'68 L35 BB Cylinder Head Torque Spec - Team Camaro Tech
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post #1 of 8 (permalink) Old May 11th, 14, 09:37 AM Thread Starter
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'68 L35 BB Cylinder Head Torque Spec

I purchased the "correct" M marking BB head bolts and began assembly of my engine. Used thread sealant on all the bolts and oil under the bolt head to get an accurate torque reading. Began to tighten the bolts in three steps. I used the 80 ft-lb spec as specified in the service manual for the final step. All was going well until one bolt wouldn't get to the final torque spec and snapped. After removing all the bolts and pulling the heads, I was fortunate that the bolt sheared with about 1/2" sticking out of the head. I was able to remove it by hand!

So I have a couple of questions:

1. Has anyone else experienced problems with these "correct" M marking head bolts shearing?
2. A web search yields a number of different head bolt torque specs. 70 ft-lb seems to come up a lot. Is 80 ft-lb too much for these bolts?

I realize that one should generally follow the recommended torque spec of the bolt manufacturer but all of the parts suppliers simply list these bolts on their website with no info on torque spec. Using an aftermarket bolt (e.g. ARP) is not an option since I'd like to keep everything as original looking as possible.

Thanks,

Bob
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post #2 of 8 (permalink) Old May 11th, 14, 09:42 AM
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Re: '68 L35 BB Cylinder Head Torque Spec

are you sure the length of the head bolts were the same as the old ones,seems wierd that after it broke you could remove it that easy.
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post #3 of 8 (permalink) Old May 11th, 14, 10:00 AM Thread Starter
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Re: '68 L35 BB Cylinder Head Torque Spec

It does indeed seem odd. I've never had a head bolt shear like this and it has me wondering if it was just a fluke or if they cannot withstand the 80 ft-lb of torque.

I measured the old bolts versus the new and they were nearly identical.

Attached is the picture of the sheared bolt.

Thanks,

Bob
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post #4 of 8 (permalink) Old May 11th, 14, 10:52 AM
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Re: '68 L35 BB Cylinder Head Torque Spec

you should have long bolts and short bolts usually the shorter bolts have a lower torque
spec check specs in manual again or check web sites for your engine
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post #5 of 8 (permalink) Old May 11th, 14, 11:28 AM Thread Starter
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Re: '68 L35 BB Cylinder Head Torque Spec

Yep. I have both long and short bolts. The short bolts are installed in the holes along the bottom of the head.

The Service manual does not differentiate a different torque spec between the two bolts unless the engine is an L-88 or L-89, which mine is not. Having said that, I have seen websites state that the long bolts get torqued to 75 ft-lb and the short bolts to 65 ft-lb regardless of whether the heads are aluminum or cast.

Bob
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post #6 of 8 (permalink) Old May 12th, 14, 11:21 AM
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Re: '68 L35 BB Cylinder Head Torque Spec

Bob, I'm thinking that this problem is easy to over think. I think your correctly marked bolts are junk and should be in the trash.
The only torque specs for BBC headbolts I've ever seen show two different values for long vs. short. And that spec should NEVER break a bolt. When you tighten them it's to insure a proper head to block seal, of course, but also to preload the bolt so it won't release tension under working conditions. That load is about 25% of breaking torque. Put those correctly marked bolts in the trash. In fact, do what engine builders do when replacing suspect rod/main bolts: smash the threads of the old ones with a hammer while holding them on a vice or anvil. It can prevent reuse and a subsequent disaster.
Disclaimer: I don't cycle through headbolts often. The set I use today are the same as I bought in 1969 to secure my new aluminum heads on my new L78. They don't have any other correct markings except GRADE 8.

One owner 69 Camaro,(yep, bought it new
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post #7 of 8 (permalink) Old May 12th, 14, 06:06 PM Thread Starter
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Re: '68 L35 BB Cylinder Head Torque Spec

Fred,
I would agree. The bolts are total junk. I'm not one to vendor bash but I've already communicated to them this morning that I want a full refund. No questions asked. They said they would take them for a full refund, no problem. This sends a big red flag to me that perhaps they've received other complaints as well.

After communicating with other board members and friends, the consensus is to just re-use the original bolts. The heads have only been off one time in the 36 years I've owned the car so it's not like they've been on and off a half dozen times.

This was one of those things that I think that many of us have done......"I've gone this far with the rebuild what's another few bucks for some bolts". Lesson learned. Could have been a whole lot worse I guess!

Thanks,

Bob

Last edited by camarocards; May 12th, 14 at 06:17 PM.
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post #8 of 8 (permalink) Old May 12th, 14, 11:27 PM
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Re: '68 L35 BB Cylinder Head Torque Spec

Sheeze Bob, a half dozen times would be a welcome rest for mine. I've lost count, and not because of old age. Well, maybe a little.
Ya know, I'll bet your head gaskets are reusable no matter what the type. If the other head is on already just replace the bolts one at a time. For the head you had to remove, just put it back real quick before it notices. (it should be OK because it hasn't been run)
Good call too on reusing the original bolts. Don't forget a video.

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1000cfm AED and HD Harold-Lunati SR, 725 lift with shaft rockers too.
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