Flywheel bolts

[Tadpole]

Hi everyone,

Just got thru doing battle with the flywheel bolts. The first two came out and the next four stripped the heads out. With welder in hand I sacrificed an old allen wrench and now all the bolts are out.
My question is are these bolts really that special to cost almost $13.00 apiece? Or can a hardened bolt be used? Just trying to learn as much as I can and to see if money could be saved. If the bolts are special so be it and I 'll order them. Thanks for any info.

[Walt Fricke]

Mike

I shudder at the cost too. But this isn't really the place to skimp. Almost like the Jesus bolt on a helicopter.

Or, if you are a risk taker and this is a street motor, reuse the ones you have. A shop owner I know used to reuse these for street motors, and said he never had a problem despite everyone from the factory on down advising not to do this. Of course, most of yours can't be reused, so you are sort of spared that temptation.

The stock torque on these is pretty high. On my 6 bolt race motors I torque them to 150 lbs/ft. Where can you get a bolt with this head and thread pitch other than at around $13? The head has to be thin to fit the space available.

Walt

[Tadpole]

Thanks Walt,
You talked me into it. This car doesn't need another potential problem down the road. I'll order the bolts as it does sound like that they are special.

[Superman]

At 6000 rpm, the flywheel turns 100 revolutions per second. The pressure plate is bolted to the flywheel and this is where the engine's torque is transferred to the transmission input shaft. Whether or not to reuse flywheel bolts is a good question, but a better question is how in the world do they stay on in the first place. I do not know anyone who reuses these. I know lots of mechanics who will reuse pressure plate bolts. I've heard of guys rebuilding motors without a torque wrench. But they all use NEW flywheel bolts.

[Walt Fricke]

Well, let's see: the torque is transmitted through the tightly clamped interface between the end of the crank and the flywheel. The stretch of the bolts creates this clamping, and it is not the shear strength of the bolts which comes into play to transmit the crank's torque (at least not directly - these are fasteners, not pins). So say the engineer types who have commented on matters like this anyway. Which is why I am careful not to get any lubricant on the interface - like stray Loctite.

From personal experience, loose flywheel bolts will not shear, at least not in the time it takes you to realize the clutch isn't completely releasing and you can't get into gear from a stop because the bolts have backed out and are rubbing the clutch disk center. I've run at least 20 minutes (a run session) in this condition, back before I figured it out. So even without clamping, their shear capacity is pretty significant.

If the bolt is stretched beyond its elastic limit, my understanding is that it now has a new, lower, elastic limit, so it can't produce as much clamping force. High quality rod bolts, which have forces trying to stretch them dynamically all their lives, can be reused if they aren't stretched too much.

With a flywheel bolt, what tension does it see beyond its initial stretching/torque? Thermal effects would be modest? And would tend to reduce rather than increase stretch as things heated up? Steels fatigue cycled below their elastic limit can last a long time. Thermal stressing is about the only cycling I can see for these bolts (removing and retightening is so low a cycle as to be nil).

So if a fastener can survive a ~50% increase in torque (I'm talking the 6 bolt crank only) as I know from my experience these can, what exactly is going to make them fail if reused a couple of times at the lower, stock, torque/stretch? Which works fine until you get up somewhere above 7,000 rpm on a 2.7? But will allow the bolt to back out if you decide to shift at 8,000 rpm routinely?

Anyway, Mike doesn't have a choice here, as he couldn't get most of his bolts to back out without stripping the hex (I bought a relatively high dollar triple square socket tool, though I have had modest success with the FLAPS kind if I keep it short out of the socket holder and grind its end flush with just a little radius on the end to ensure maximum gripping surface).

Mind you, if Porsche says don't do something, that's pretty important. I'm not on a crusade to get folks to reuse these bolts routinely. But I wouldn't be a DIY guy if I didn't wonder what was behind things.

I've seen, in person and more often via lists like this, failed rod bolts. I even think they have a characteristic necked down look on each side of the fracture surface. But never a failed flywheel bolt. The ones which backed out were a bit beaten up about the exposed threads, of course. As the torque wrench approaches 150 lbs/ft, I keep waiting for that feeling you get when a fastener is shearing. Lord knows, I've had that inconvenient feeling often enough. Not with these bolts, though.

Walt

[RWebb]

Porsche had a TSB out on these things - I posted the gist of it on an old thread.

But, I just stay away from the edge of a cornice, and I just use NEW, FACTORY bolts. Makes things simple.

FWIW, I am a member of the Red Loctite Club too.

[Ken911]

Kind of strange every other flywheel bolt is a dollar or two except the ones for your car. I wondered why so much, the ones for my turbo are 1.50 and I replace them every time. those and the pressure plate bolts stretch. so the second time they are used the torque may be the same but the stretch and the clamping force will be different and closer to the yield strength on the bolts.

[Walt Fricke]

The 9 bolt flywheels use smaller bolts with less torque. That larger diameter crank end must change the demands on the fasteners for the better.