Moparts Tech Archive

Motor General

how do you calc reciprocating weight?

well how do you measur piston weight? does it include the rod weight? how do you calculate the throw weight? how do you know how much to add or subtract?

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Bob weight on a V8 is calculated as 1/2 of the reciprocating weight, and all of the rotating weight.

Rotating weight includes the big end of the rod, rod bearing and oil.

Reciprocating weight includes the small end of the rod, piston, pin, rings and locks.

Double the figure for the second rod and piston combo on that journal and you have bob weight.

It this different for a 4 cyl? I'm building a 1275 MG modor...

Yes, inline and pancake engines are different. On an inline, like a 4 cylinder, you just make all the parts weigh the same and then the crank is spun and balanced all by itself. Because all of the throws are 180 to each other they dont require bob weights.

No, you still have to separate the reciprocating and rotating components and make them match. Total weight of all parts for one cylinder may be close enough, but it would be an accident.

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