Calculating Bob Weight Continued
 The lightest piston is placed
back on the scale. The result shown on
the scale (639.5 grams) is the sum of the lightest piston
and the weight
of the pin.
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 In our case, the Probe pistons and pins ordered were all
within spec. No material needed to be removed to equalize
the weight of the piston and pin combination. If material
did need to be removed, this photo shows a common area
on the underside of the piston to bring it within spec.
See red square. |
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 Next, the weight of one ring set is measured.
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 Here, the "big end" of the rod assembly is calculated,
but it IS NOT part of the reciprocating weight calculation.
Once the rod is balanced on the scale with a special jig,
the scale is tared. For information on rod preparation,
reconditioning, and balancing be sure to see the FordMuscle
article Roddin'
Real. |
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 With the scale tared to zero, placing the whole rod on
the scale now provides the weight of the "small end"
which IS part of the reciprocating weight calculation. |
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 Rotating
Weight
With regard to crankshaft bob weight, rotating weight refers
to the centrifugal forces exerted
on the crankshaft from the rotating mass of the "big ends"
of the connecting rods and the rod bearings. The
following table shows the values of each and the total for one
of our engine's rod journals. As mentioned in the opening portion
of this section, the "Times Rods" multiplier of two
accounts for two rods per journal.
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Rotating
Weight - Ford 400 Build
(all values in grams)
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Rod Big End
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621.5
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Rod Bearing +
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45
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Total =
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666.5
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Times Rods x
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2
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Rotating Weight =
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1333 grams
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The following photos show how the
rotating weight calculation was actually performed at the shop.
The weight of the rod's "big end" was already
calculated as a necessary step to determine the weight
of the rod's "small end". |
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And here, the rod bearings are also factored in. |
Assembling the Bob Weights
When the reciprocating weight and rotating weight are calculated,
one final calculation provides the necessary value to begin
creating the bob weights. You may be surprised to know that
at this point, a good portion of the work involved in balancing
a rotating assembly is done. The following chart shows the sum
for one rod journal's reciprocating weight and rotating weight.
An estimated weight of the motor oil, which will be slung against
the underside of the piston in final operation is also included.
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Bob Weight
- Ford 400 Build
(all values in grams)
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Reciprocating Weight
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912.5 |
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Rotating Weight +
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1333
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Oil +
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4
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Bob Weight =
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2249.5 grams
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The following photos show one bob
weight being incrementally weighed to reach 2250 grams.
This procedure is repeated 3 additional
times, one bob weight for each rod journal.
An aluminum clamp and a selection of brass weights are
combined to reach the target bob weight. |
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Small washers help dial in the final gram weights. |
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 (On
the Balancing Machine, Balancing the Pressure Plate) |
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