Big-end Journal Wall thickness

Possum · Jan 24, 2017

I tried to post this late last night & lost it into the void...so apologies if it is repeated elsewhere; I am better on a micrometer than a computer.

Does anybody have a 600cc &/or a 650cc crankshaft pulled apart?

I am interested in knowing the inside diameter of the hollow core of the crank-pin (Big-end) journals, to see if Norton altered the wall thickness of the journal, or just the outside diameter & stroke of the journal.

For example, the 500cc crank-pin journal is 1.500" diameter with a core of 1.000" diameter.

Rohan · Jan 24, 2017

Can't help with the 650cc stuff, although that was discussed here somewhere a while back.
The 650cc crank was a lot stronger than previous.

The 600cc Model 99 cranks used the same crank journal - and rods - as the 500.
Once they were hotrodded a bit, thats why they started having crank problems.
What was good for a mild 500cc was rather marginal for a 600 cc.
As all the old timers chime in with as soon as this subject is raised.
They weren't known as detonators for nothing ....

Oddly too, hollow tubes can be stronger than solid shafts, wall thickness and hollow size depending.
Graphing it shows a web of crossing lines/curves, not a simple exercise and not one size/ratio will always be stronger...

I seem to recall also some discussion about internal machine marks and stress raisers and poor machining,
so all food for thought... (might have been Commando though ?).
hth.

Possum · Jan 27, 2017

Seems the core diameter did not change from 500 to 650; just the journal diameter from 600 to 750.

So the hairline crack in the journal radius of the 650 crank that I have recently acquired, is the result of the core being increased to 1-1/4" diameter. Presumably in an attempt to reduce weight...

Another doorstop!

Burgs · Jan 28, 2017

Hi Possum
I have a 650SS crank that I can strip down and measure if you still require it?
It's a 1963 year model and I have to strip it down anyway.

Best regards
Burgs

Rohan · Jan 29, 2017

Possum said:
Seems the core diameter did not change from 500 to 650; just the journal diameter from 600 to 750.

Just to be clear here, precisely what is this 'core diameter' you mention ?

And, is not the 650 near about the same as the 750 ?

Possum · Feb 24, 2017

I measured 500cc, 600cc & 650cc and found them all to have a 1" I.D. big-end journal core.

Rohan · Feb 24, 2017

Which actually means the 650 has a lot more metal there than the small journal 500 and 600 versions...

wot · Mar 2, 2017

Some sort of British engineering?

Rohan said:
hollow tubes can be stronger than solid shafts

Rohan · Mar 2, 2017

Not at all - have a look in any standard engineering textbook.

Having a hole through the middle distributes the stresses around the shaft.
Depending on the dimensions, this means a large circular hollow shaft is usually (much) stronger
than a smaller solid shaft, for the same metal.
Ribs fillets and fillers in aircraft often have hole(s) in them for the same reason.

dynodave · Mar 2, 2017

Possum said:
I measured 500cc, 600cc & 650cc and found them all to have a 1" I.D. big-end journal core.

A few years ago I asked around about 88SS cranks rumored being different. I had measured mine and on BenG face book page and he also emailed me confirmed the smaller diameter core for the uprated SS crank core size but I failed to write the size down.

My (bogus) 88SS crank is standard so I am not going to tune all the way to hopefully give it a chance to live and not blow up.

wot · Mar 13, 2017

Rohan said:
Not at all - have a look in any standard engineering textbook.

Having a hole through the middle distributes the stresses around the shaft.
Depending on the dimensions, this means a large circular hollow shaft is usually (much) stronger
than a smaller solid shaft, for the same metal.
Ribs fillets and fillers in aircraft often have hole(s) in them for the same reason.

Amazing, you drill a hole in a round bar and that makes is stronger.

So a 1.750 diameter bar with a 1.000 diameter hole through it is stronger that a solid 1.750 diameter bar? And this hole "distributes stresses" to add strength? Does it get stronger with larger holes? How about a 1.625 diameter hole, leaving 0.062 wall thickness?

Maybe you should get that 'standard engineering textbook' out and calculate the section modulus of a solid 1.750 diameter as opposed to a 1.750 diameter with a 1.000 diameter hole.

Rohan · Mar 13, 2017

Per weight of metal, have you done the sums ???

Don't forget there HAS to be a hole through it - to supply oil to the bigends.
And to allow for sludging up and still supply oil.
And the overall weight also has to meet the balance factor requirements....

How about a 1.625 diameter hole, leaving 0.062 wall thickness?

Obviously there are limits - do the sums, and see how they graph out.

P.S. Not unrelated, have you ever noticed that (for many years) motorcycle frames
were made from large hollow tubing. As distinct from from small diam solid rod.
For the same metal, do the sums....

worntorn · Mar 16, 2017

Ocean going ships use hollow drive shafts. At one time solid shafts were used but breakage became a problem as horsepower levels rose. A hollow shaft solved the problem.

Glen