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 recently reminded that early Miatas had a problem with the valve drive gear bolt coming loose; we used to lose Miata crankshafts often in racing, and checking the crankshaft bolt became a regular pre-weekend prep item. The design was fixed after '91 with a longer nose and bigger bolt.

The Miata design is different, in that it's a straight fit and the keyway drives the pulley. But a buddy reminded me of the DIY trick for "repairing" the situation if you're on a budget.

https://www.miata.net/garage/hsue/Loctite%2...%20Part%201.htm

And a video from Loctite: https://www.youtube.com/watch?v=LLSLtr0ECbc...%AENorthAmerica

I'm leaning toward small files to knock down the high bits on the backside of the key damage and then valve-lapping-compound the taper all the way around with a replacement hub (and bolt and key). Chris is leaning toward having me schlep the assembly to his shop for bronze TIG, but I'm wondering, given a good taper, if something as simple as Loctite 660 to stabilize the key would be fine. Loctite lists the shear strength of 660 as approaching 3000 psi, which is more than adequate. What I can't tell if the keyway would removable; I'm guessing not.

Let's check those bolts, team. It's not hard at all to do and I fear there may be more of us out there...23 ft-lbs and use Loctite 243 on the bolt. - GA

[Superhawk996]

What I can't tell if the keyway would removable; I'm guessing not.

Loctite site says 500F heat to remove the key.

I have no experience with that product but sure looks interesting and appropriate for the application. I would imagine you’ll still get minor variation in timing depending on how perpendicular the key is to the crank.

I remember the Miata fiasco vividly - had a friend with a 90’. Mine was a 91’ specifically to avoid that issue. Pretty rare for the Japanese to screw up the engineering that badly but it happened - probably in the rush to get it to market. Put 220k miles on my 91 and never had an issue after they fixed it. Completely different design than a taper though!

https://www.youtube.com/watch?v=6-m2IT4Xojo