Help with Commando Crank

Norton46 · Apr 2, 2018

Hi
I bought a 74 Commando to use for the basis of an E-Start bike. The bike had not been running for an unknown period of time. I did basic service to it and installed new carbs to replace a Mikuni. It seems to be a good runner, perhaps a big quicker feeling on acceleration. While test riding the left exhaust thread gave up the ghost and I removed head and cylinder, as I wanted to inspect the cam. Cam looks fine, but I found someone has modified the crankshaft with additional drillings.
I need help in figuring a couple things.
how much weight has been removed?
how this has affected the balance factor?
There are 23 drillings to the crank. 6 of them are 1/2' diameter and 17 are 3/8" diameter. They range in depth from .338" to .807".

Thanks in advance.

comnoz · Apr 2, 2018

I might guess the balance factor has been moved up to around the 65% area. I is not likely to make a big difference in perceived vibration since bikes with Iso mounts are not real sensitive to balance factor.
It may even feel better than it did stock if they did a dynamic balance when they raised the balance factor.. Jim

texasSlick · Apr 2, 2018

It looks like a re-balance job. You can estimate the weight of metal removed by filling the holes with water, then collecting and measuring the volume of the water.

Then weight (grams) = volume (cc) x 8, where 8 is the average specific gravity of mild steel.

Slick

olympus · Apr 2, 2018

Funnily enough I have just been in contact with Bassett Down to have my crank balanced

Hobot · Apr 2, 2018

hobot way to find the holes mising volume would be fill with candle wax flush, then melt out to measure then convert to cast iron mass. Lighter flywheel counter weight rises BF for more horizontal oval engine oscillation, that may or may not matter with isolastics. Might consider JMS lighter rods/pistons if seems too far off mid 50's BF.

worntorn · Apr 2, 2018

Did it run smoothly above 3000 rpm when you test rode it?
If so, the bf must be ok.

Glen

Hobot · Apr 2, 2018

I'd assume- as the drillings so extensive/expensive and organized, its not willynilly DIY but was professional shop BF balanced better than factory so just needs a spin balancing session to double check and refine dynamic balance, if not already about good as it gets. I say if rest of engine well used then likely good to go as is and spend on other more delicate wear prone or upgrade components. Straightness from ends run out would be my main concern then journals and inner race seats deminsions or DS taper intact. If oil flows freely out the journal oil holes I'd forget about splitting for sludge trap cleaning as will fill right back up in 7000 miles or so to about same best streamling, sort of like D shaped exhaust ports funneled to oil exits, as its rather *Difficult* to get or brave level of properly over torqued, staked or lock plate factory nip up task done as well or as stable again. I've seen red locktite turn to lubricating dust in crank fasteners so ok to use but only bolt stretch and or locking fasteners mechanically lasts long.

Dances with Shrapnel · Apr 2, 2018

What worntorn said. If it felt decent then leave well enough alone.
Only comment I have is that the flywheel looks like it had been turned or ground down a bit. I know this mark from the factory - probably some fine adjustments but look to see that the previous owner did not turn down the outside perimeter of the crank. If too much is taken off the flywheel becomes more prone to grenading. I'd only experienced, seen and heard of these flywheels grenading under race conditions so probably ok. If you tend to regularly wring the motor out I would think twice if the perimeter was turned down too much.

Hobot · Apr 2, 2018

Crank failure has been reported at idle of 850 in drive way to launch out front glace off car across street entering picture window destroying furniture and wall. Racing certainly more risky but mature people are constantly scared once knowing facts of life. Not likely a worry unless redlining a lot. Does not seem much surface relieved, compared to what racers did in that era.

Dances with Shrapnel · Apr 2, 2018

Can't really tell from this picture if anything more than what the factory removed. Another view at the rod journals would be more telling.

A crank failure is mere child's play compared to a catastrophic flywheel failure. Been there, done that and had a front row seat (second row on the grid) when a well known member of this forum grenaded his Commando in a spectacular fashion. Did I mention that they are all spectacular.

acotrel · Apr 2, 2018

When I rebalanced my crank to suit the rigid mounting, I fitted a steel plug in the hole in the flywheel. Then I started to drill the other side, realised what I was doing and stopped. It came out at 72% balance factor which is extremely smooth at 7000 RPM. If I wanted to rev higher, I would change the crank. If the Commando crank did not have the hole in the flywheel, it would probably be perfect for an Atlas.

