Engine balancing question...

I just weighed my new Brooks rods in preparation for balancing. I show a high-to-low of 1.7 grams. I haven't weighed the pistons yet, but assume there is a small variance there as well. Is it common on these blown hemis to grind material from the rods and pistons to try to make them "0" variance?

-Scouder

 

You can get very intricate with it. Many do. But this aint no pro stock engine, and the piston picks up 20 grams of oil on the first revolutions when finally running anyway. But I have seen rods break from a certain spot that was ground and left a sharp spot. (i.e. A place for a crack to begin.)

 

So am I wasting my time worrying about a 1.7 gram difference in total rod weight?

I am going to send off a rod and piston assembly (with rings and beaings) to have a bobweight calculated. Should I just send an average rod and piston, so the variance either way accross all 8 would be minimal from the calculated bobweight?

-Scouder

 

whe you have a crank ballanced they will want you to send them a piston pin rod with bearing rings and buttons to get your bob weight I have never found a brooks rod to very but I would never take any off a rod, what you might do is call lance flanikin and ask him..Dave

 

I would say that you got a very good set. An awful lot that come through here are not so wonderful weight wise. Most of the time you can only rub about 2 grams off of AL rods as there isn't enough meat to work with. While most any steel rod set can be taken to 1/2/-0g I often have to settle for a 2g spread on al....if I can.

What Mike Canter is referring to is that you want to weigh the big ends and the small ends separately. Here is what you do in this case.
- Match up a heavy big end with a light big end for each rod journal.
- Match up a heavy piston and pin with a light small end.
That will give you a better combo than most.
- Also note that pins can vary so you can move a light pin over to a heavy piston to get a better average.

Tell your balancer whether or not you included oil in your BW figure. 5g is the norm. Otherwise he may add a little that you don't expect to your figure.

Dave Koehler
http://www.koehlerinjection.com

 

Make sure you send everything.

Piston, pin, buttons, rings, bearings, rod.

 

Thanks guys. I'm gonna begin re-assembly after the holidays, then the questions will REALLY start!

-Scouder

 

Its very common to find rod sets with 5-8 grams variance between them but I would never grind on an aluminum rod . just match in pairs as close as posible ...i know for experience as I machine and balance cranks in our shop. for naturally aspirated engine we overbalance by 7 grams and blower motors high RPM we use 15 to 20 grams as oil will be everywhere.

 

You put the heaviest rod with the lightest rod on the same throw you only have a difference on the throw of .85 of a gram.
Then you measure all your wrist pins and pistons do the same thing. Heaviest piston goes with the lightest pin.
Then do the rings and do the same thing.
Your total difference in pairs will shrink down to an insignificant amount with out any grinding.
Many would consider .85 to be insignificant.
I prefer to be match parts up to .50 or less. No balance shop is going to bother with that, so I balance group my own parts.

Do the rings after you set the gaps for their bores or everything will change as they are your most dense material the you will be changing the weight of.

 

Required reading

http://victorylibrary.com/mopar/crank-bal-c.htm