E: Engine Crankshaft Balance Method

LoneStar

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Hello all,

I'm rebuilding my Series C Black Shadow with new pistons and rods, due to a catastrophic failure last summer. I now have the crank assembled with the rods, and need to check the balance. Based on a consensus of informed opinion, I’m targeting a 50% balance factor.

After doing a lot of reading, I think I understand what needs to be done to reach this figure. But, I’ve made stupid mistakes before and it would be inconvenient to tear down the engine again to correct one. So I’d like to outline my logic, and ask if anyone sees errors or faulty assumptions.

Here are the basic figures:

pistons: 417g each, with rings and pin

rod reciprocating weight: 187g

This gives total reciprocating weight on the crank of 1208g

The idea is to balance 50% of this reciprocating weight, or 604g. This means with 604g reciprocating weight, the crank should be balanced, and stop at any location when rotated on bearings.

To test this, I subtracted the 374g reciprocating weight of the rod small ends, as they’re already in place, and attached 230g to the small ends to reach the 604g figure.

Unfortunately, but not surprisingly, the crank wasn’t in balance at this point: the crankpin side was heavier. This means I need to add weight to the flywheels opposite the crankpin, so I attached weights there until the crank was in balance. This took 113g.

The next question is how to implement this on a permanent basis. The accepted method appears to be inserting plugs of heavy metal, such as tungsten, into holes drilled near the flywheel rim. So, how much tungsten is required?

The weight required will differ with the distance from the center of rotation. My test weight was on the outside of the rim, at a radius of 4.2”. The holes to be drilled for plugs will be c. 3.5” out, so I increased the weight requirement by the ratio – multiply by 1.2. This gives a weight requirement of 135g.

Now in order to insert the plugs, holes must be drilled. This has the unfortunate effect of removing steel, so requiring more tungsten to compensate. The density of tungsten is 19.6g/cc, while steel is 7.85. Based on this, the effective density of tungsten when replacing steel is 11.75g/cc. Therefore the weight of the tungsten plugs will be the required net weight increase x (19.6/11.75), or 225g.

Is this all correct?

The rods and pistons are both Carrillo, and I’m told by the supplier that it’s normal to need to add weight to the flywheels when balancing them.

Thanks!

Dave
 

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Albervin

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Oh dear. There is catastrophic and then this. I hope the crankcases survived. There are already holes in the flywheels so you could possibly fill some of them with a less dense material. This will involve physics that I forgot over forty years ago. I would seek the help of a race shop with experience in these things. That is exactly what I did with my old car. Surprisingly not expensive as they have all the machinery, dials and gauges. I wish you good luck in getting the beast back on the road.
 

Bill Thomas

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I am interested because I have to do a Comet.
Not sure of the Sums ??, YouTube gives a few different ways !!.
But if it's heavy at the C/Pin, Drill holes either side of the Pin.
I did it to my Twin 40 plus years ago, But have forgot.
David Bowen told us, The factory drilled out the top 2 outside holes to 17/32 " and the 4 below to 7/8 " when our engines are used in cars with standard crank pins.
Try and keep the swarf out of the bigend bearing !!.
Have a read of "Tuning for speed ".
Good luck, Let us know how you get on, Bill.
 

Bill Thomas

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Just seen the photo !! How did that happen ?, Never seen that before.

Had to go a bit MAD with mine, After taking 1/4" off the side of each wheel, Looking for more rev's
for Racing.
But it worked !.
 

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LoneStar

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The crankcases, miraculously, survived with very minor damage.

On the existing holes, I would need more of them - for which there is not much room. Filling them would have the opposite effect to what's needed. Martyn's idea of drilling the circumference near the crankpin might work, although I suspect it would take too much drilling to remove the necessary weight. Pretty much all area near the crankpin where a hole can be drilled, it already has been - see pic. I'm not sure if that's the factory treatment or if some holes were added later.

I don't know the balance factor on the old assembly, as pictured. The crank and rods were apparently redone decades before I acquired the bike, based on receipts I inherited - original Vincent rods, and an Alpha big end. The pistons were Omega cast 8:1. I can't check the previous factor now, as the crank is together again with new rods.

Carrillo rods are heavier than Vincent (660g vs. 620g with races), although the reciprocating weight is lower (187g vs. 226g). This seems to result in the need for extra weight opposite the pin. Coventry Spares tells me that when they have cranks balanced with Carrillo rods, they always need plugs to add weight across from the pin.
 

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LoneStar

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Bill,

I don't really know! I was riding along around 70 mph in third, minding my own business, when it died. It looks like one gudgeon pin boss broke away from the piston, causing the bend in the rod. The pin stayed in the small end for a while, as you can see the marks its end made when crashing into the bottom of the piston crown. The piston shows some signs of seizure, but not that severe - and in any case, a seizure is supposed to drag you gracefully to a halt, not break the piston apart...

Dave
 

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Bill Thomas

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I would open out the top 3 holes.

I think you were lucky !!, The rods snap there and the bit that is left bangs a hole in the cases.

What make piston was that ?.
 

LoneStar

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Bill,

True, there's some room for that. I'll investigate. Perhaps a risk of weakening the flywheel too much, though?

Dave
 
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