DASHTI

i have a question
my cranckshaft bob white 1800g
my piston and pin 620 g
insert 45 g
big end 394 g
small end 151 g
oil 4 g
locks 11 g
total bob weight 1690 g
why do the crankshaft do i need the balance or not the balance
thanks

rsz288

It is not just the weight of all the components hanging on the journal. There is reciprocating mass and rotating mass to be taken into account.

There is a formula to work out what the influence of the big end and small end have in terms of the actual bob weight to be applied to the crank for balancing purposes.

As I am on my 3rd glass of wine, I cant remember where to locate the details, so will defer to someone who is more on the ball..... And might drop in here and comment. hic......... My apologies.

Quick Carl

You have to also include the ring set weights. From the post It looks like someone has calculated a bobweight of 1690 G's, because the weights given wouldn't work out to that when allowing for reciprocating vs. rotating weights. If the 1690 g bobweight is correct and the crank is balanced at 1800 grams then yes you should balance it. I like to add a few percent for stability in high HP engines when we balance, but that's way off.

Mighty-SS

iTrader: (19)

Always balance it !!! You will save a lot of time and less problems in the future...

Tom@SpeedInc

iTrader: (5)

You have to double the big end of the rod weight (and bearing too)

also, 11g on the piston lock? better check that again.