Hobot · Apr 2, 2018

Aw shoot all motorcyclers must ask themselves how luck do ya feel punk, before heading out in public, so similar here, if feeling lucky with intact engine guts supporting this, forgetaboutit, but if time/money to spare, ain't no short cuts but take crank out for proper exam and decision making which of course will then mean new bearings and rod shells etc. If not fractured or over red lined routinely, just short of it much as ya like, a Norton bottom end is dam robust to last 60k miles. Lighter/higher BF are used in solid mounts for less vibes so can't see why wouldn't rubber mount isolate even sooner than normal if your BF is skewed upward. Love/hate the philosophical mechanical wisdom vs wasted expense/effort dilemmas. To follow the best advice y'd put a steel flywheel in there but i sure ain't in my stock '72 Cdo I run to edge of redline pretty often, which is why I settled on Commando for its kix.

acotrel · Apr 3, 2018

If the crank balance factor is higher than usual and you are still using isolastics, you should not have a problem unless the flywheel lets go. The isolastics don't know if the vibes are low frequency or high frequency.

robs ss · Apr 3, 2018

acotrel said:
If the crank balance factor is higher than usual and you are still using isolastics, you should not have a problem unless the flywheel lets go. The isolastics don't know if the vibes are low frequency or high frequency.

It is my belief that higher BF does not raise the frequency of vibration (rpm does that) it alters the magnitude and location, within a single revolution, of the vibration.
Higher BF trades off by reducing 0 & 180 degree vibration force at the expense of increasing vibration force at 90 & 270 degrees. So less up & down but more fore and aft.

I suppose if you were starting with a BF of zero percent, any increase in BF creates the fore/aft vibration so in fact doubles the frequency - although I'm not aware of anyone operating a Norton twin with a BF of zero

Fast Eddie · Apr 4, 2018

I wasn’t gonna post as I don’t want to unnecessarily add to your stress!

I imagine that if you want to ride your Cdo as a sensible classic bike owner, that crank will last you, trouble free, forever.

However, if it were mine, I’d be worried about revving it hard. There are a lot of holes in it, some of them worryingly close together, I would be worried about it being more prone to cracks forming, with rather dire consequences.

So, IMHO, if it’s for steady use, nail it back together and forget about it.

If you intend to put it to hard use, I would replace the flywheel at least.

All only IMHO of course.

oldmikew · Apr 4, 2018

Dances with Shrapnel said:
Can't really tell from this picture if anything more than what the factory removed. Another view at the rod journals would be more telling.

A crank failure is mere child's play compared to a catastrophic flywheel failure. Been there, done that and had a front row seat (second row on the grid) when a well known member of this forum grenaded his Commando in a spectacular fashion. Did I mention that they are all spectacular.

Hmm rod through cases at 80mph was spectacular enough for me-not on a Commando though. I think if its had metal taken off and that is why it has been rebalanced ,then I would view it with suspician. New flywheels are not that expensive and there are those on Ebay. Should you go this route do get new bolts and nuts. What bothers me in part is that the pattern is not symmetrical . Suggests whoever did it has not done it too often..And its a fruitful source of cracks

robs ss · Apr 4, 2018

Everyone has their own threshold of acceptable risk.
If it were me, I would not use that flywheel - I would never really enjoy riding with that nagging feeling in the back of my mind.
We've got enough issues with "unknowns" without ignoring "knowns".
I'd recommend new flywheel, careful inspection/correction of condition and radii of journals etc, new bolts/studs/nuts (as per oldmikew) dynamic balance ($200?) to your chosen BF.
That way you know you've got a good crank and can enjoy the bike.
Cheers
Rob

ashman · Apr 4, 2018

I have my crank balanced at 72% for my Featherbed frame and I can tell you it don't have that many drill holes in the crank and not all over the place like yours, my motor runs pretty smooth for a hard mount motor from low revs to high revs even with the hot cam and other work done to it.

Ashley

Dances with Shrapnel · Apr 4, 2018

I don't want to be the alarmist here, just the cautionist (I just made up that word). From my experience, where you get in trouble with trimming masss off of the stock flywheel is when the flywheel flange near the rod end cheek bolt circle flanges gets reduced or eliminated, this is where I have seen the catastrophic failures. The holes drilled around the perimeter are in my opinion, rather benign.

Help with Commando Crank

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