Flywheel/crankshaft dowel

[RichardHawkins]

I have bought a new flywheel and thought I may as well have one with a dowel, as the crankshaft has a hole for a dowel. Ignoring my own advice on all things Lotus, check and measure everything, I assumed the hole in the crank and flywheel would be the same diameter. The dowel I bought is 3/8” as is the hole in the flywheel, but the hole in the crankshaft appears to be 7/16”. I say appears to be because I have always struggled to measure internal diameters.

Has anyone else come across this? Which diameter is correct? Do I need a stepped dowel?

Richard Hawkins

[alan.barker]

Do you need to fit a Dowel
Alan

[RichardHawkins]

Alan,

Probably not, but as the crank had a dowel hole and the flywheel was offered with a dowel hole I thought I would.

Richard Hawkins

[Craven]

I need to double check, I’ve got some new ones somewhere if you get stuck, it’s a stepped dowel made like this to ensure it can’t come out.
Larger diameter will not pass through the flywheel, most dowels are made this way, flywheel to clutch cover is the same they can’t come out as well.

[RichardHawkins]

Craven,

Thanks, that makes sense. I had noticed the clutch pressure plate dowels, but not joined the dots to the crank/flywheel dowel.

Richard Hawkins

[HCA]

Why are there, sometimes, two dowels?

[Craven]

Even when Ford went to 6 bolt flywheels under some condition, high torque, the clamping force was insufficient and bolt shear occurred a single dowel was added ok most times, I’ve seen 3 dowels used; of course there is always a 12 bolt flywheel.

[HCA]

Outside of bus engines and the like, I find it hard to comprehend flywheel bolt sheer on small car engines, but know that it is not an uncommon issue. I would have thought there many items down the transmission line that would sheer ahead of six hefty bolts to the crankshaft!

I have seen one instance - on a beefy V8 where the experts put it down to the assembler oiling the threads…

I thought I’d pay lip service to this possibility and ordered three of my bolts to be ARP type. Maybe I should make it all six now!

[alan.barker]

On my Sprint no Dowel.
Hole in Crank for Dowel but no Hole in Flywheel. Been ok like this since 1972 dob.
Alan

[rgh0]

Outside of bus engines and the like, I find it hard to comprehend flywheel bolt sheer on small car engines, but know that it is not an uncommon issue. I would have thought there many items down the transmission line that would sheer ahead of six hefty bolts to the crankshaft!

I have seen one instance - on a beefy V8 where the experts put it down to the assembler oiling the threads…

I thought I’d pay lip service to this possibility and ordered three of my bolts to be ARP type. Maybe I should make it all six now!

Hi Hal

I would not mix the bolts i would use either original Ford or ARP

cheers
Rohan

[HCA]

OK Rohan - I hear you Is this all to do with differing stretch abilities by any chance?

I did wonder at the time if mixing the bolts, the work would be down to the three bolts best able to keep the clamp force that would end up no stronger than an original four bolt set-up...but then I looked at it objectively and with it being only a 150bhp road engine and no intention of pulling a double horsebox, what could go wrong! - especially as there two dowels holding it all in place...

But I shall change my order! Thanks!

[Andy8421]

Outside of bus engines and the like, I find it hard to comprehend flywheel bolt sheer on small car engines, but know that it is not an uncommon issue. I would have thought there many items down the transmission line that would sheer ahead of six hefty bolts to the crankshaft!

I have seen one instance - on a beefy V8 where the experts put it down to the assembler oiling the threads…

I thought I’d pay lip service to this possibility and ordered three of my bolts to be ARP type. Maybe I should make it all six now!

Its the oscillating torsional forces that do the bolts in. Every time a piston comes up to compression, the flywheel is slowed slightly, every time a cylinder fires, the flywheel is accelerated slightly. Its the back and forth stress that fatigues the bolts. Anything downstream of the flywheel has the peak torque loads smoothed by the inertial damping effect of the flywheel and aren't so exposed to this problem.

I recall reading that the first outing of Duckworth's new DFV engine was marred by the timing gears destroying themselves because of this effect. The angular peak load was many times the average torque required to turn the camshafts, and Duckworth had to design a quill spring torsion absorber to reduce the peak loads experienced by the gears.

Edit: Apparently the problem still exists and modern F1 engines have a similar vibration absorber to Duckworth's original design.

https://www.f1technical.net/forum/viewtopic.php?t=26642

[HCA]

Wow! Talk of learning something new every day

Eminent sense though once I read it! I can see clearer now why oiling the threads will only help the oscillation do its job...

ARPs then

Thanks Andy!

[RichardHawkins]

Andy,

Thanks for the fascinating insight. I had no idea I was opening up such a topic. I do have a bit of industrial experience of gear failure which I left to those who had much more knowledge. Two helical gears in mesh being driven by an electric motor, one gear was more flexible than the other, and kept breaking, the answer was “a helix angle correction” which I understood to mean that the gears meshed correctly under load. Way above my maths.

Richard Hawkins

[2cams70]

For bolts that operate dry It is actually best to oil the threads before you install the bolts. I use a light grade "singer" machine oil - i.e something with little anti friction properties that will influence the stretch versus torque relationship.

The general rules are:
1. If the OEM recommends a lubricant for the threads whilst tightening use the lubricant specified when tightening.
2. If no lubricant is specified and the bolts operate in a dry environment use a light grade oil such as the singer machine oil. Dry thread torque normally assumes just a light coating of rust prevention oil (i.e oil such as "Singer")
3. If no lubricant is specified and the bolts operate in a wet environment submerged in oil use the same lubricant on the threads as the bolts operate in when they are in service.