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Walt Fricke Walt Fricke is offline
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Join Date: May 2004
Location: Boulder, Colorado
Posts: 5,063
Porsche Crest

I'm a little surprised that all of your bolts didn't hold up with the 88 lbs/ft (versus 66). I say that because on my 2.7 motors I routinely torque the larger (12mm) bolts to 150 lbs/ft despite a factory torque of 110. If you spin your engine above the factory redline with any frequency on these 70.4mm 6 bolt cranks you need the extra bolt torque or the bolts will back out. But I've never had that sinking feeling you get when something starts to give when torquing a bolt. I looked up the supposed max torque for these bolts (Maryland Metric had a chart) and it was around 137 lbs/ft.

My conclusion was that torque is set rather conservatively in most applications. MM's chart shows that for grade 12.9 10mm bolts, coarse thread max recommended torque is 98 Nm/72 lbs-ft, and for fine it is 104/76. So you are over, but not by a whole lot.

With your 9 bolt crank that issue seems to have been resolved, and one does not hear of the need for tricks to keep the bolts in place.

I am pretty sure the factory manual says to replace these bolts. If it didn't I suspect I'd be reusing mine (at least I would have before I started overtorquing them on purpose). A shop owner once told me he routinely reused these bolts on street motors, though, and had had no problems at stock torques. But because of the overtorquing I have a bag of them I try to use for other purposes less critical than the flywheel.

But perhaps Porsche decided that with the lower torque and the 9 bolts it was no longer necessary to replace these for the 9 bolt cranks?

Myself, having had one start to fail I'd be inclined to replace them all. Sure, having different torque on only one of 9 may not mean anything. And perhaps that one bolt was a hair off its nominal strength. But why take the risk? As I recall, buying 9 of these smaller bolts was less expensive than six of the 12mm ones for earlier cranks.

Walt Fricke
Old 09-06-2005, 10:16 AM
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