whoa baby! Rocker bolt stretch

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Ironmike

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Man. Trying to button everything up today. Everything going pretty good this build, and I run across this.

Torqueing my rocker shafts and things just didn't feel right. Just COULDN'T reach 30 ft lbs. Thought something might be stripping and ran across this.
Check out the rocker shaft holdown bolt on the right. Not a very good pic, but I hope you can see the amount of streeeech. Glad it didn't break. It actually has a "waist".

I think the combination of 565lbs open pressure and the fact that these are probably factory(old) bolts caused the problem. I'm running Hughes 1.6 rockers with their shafts.

Now. Should I shell out for their Billet holdown kit, or just go find some new grade 8 bolts.

Sure appreciate your advice, as always!
 

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30 LB FT is just about TWICE their torque spec. 15 is what all mine get unless they are aftermarket or studs.
 
Looks like one bolt was actually longer. Granted the threads are stretched but the shank length is not even close between the two bolts.
 
RustyRatRod said:
30 LB FT is just about TWICE their torque spec. 15 is what all mine get unless they are aftermarket or studs.
Click to expand...

IIRC, the factory service manual has the wrong spec for that bolt (too high).
 
autoxcuda said:
IIRC, the factory service manual has the wrong spec for that bolt (too high).
Click to expand...

The 69 FSM, I'll guess earliers as well, have the wrong bolt size listed as a 3/8 when it's 5/16. Most of the sites that glom info from the same rotten source have the incorrect 30# listed. 70 and later FSM got it straightened out

If you build stuff please pay attention to the bolt sizes so you don't overtorque stuff.

The correct torque is 200in/# divided by 12 or about 17ft/#
 
The book I use when I assemble engines is about 40 years old (and falling apart) and it lists the torque spec for those at 15 lbs for the 318 and 30lbs for all the other small blocks. Guess I'm not going 30 on the 340!
 
That's why you have to watch when reusing bolts. Each time that they are torqued, they stretch slightly. Eventually they stretch enough to where they "neck down" as shown in the bolt on the right of the first picture. this happens just before the bolt will fracture/break. Once this starts, the strength of the bolt is now compromised....
 
If they are torqued properly, the bolts will return to the original dimension assuming they are being loaded within their tolerance. Rocker shft bots (grade 5, 5/16 shaft) are nowehere near capacity at 18 pound feet. They are beyond it at double...lol. You should also check the rocker shaft - if it's factory stock, and you're running a roller rocker, you may have squished it out of round. Thickwall shafts will just pull the threads or break the bolt. But thin shafts can deform.
 
Nice example of plastic deformation. Throw them away. Correct spec for all small block rocker shaft hold downs is 200 inch pounds, from the 1973 Dodge Service Manual. 30 foot pounds is for big blocks with 3/8 bolts.
 
No, it was a comedy of errors.

1) I added shims to the shaft to compensate for milling the head and block but didn't use a tapered thickness shim so the all the pressure was on the pedestal tips.

2) I dialed in the wrong torque value from the '69 service manual.

3) I determined after the fact that the torque wrench I used was inconsistent.

A total screw up on my part.

An expensive lesson learned.
 
how did that adage go?
tighten it until it snaps then back it off half a turn?
 
Factory rocker bolts on my 360 seemed waaay too soft when I tightened them so I replaced them with grade 8's. which seems to work just fine. Torque specs in the book I had were out to lunch as well.
 
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