Manufacturing marks or wear?

[Ruairidh Dunford]

Thank you for all your thoughts so far.

This centre plate is from a very early engine (number 5XXX), and the spline is the shortest of the various lengths over production.

Further investigation has revealed an 1 5/16” crank fitted that has had the taper modified to take the original early flywheel.

I am waiting for a photo of the gearbox it was removed from.

[mk1-mark]

I think people are being distracted by the apparent "new" look of these marks.  Just because they aren't "old looking" doesn't mean they aren't old.  The part where the marks ar is well greased & has not been hardened, meaning the marks could potentially stay bright for years.  The radial nature & uniformity of these marks tell me that they were not made by something being worn or incorrectly fitted.  Each mark is "radial" in nature, moving out from the centre or toward the centre, they are not lateral chatter marks.

I am, as I said in my first post on this, absolutely convinced that they are machining marks from the spline cutting process, I inadvertently described them as broach marks, what I meant was that they were made by a gear cutting milling bit. Something although not identical, along these lines.

[Ruairidh Dunford]

Every idea I’ve read on the thread makes sense to me - thanks to everyone who is contributing, much appreciated.

[Bob Culver]

Whilst on the topic, I have always been somewhat surprised by this component. On the other side of the flywheel the much more thick crank is prone to fail.  Yet this  item not much more than 5/8 with no clutch cushioning springs seems fine on the most powerful engines, even with lightened flywheels. And this despite many having been subject to very worn engine bearings. The engine may not produce great torque but most cars have endured many violent clutch engagements. In the folly of youth I had one built up by hard chrome, reckoned to promote failure, but still no problems. I guess it goes to prove the fatigue resistance of case hardened parts.
 
Rather than worry about machining marks, the splines should be inspected for a good fit, and the rivets for security. It was common to find splines very seriously worn. When either is loose car develops a driveline clatter especially noticeable at low speed. And a serious wear step can cause clutch disengagement problems.  I gather some have renovated using carbide drills.
 
I wonder who designed such components. Edge was young to have attended tertiary classes. He may have covered draughting at secondary school, but unlikley to extend to necessary design level.

[Scarlet O'Hara]

Would the splines not be cut prior to any heat treatment therefore isn't it reasonable to expect that they'd be the same surface colour as the rest of the component if made during manufacture?

Rory has now determined that the flywheel taper has been enlarged to fit onto the bigger crank. Was this crank installed with the narrower AC front bearings which would require spacers to be fitted either side of the bearings however, possibly a spacer has only been fitted at the front which would move the crank backwards and therefore the clutch plate nearer the gearbox. Rory are the little ends of the conrods sitting central in the pistons or are they forward of center?

I'm in the wear camp on this one.

[Ruairidh Dunford]

Unusually it is the crankshaft taper that has been modified to allow the unmodified early flywheel to fit.

[AustinWood]

Modifying the crank would be a good solution if the original taper was in bad condition. Solves the flywheel problem and also rescues a bad crank.
Bob, I think the crankshaft breaks because of whip and vibrations while running which are damped by the mass of the flywheel. This weakens the crank. By the time the drive reaches the splines oscillations have been damped by the flywheel.

[Ruairidh Dunford]

Here are the photos of the gearbox it came out of, no signs of fretting whatsoever - as far as I can see...

   

[Chris KC]

Did you also see the release bearing assy. Ruairidh? There are some 'inventive' owner mods of these around.

[Duncan Grimmond]

Given that the scars are all almost identical in a slightly asymmetrical scallop form I'd guess that the same mistake has occurred at the same moment of each spline machining process. The marks appear to have been made by perhaps three different cutters or the same cutter at three different settings. Without having the object in my hands that is as far as my forensics will take me.
I'm sure a regular machinist could be more specific if he had the item in hand and a good magnifying glass for closer inspection.
Could it be that someone "liberated" a factory part prior to machining and did it himself at home?
But why?

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As the scars are virtually identical and form an asymmetrical scallop shape in line with each spline I think they must have been formed by a repeated operator error or a malfunction of machine repetition.
Having the thing in your hand with a good magnifying glass would help with the forensics.
Could an unmachined part have been "liberated" and finished at home? Why?

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Don't know what happened ther.. I posted, the post vanished, I rewrote and reposted, both appear. Could the machinist have had the same problem 80 years ago and has come back to haunt me?