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Old 02-15-2010, 07:03 PM   #1
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Default Bolts, classing, hardness, and strength...

So at some point in the near future I'm going to be removing my timing wheel to do some minor modifications. I've decided to go ahead and purchase new bolts head of time for when I re-install it, as I hate re-using fasteners in super-critical applications like this, especially when they're only 40-50 cents each.

For review, I'm talking about the four 6mm x 70mm bolts that hold my 36-1 wheel and its aluminum hub (and the crank pulley) onto the front of the crankshaft:



(ignore the large bolt in the middle in that image- I only put that one in when I need to turn the crank with a wrench. It's not part of the main structure of the system)


I have a couple of options here. I can either get zinc-plated class 10.9 hex-head bolts (which is what I have in there now) or I can get unplated class 12.9 socket-head bolts.

I like the idea of using bolts with a higher yield strength, which I'll be able to torque to a higher preload. I'm slightly concerned that the 12.9 bolts' greater hardness may cause them to be more likely to fail catastrophically in what has got to be a brutally high-stress environment. And I'm slightly more concerned that using unplated hardware in this environment is just an incredibly stupid idea in general.

Do the Mech-Es among us have any thoughts on the matter?
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Old 02-15-2010, 07:06 PM   #2
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Do they still look that good? I don't see the problem with re-using a good bolt. What are they torqued to? Surely not too much. As to what I would replace them with, I would stick with what you know and just buy new duplicates of what seem to be doing their job, if they are only $.50 each.
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Old 02-15-2010, 07:37 PM   #3
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I'm not re-using them. Bolts like this get torqued once, then thrown away.

The ones that are in now look fine. Can't say the same for the wheel itself (covered in rust) but I see no indication that the hardware has suffered in the least.

I'm definitely leaning towards replacing with same. Just trying to convince myself it's the right thing to do. Several folks speculated when I first did this that the bolts would fail leading to the destruction of the crankshaft (and / or a spiked, spinning wheel coming out through the top of my hood) and for some reason I'm still paranoid about it two years later.
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Old 02-15-2010, 07:50 PM   #4
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Default the post where I go into far too much detail.

bolts clamping two flat pieces together are primarily in tension, not shear. They do their work by acting as super high rate springs when you torque them. the main force preventing relative motion is friction between the mating part interface.

clamp load obviously increases with bolt torque.

and increases in bolt tensile strength mean higher clamp load capacities.

the FSM says 8.7-13 ft-lbs for the four pulley bolts.

based on these charts:
US & Metric Bolt Torque by Grade
http://www.rpmmech.com/docs/tightening_torque.pdf

it appears they're using a lower grade bolt. If you want to "uprate" that, an 8.8 should be plenty, but I wouldn't torque it past the 13 ft-lb limit for fear of stripping threads.

You can look at the second chart to get a feel for the clamp loads involved if you want. multiply the clamp load by the number of bolts for the total clamp load. what's that work out to, 3-4000 lbs clamp load?

so even with a higher rated bolt, you're not getting more clamp load at the same torque, just more factor of safety.
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Old 02-15-2010, 10:25 PM   #5
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If 4 of those bolts can't hold 13lb-ft each, something is seriously wrong. I'd stick with what you have, unless you can get something with a higher rating just as easily.
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Old 02-15-2010, 10:37 PM   #6
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I doubt that it is a very high stress environment at all. Especially when all for bolts are torqued similarly. No relevant sheer will be acting on them on startup. The aluminum spacer and toothed wheel aren't heavy enough to cause any. Since crank pulley and toothed wheel will spin at same velocity, again, no sheer. New bolts (for your piece of mind), torque to spec and forget.
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Old 02-15-2010, 10:47 PM   #7
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If you actually need to throw them out after each use, you're tightening them too much, or being paranoid. You'll start sacrificing too much elasticity when you go to a higher grade. Get the standard 8.8, long enough to engage all the threads, and torque properly. I think fsm says use loctite, but you shouldn't have. Might help put your mind at ease though.
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Old 02-15-2010, 11:21 PM   #8
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Quote:
Originally Posted by curly View Post
(...) or being paranoid.
This.

Sometimes, the things I build have a tendency not to work in rather dramatic ways. It doesn't happen often, but when it does, I prefer that it not result in the destruction of my engine.

New 10.9 bolts it is.
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Old 02-15-2010, 11:32 PM   #9
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also joe, the longer the bolt, the higher its stretch capacity. if those bolts are longer than the original ones in the pulley, then you're even better off.
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Old 02-16-2010, 12:13 AM   #10
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On my old application with a 60-2 wheel I drilled and pinned the wheel with roll pins. They are very strong and take all the side load stress off the bolts so you don't have to worry about vibration working on them. The bolts then just hold the wheel on.

