What is the "Bob-weight" of an engine?

[CoupeBoy]

I was out looking for information on something completely unrelated when I stumbled across this thread on a forum dedicated to another engine of the Ford family. And since I have never read anything about this on here I thought some others might find it interesting

http://www.network54.com/Forum/thread?forumid=85220&messageid=1088637255

But that does make me wonder would we be ok to get this into close approximation ourselves before the engine builder balances everything or is this one of those black arts things that should be left to the professionals?

-ron

 

[rbohm]

8) when balancing an engine, you start by first rebuilding the con rods. then you balance the big end weights, then the whole rod weight. then you balance the pistons to each other weight wise. now is where the bob weights come in. the rod, piston, rod bearing, and ring set are all weighed and that number recorded. you then fill the bob weight untill it weighs the same as the piston/rod combo. the bob weight is then attached to the crank which is then spun(bob weights on all rod journals) and the crank is balanced accordingly(if the crank is externally balanced then the flexplate/flywheel and balancer need to be included as well).

 

[Thad]

Bob weight for blancing purposes duplicate the effect of rods and pistons weight on the rotating crank.

First reduce piston weights to the same weight of the lightest piston by milling material fron the balance pads on the underside of the piston.

Reduce the weight of each rod big end to the same weight of the lightest rod big end by removing material from big end balance pad.

Reduce the total weight of each rod to the same weight of the lightest rod by removing material from small end balance pad.

Rods are first weighed by suspending the small end on a pivot with the big end on the scale. Refered to as "Big End Weight" A second weight recorded for each rod is "Total Rod Weight"

To derive Bob Weight add Big End Weight for one rod (the BEW are now the same for all rods) with the weight of one rod bearing. This is "Rotating Weight"

Add the difference between Total Rod Weight and Big End Weight to the weight of 1 piston and 1 ring set.
DIVIDE by 2, this is "Reciprocating Weight".

Rotating Weight + Reciprocating Weight = Bob Weight.

Remember there can be a pop quiz tomorro.