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

[technicalninja]

@Superhawk996

I didn't intend to insinuate that you were wrong.

I was referring to me!

You are an absolutely top-notch source for knowledge.

You will go through the trouble of looking stuff up like no-one else I know.

I really appreciate your input.



On the loosening up of the crank bolt BS.

In 40 years of building stuff (very limited VW and Porsche) I've found as you improve horsepower you commonly need more vibration attenuation than less.

In the Miata world the stock balancer is SO bad many tuners want an aftermarket balancer BEFORE a tuning session. The stock balancer is not good enough for double or triple the horsepower.
Now, I'm tripling power on these, most of the T4 work on here doesn't triple the power so the need for increased balancing will be less.

I've seen multiple threads in the short time I've been on here regarding crank noses being damaged. Many of these could be from improper mounting but there are enough of them that makes me think "maybe there's more to this than we realize".

I like my street-based vehicles to have excessive damping.
I'm also a believer in full weight flywheels to increase transmission life...

Thanks again for a scientific look at the balancing requirements for an H layout engine.
It has been informative!

[Superhawk996]

I've seen multiple threads in the short time I've been on here regarding crank noses being damaged. Many of these could be from improper mounting but there are enough of them that makes me think "maybe there's more to this than we realize".

I think part of that is just due to your short time on the site.

Like you - I’m new too, having only been here since late 2018.

There have been two instances of crank nose loosening that got a lot of traffic in the last 9 months or so which makes it seem like it occurring often. But honestly, I don’t think I’ve seen more than 3 or four instances since 2019. Have there been more? Certainly. But, probably not a regular occurrence by any stretch of the imagination.

Like so many things, it’s partly an issue of people not understanding how things are designed to work.

What do I mean by that?

In the case of a taper, people simply think tightening the joint to spec is good enough. But that is 100% contingent on having a taper that is clean, dry, and has good interference fit between the two parts as they would have been when fresh back in 1970s when taken out of the dunnage for the engines initial build.

I think as parts get older and more beat up, we will see this issue more often. Especially with the home built engines where good intentions may not be enough to make up for a lack of attention to detail (not aimed at anyone in particular - especially OP).

For some reason, lots of people don’t have a real appreciation for how precise fits in an engine are. Likewise many don’t have an appreciation for how easily steel can be damaged.

I’ve worked in engine plants and I can tell you horror stories - mostly related to new hires that don’t have an appreciation for the precision machining that parts have and how they need to be protected from damage. There is a lot of work and training done to impart knowledge of how important it is to handle parts with care, yet you still see instances where someone isn’t thinking and damage can happen.

To some extent we all have to fight the urge to just get a job done and we need to focus on doing it right. I know I’m guilty of this sometimes.

Anyway, just a point of view for consideration about how common this issue may or may not be.