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

[Superhawk996]

Not trying to throw anyone under the bus so to speak. (IMG:style_emoticons/default/laugh.gif)

Checking torque after the fact with a typical clicker wrench isn’t necessarily an indication that the hub is loose or that it wasn’t torqued properly on assembly. I fully believe that the race car engine in particular was built properly.

I don’t want to go into a deep dive on how fastener toque specs and subsequent audit torque checks are established but it will suffice to say that we would need way more than a sample size of two or three fasteners.

Likewise simply jacking up the torque may or may not help. If the fastener torque spec is close to yield, increasing it arbitrarily can take it over yield and there will certainly be some loss of clamp load if the fastener wasn’t designed for a torque to yield application. I don’t think that is the case here but I don’t have the fastener data to know one way or the other since I don’t know what grade the hub fastener was as OEM or if someone swapped it out for something lesser along the way (doubt it).

At this point I’m deep in the weeds and simply hoping that the fix holds up for the long haul.