I got e-mail last week from a guy named Warren, of Heber. He is thinking about building a coupe and wanted to come by, look at the chassis, sit in it, etc. So on Saturday I went out and spent a couple of hours cleaning up my shop and getting everything looking good. Show-and-tell went great, and the clean shop got me inspired to do some work on the cars.
First, of course, was swapping the winter tires back to the summer tires on the van. Then I fixed the coolant leak in my RX-7. (I hope!) But finally, I went to install my intake manifold, which has just been sitting on top of my motor since I did the cam swap.
The intake manifold has 12 bolts, and you're supposed to tighten them in a specific sequence, first to 8 ft-lbs, then 16, then 22-24. I don't think my torque wrench is trustworthy at 8 ft-lbs, but I just went snug, then 16 then started to do 24 and SNAP!
One of the Grade 8 manifold bolts broke. Brittle failure, on a 5/16" bolt at something less than 24 ft-lbs. Weird. OK, let's do a little calculation. A 5/16" bolt should have a stress area of .0524 in^2. Grade 8 means minimum tensile strength of 150,000 psi. So that's a maximum load of 7860 lbs. Now according to my sources, a torque of 25 ft-lbs on a dry 5/16"-18 bolt gives a clamp load of 4720 lb. But assume my torque wrench is off by 20% to give and the threads were lubricated by some residual oil. Now we've got 7550 lbs. Hmm. That's getting close.
Well, now I'm paranoid about the other bolts, so I ordered a complete set. Haven't pulled the manifold back off, so I'm not sure exactly how difficult it is going to be to remove the broken bolt. But I'm hoping that it should come out pretty easily, since it's not like a bolt that was frozen and snapped off when I was trying to remove it. It shouldn't even be tight in there.