Crankshaft Hub Bolt Loose? Torque spec?, Sigh... Options

[GregAmy]

Is it a known issue for the fan hub bolt to loosen?

Backstory: car started running like hell last Spring on my Microsquirt conversion. Wouldn't rev, #3 CHT going vertical, very hot exhaust. WTF, over? I checked some basic stuff, nothing found.

Drove it again a week later and it seemed to be getting worse. Checked the ECU logs and all seemed normal, TPS, MPS, all working as expected. I even went back to a tune that worked great in November (only difference was cold startup stuff, which I can test/adjust in the Spring when it's colder), but still even worse. Checked timing with an adjustable light at full advance, spot on for what the ignition map was calling for.

I parked the car. Life was busy this year and I just didn't have time to deal with it. But I trotted it out again in September, and it was just as bad. Since I had problems with the IGN4VW coil a year or so ago (mounted in a hot place and was cutting out), I replaced the coil with one from NAPA.

Suddenly the car would not start at all. And when I looked at the ECU logs during cranking, I was seeing 0 RPMs. Bad crank position sensor?

Today I put the car on the lift and visually inspected the crank position sensor (as well as I could, anyway; the toothed gear replaces the A/C spacer) and the spacing of the CPS tip to the teeth was clearly too large; whereas it should be about 50-thou, it was actually around 1/8"-3/16". I pried on the sensor bracket with a screwdriver, and it was solidly mounted, no looseness.

I know I spent a LOT of time getting that sensor set just right during the install (you can't get it it with the engine installed). So I knew something was up. The sensor was solidly mounted, the toothed wheel was not loose, so there was only one thing to check...

..and yup, the hub bolt was loose, allowing the hub to walk outward. Sigh...

I tightened it down - best spec I can find in the interwebs is 23ft-lbs - and of course the car fired right up and drives fine. I hope this didn't cause any other damage...

Is this common? I've never seen it before. The engine was built in California and I truly don't recall who installed the hub, them or me. It's torqued down right now and I'm going to check it on a regular basis (I thought about pulling the bolt and putting Blue Loc-Tite on it, but if I lost that bolt in there then I'd be toast and would have to pull the engine to get it back. Arthritis in the thumbs and tennis elbow so NFW I'm tempting that Fate...)

Oh, and then I broke the door cam actuator when I was putting it back in the garage..it just never ends, does it...? - GA

[GregAmy]

I was having a dinner conversation with a couple buddies last night about this problem, how the tapers came loose and damaged the crankshaft.

One buddy - a Chevy LS guy - started making fun of old Germans with wonky engineering ideas and why didn't they just do like 'Merica and put a press fit and key there.

The other buddy drives a late-model BMW E-something (M4? I don't know those cars) just a few years old. He noted that the BMW line is still using that taper fit for driving the cam belts, without a keyway, and is having problem with guys modding their engines to the point where when suddenly downshifting the crank hubs are spinning and destroying multi-$10Kengines. The aftermarket "fix" is apparently to drill the crankshaft for a pin/key to stop that.

And that got my brain to remembering when I did my camshafts belt on my '00 Audi S4 at 100k miles, some 14 years ago. I remember that job with dread. The cam pulleys were taper-fit only, no drive keys. You'd set the engine to TDC and remove the tensioners and the belt, then loosen the bolts holding on the gears; you'd put the new belt and idler pulleys on and then setup this elaborate (rented) alignment tool with bars and pins that would locate all the engine drive components - crankshaft, camshafts, whatever was in there - at a specific rotation. And then you'd tighten the pulleys down to spec.

I remember thinking at the time that my entire investment in that car was wholly dependent on a bunch of taper-fit pulleys being properly secured by bolts...and if anything slipped I'd be funked...but nothing ever did.

Amazing how thoughts like that get buried away in your head, only to be exhumed at interesting times...

So ze Germans are still onboard with this taper fit thing even today. - GA

[930cabman]

I was having a dinner conversation with a couple buddies last night about this problem, how the tapers came loose and damaged the crankshaft.

One buddy - a Checvy LS guy - started making fun of old Germans with wonky engineering ideas and why didn't they just do like 'Merica and put a press fit and key there.

The other buddy drives a late-model BMW E-something (M4? I don't know those cars) just a few years old. He noted that the BMW line is still using that taper fit for driving teh cam belts, without a keyway, and is having problem with guys modding their engines to the point where when suddenly downshifting the crank hubs are spinning and destroying multi-$10Kengines. The aftermarket "fix" is apparently to drill the crankshaft for a pin/key to stop that.

And that got my brain to remembering when I did my camshafts belt on my '00 Audi S4 at 100k miles, some 14 years ago. I remember that job with dread. The cam pulleys were taper-fit only, no drive keys. You'd set the engine to TDC and remove the tensioners and the belt, then loosen the bolts holding on the gears; you'd put the new belt and idler pulleys on and then setup this elaborate (rented) alignment tool with bars and pins that would locate all the engine drive components - crankshaft, camshafts, whatever was in there - at a specific rotation. And then you'd tighten the pulleys down to spec.

I remember thinking at the time that my entire investment in that car was wholly dependent on a bunch of taper-fit pulleys being properly secured by bolts...and if anything slipped I'd be funked...but nothing ever did.

Amazing how thoughts like that get buried away in your head, only to be exhumed at interesting times...

So ze Germans are still onboard with this taper fit thing even today. - GA

Exactly why myself and many more just love the simplicity of our 1/2+ century old sporting machines.