Engine balancing question
 

I recently had my rotating assembly balanced in a BBC. I have decided since then to change the rods. do I need to rebalace the entire assembly again for crank counterweight? I changed rods from 6.625 to 6.535 lenght.
Thanks for responses.
Ken

Re: Engine balancing question
 

yes

Re: Engine balancing question
 

I hope you changed pistons, they may sit a little low in the cylinder. I would re-balance.

Re: Engine balancing question
 

What he said ^^^

Re: Engine balancing question
 

Unless by some chance you know the weight of the existing rods and by some miracle the weight of the new rods is the same. And of course that would have to include the new pistons too.

Re: Engine balancing question
 

Thanks for the input. I am using the same pistons to get the desired compression. the longer rods were a mistake in what was shipped. should have measured before I started.

Re: Engine balancing question
 

Make sure the crank is cam cut to clear the bottom of the pistons.

Re: Engine balancing question
 

The shop that did the initial balance work should still have the work sheet with all the component weights. the machinist will just need to weigh and balance the rods, and then balance the crank with the corrected ring/piston/pin/rod/bearing weight.
The short answer is as everyone else has said, yes. Don't cheat now.

Re: Engine balancing question
 

I would think that if you can get the new rods to weigh the same as the old rods you should be OK.
I believe that the big end weight has to be the same also.

Re: Engine balancing question
 

Tinkerer, are you thinking that you can add weight to a rod to make the 6.535 rod weigh the same as the 6.625 rod did?

Re: Engine balancing question
 

NO - He may have just gotten lucky and the original rods ( 6.625 ) were light and the ( 6.535 rods were heavy out of the box.

Re: Engine balancing question
 

Making the total rod weight end up the same, as well as the big ends and small ends all end up the same is a real long shot. If it were me I'd just have the shop balance the rods and spin the crank.

Re: Engine balancing question
 

total rod weight being the same - maybe.
big end weight being the same, too? not likely.

Both weights factor into the balance.
You already knew that, you were just distracted.

Re: Engine balancing question
 

BADGIRL
Happy new year to all the oso crew some things are correct and others are'nt what you need to do is recheck all the weights over your rod rotating rod reciprocating piston and pin rings brgs and assure yourself that the bob weight does not vary more than 10 grams less the bob weight can not be heavier then the original . If you have a larger difference than 10 grams less than the original I would recommender to rebalance the entire assembly not just the crank sometimes different weight affects different areas of the assembly more or less include your drive plate also if you have to rebalance.You might be able to check your bob weight yourself if your balance shop gave you a balance card with your weights you can compare them to your new weights and do the math if you need any help please feel free to call or email me anytime Sincerely Yours Laz Mesa P.S. Please stop down and see us at the Miami boat show in Feb. booth 4184

Re: Engine balancing question
 

M.B., that's why I recommended that the original balancing shop be asked if they still had the work sheet from the first balance job. It will have all the component weights on it to save some time and effort that would be spent verifying individual components. Just my two cents.

Re: Engine balancing question
 

Thanks everyone, I now know which direction to go.

Re: Engine balancing question
 

Hey Laz ... just ballpark figure ...all this already apart ...what kinda $$ we talkin to Balance an entire assemby ? Seems foolish to go this far without doing it ?

Re: Engine balancing question
 

Amen to that... he is the best when it comes to balancing, he does it for many big time engine builders all over the nation.