I can't tell from that picture what keeps the wheel centered?
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Old 02-16-2010, 03:20 AM   #11
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Quote:
Originally Posted by rweatherford View Post
I can't tell from that picture what keeps the wheel centered?
The inside of the aluminum hub self-centers on the large crank bolt. The wheel self-centers on a raised section in the middle of the hub (beneath the large bolt and washer in the above pic.) Details here.
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Old 02-16-2010, 10:24 AM   #12
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If by socket bolt, you mean one of these guys here


Then I have experienced socket bolt failure in this application. It occurred on the dyno. The head of one bolt sheared which allowed the timing wheel to begin separating from the crank pulley--creating an awful noise that sounded a lot like a really bad lower engine problem. 2 other bolts loosened up. Bolt 4 was tight.

The theory/speculation is that the bolts were much harder than the soft aluminum trigger wheel underneath. As the bolts hammered on the trigger wheel, it wore material away, allowing vibration to eventually destroy the 1 bolt and back out the other 2. As I found out later, I am not the only person who has experienced this, and the ultimate solution is install a large steel washer ring behind all 4 bolts.

So my experience is that the bolts themselves can fail, but the key factor is in the mating surface--not the bolt itself.
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Old 02-16-2010, 11:54 AM   #13
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The TEC2 folks from 2001 had the same experience as Ben with the same solution (larger washers).
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Old 02-16-2010, 01:29 PM   #14
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Interesting stuff.

Yes, those were the socket bolts I was considering as an alternative to the ones I currently have. In fact, if you look at the pictures I took during the mock-up of my original build, there is a set of them sitting in the stackup. I wound up not using them as the particular ones I'd bought were too short.

In my case, the mating surface for the bolts (apart from some common cut-washers) is the 36-1 timing wheel, which is made from mild steel about 3/16" thick. This wheel sits on top of the almost completely solid face of the aluminum center hub, which in turn sits on top of the crankshaft pulley, in the place where the large steel washer with five holes sat in the stock configuration.

I'm not concerned about deformation of the mating surfaces.

My only concern (and this is purely speculative) is that any imbalance in the hub/wheel combination will, at high RPM, attempt to place a bending load on the assembly (IOW- force it outward to one side.)

If the bolts, which are in fact quite long, are able to stretch, then the assembly will be able to yield to these forces and displace itself from centerline, this increasing the load, etc.

I'm just being paranoid, I admit.
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Old 02-16-2010, 02:07 PM   #15
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Quote:
Originally Posted by Joe Perez View Post
My only concern (and this is purely speculative) is that any imbalance in the hub/wheel combination will, at high RPM, attempt to place a bending load on the assembly (IOW- force it outward to one side.)
Seems like a valid concern. But I can't imagine why your assembly would be out of balance.

Isn't the whole configuration slightly out of balance anyway due to the forces required to drive the accessories?
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Old 02-16-2010, 02:48 PM   #16
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Should be able to take it to a decent shop and have the whole assembly balanced. And by whole assembly, I mean pressure plate, flywheel, crank, timing sprocket, pulley, aluminum hub, and timing wheel, with all hardware.
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Old 02-16-2010, 03:32 PM   #17
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The biggest forces on it are not imbalance, but probably crankshaft torsional vibration. (which the ATI damper reduces BTW).
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Old 02-16-2010, 09:37 PM   #18
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I vote for vibration and since I had one on a VERY high NVH motor (DIY Turbocharged 0Olds Quad4) with an aluminum crank pulley, I still vote for a positive lock in the rotational axis with something.

I used roll pins with very good results after one failure from bolts loosening and deforming the surface under the bolt face.

I was a TEC2 user as well.
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Old 02-16-2010, 09:43 PM   #19
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Be scared! BE VERY SCARED! You will have a ninja throwing star sticking out of your hood in a month.
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Old 02-16-2010, 10:53 PM   #20
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Quote:
Originally Posted by curly View Post
And by whole assembly, I mean pressure plate, flywheel, crank, timing sprocket, pulley, aluminum hub, and timing wheel, with all hardware.
Yeah, well, given that the crankshaft is sort of inside the engine just at the moment, I kinda don't see that happening.

Quote:
Originally Posted by rweatherford View Post
I still vote for a positive lock in the rotational axis with something.
Provided by the stock pin pressed into the hub thingy that sits in front of the crank. It's not quite an interference fit, but it's snug.

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Be scared! BE VERY SCARED! You will have a ninja throwing star sticking out of your hood in a month.
Heh. It's been fine for close to two years, but it's funny that This Thread should come up today.
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