" this design flaw, they did - it's called the 2JZ.

 

[7M-KDL]

well i have had this conversation with many people, many times. while i know the first BHG's are due to the improper torque from the factory. however, to me, it seems like the gasket is still poorly designed. there are holes in the gasket that do not match with anything on the block/head in multiple places. also, there are some holes that cover up half of the hole in the block/head. these unnecessary holes seem like they would cause a weak point. maybe there is some reason for them being like that that i just dont understand. i also do not understand(if those holes do not serve a purpose) why all of the aftermarket manufacturers of HG's for the 7M dont simply leave the material there. all of the aftermarket gaskets are the same design(minus being metal) as the factory gasket, why not improve the design, they have to make them from scratch anyway.

 

[Poodles]

Yep, Clifton, used to be around here too. I dunno why he'd hate on the 7M considering how that engine in his car lived a hard life and took it for a good long time.

It's the nature of the engine. Long aluminum head, all it takes is an overheat (which will impact the exhaust side first) to kill it.

Retorquing a leaking stock headgasket will NEVER work because of the construction of the gasket. When you blow it, it deforms the sealing rings and moves them out of place, no amount of torque will seal them because they're not where they need to be to actually seal!

Honestly his "better options" will all take more work than the 7M to achieve the same goals. A 1UZ is great in stock form, trying to turbo it and building it for performance is another matter as it doesn't have the aftermarket support.

 

[upgradedsupra]

Good post Mike.

Duane

 

[shaeff]

I've never blown a 7M headgasket, and I've done some stupid things in the past. LOL.

 

[kamikazemkiii]

Im glad you started this thread ive always wondered what make the 7m's blow HG's. I was going to make this kind of thead but i guess u beat me

 

[Poodles]

Also, keep in mind most are blowing HG's well after 100K miles which is far above the MTBF design point of a car...

 

[radiod]

I know of someone who bought a mkiii with a 7M that was built to 400rwhp by the previous owner...and he hasn't had to do more than just regular maintenance for almost the last 100,000km. He's had the car for 7 years as a daily driver. If that's not proof that once you take care of the "head gasket flaw" the engine is reliable, I don't know what is. Stock bottom end, just upgraded CT-26 and supporting fuel mods, intake, and exhaust.

 

[GrimJack]

I've done some deliberate testing.

Using a new stock Toyota gasket, I was pushing 18psi on an upgraded CT, still with stock fuel and intercooler, and it held fine. When I went higher than that, I toasted the gasket... on every cylinder... and shattered 2 pistons, and melted one more, and roasted two bearings.

Disclaimer: Your mileage may vary. Don't take this as permission, or a guarantee, that your car can do the same. I deliberately blew up an engine because I had a new one with forged internals waiting in the garage, assembled, on a stand, and I didn't care about the one in the car.

 

[bigaaron]

I know of someone who bought a mkiii with a 7M that was built to 400rwhp by the previous owner...and he hasn't had to do more than just regular maintenance for almost the last 100,000km. He's had the car for 7 years as a daily driver. If that's not proof that once you take care of the "head gasket flaw" the engine is reliable, I don't know what is. Stock bottom end, just upgraded CT-26 and supporting fuel mods, intake, and exhaust.

Jose put 285k miles on his 1jz on the original turbos, and when it finally lost a little compression on the #6 cylinder, it got torn down and we saw that all it needed was a new set of rings! The bearings looked like 7M bearings do with 10k miles on them. The torture that engine was put through daily was something that no 7M could ever withstand. THAT is reliable.

 

[Supracentral]

My MKIII has 189,000 miles on it. It's running:

Upgraded 57 trim CT-26
Ported elbow.
Greddy RS BOV
Stock (new version) head gasket.
ARP Studs
Lexus AFM.
RC 550 injectors
APEXi S-AFC
MKIV Fuel pump
HKS Dragger Exhaust.
HKS Downpipe.
HKS EVC3 boost controller.

Daily driver at 1 bar of boost. (And anyone who has ridden with me knows that I don't drive slow...) I blew the head gasket two months ago after a failure in the boost controller allowed an overboost. It spiked to 21 psi on pump gas detonated and blew (I believe) #1 cylinder on the HG. I haven't bothered fixing it as I have too many cars and other things I want to concentrate my money on. (Car will be parted out soon - I'll post it in the classifieds)

A well tuned 7M can survive on a stock HG for a very long time provided the driver isn't a complete idiot.

Yes, there is a design flaw in the 7M head gasket. But it's easily worked around, and the motor can be reliable.

 

[ValgeKotkas]

What about the extra cooling passage that can be drilled but isn't there stock in the block near the 6th cyl? (Possible heat spot?)

(I remember here being a thread and there was one just some time ago in the German forums so that's why I wanted to ask)

 

[Keros]

The 7M's BHG problem isn't unique. Go surf some Land Cruiser forums and see what kind of fun the F and FZ engines are. Jeep forums complain alot about BHG's in some of their straight sixes. The lesson I've seen is that all I6 bi-metal engines are succeptable to blown headgaskets because of the reasons Mike pointed out. Some engines much more than others, but nontheless, the same theory applies to any engine with a cast iron block and aluminum head.

Why bother with a straight 6 engine, you ask? There's lots of reasons they're awesome... but as has been said, there's alot of fixes for the BHG risk. Almost every 7M with a factory installed headgasket now has atleast one hundred to one hundred and fifty thousand miles on it... stuff just doesn't last forever.

 

[karlmk3]

ya but toyota makes one hell of an engine they last and there strong its not like where playing with some other stuff. hondas they are on a whole nother issue with problums vtec haha and they you got mitsubish and there glass transmisions
toyota did right like said above most inline 6 have problums with head gaskets

 

[Mk3runner]

I seen myself a stock HG on a supra at 160k still hold fine. but retorque of the head has been done also.

 

[Angry7M]

Really nice info guys, thanks for the answers.

 

[iceberg319]

I ran 4 years on a ct26@13psi and the past year on a 61mm t4 turbo, tubular manifold, external gate and the engine held up fine on stock updated headgasket and arps. This was all done on a 7MGE also with higher compression. I dont think there is any problem with the 7M design as long as proper maintenance has been taken care of.

 

[oldsking]

I agree I am going with a normal head gasket as I will not be going over 400hp with this motor. It's all in the maintenance, tuning and build which I have faith I can do Very good info guys and I will def be keeping this all in mind when I start building in the next couple months.

but when I go 2JZ I know I'll be fine