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
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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