Balancing Connecting Rods

[Hubert_25-25]

Buick Friends, 

I posted this in the technical section, but someone here may have experienced this with their Buick motor, or may want this advise when they go to rebuild their motor.  This is the posting.   Thanks as always for your help.    Hugh

http://forums.aaca.org/topic/316231-balancing-connecting-rods/

 

[Brian_Heil]

Hugh

 

Do you know what mass ring weight the crank was balanced to?  This is what you want to make the large end rod masses match. Cotter pins included. 

 

Typically, the ring weights (a heavy bolted two piece affair that matches the large end of the rod) are made (all 6 in this case) to match the lightest large end you have. Bolt all 6 on and put the crank on the balance machine. 

 

The light ends of the rod, the reciprocating mass, you want to be all common so they cancel. Include the bolts.  I’ve seen the bolts trimmed to help match mass. 

 

It it would appear on the large end of the rod there are two small balance pads per side. The two flat spots just inboard of the bolt heads on the forging.  Both sides of the rod should have these two small pads. The small end would have been balanced by machining its width on both sides. 

 

If if you can’t do any machining, put the heaviest large end rods on the center two journals to minimize the rotating couple they induce. The other good news is these are big heavy rods so look at your error as a percentage of the total mass of a rod. 15g / 1000 g is only 1.5% error on a very low revving engine. 

 

What about your reciprocating piston mass?

Edited October 1, 2018 by Brian_Heil (see edit history)

[Brian_Heil]

You can often mix and match piston mass (reciprocating) with small rod end mass (also a reciprocating mass)  to get all six close/matched. 

 

There should be be some mass machined adjustment pads on the inside of the piston on the pin bosses both sides. 

[Hubert_25-25]

Brian, 

    I took another pass at this and left my notes on the technical forum.  Your notes are helpful and add to my understanding.  I understand that they do not do the ring weights on straight 4/6/8 engines.  I have 2 options of either balancing each connecting rod individually or pairing them up.  I need to find some washers to make this work.  There is almost nothing to be able to grind on these early connecting rods.  He already balance matched the pistons/rings/wrist pins, so I need to just work with the bolt on the wrist pin end. 

[bradsan]

Hugh

 

You certainly got my interest so I had a look at this guy's video which puts your 13 gram variance into perspective. Wow. But then you'll never hit 5000-6000 rpm with the Buick...I hope!

https://www.youtube.com/watch?v=QLpiF0E0EJU

 

For our Buicks and the small ends, what about reducing weight by replacing the wrist pin bolt with a low profile head instead of the Buick high head bolt? Couple of grams

 

How does one account for any piston weight difference or is dealt with separately?

 

Will you be porting and polishing the head too??

 

Brad

[Hubert_25-25]

Brad,

 It is not a 13 gram variance in the connecting rods, it is 19 grams.   That is a good video to watch as well.  

I have not even talked about the piston replacement.  This was straightforward.  I simply bought new aluminum pistons/rings/wristpins/wrist pin bolts and lock washers.   You just can't do that with the connecting rods.

The original cast iron pistons, rings, and wrist pins weigh 764 to 804 grams.  These were apparently never balanced, as the lightest to heaviest piston difference is 40 grams!  The new aluminum piston assemblies (balanced) weigh 542 grams each.   Huge weight savings, and very little effort to match the weight.

To make matters worse,  the second lightest piston was on the lightest connecting rod.  This combination pushed the imbalance of the rotary mass to 44 grams.   Since the pairs are 1&6, 3&4, and 2&5, the difference in weights on the three 120 degree journals varied by 28 grams.   This thing was a mess.   

 

I also bought replacement wrist pin bolts with a drilled head so that I could safety wire the bolt.  It does help to have a little lower bolt head, and not to have cheap lock washers.     

I really don't know enough about what engine rev this really matters at, but if "vibrationless beyond belief" was the motto of the day, that is what a big flywheel is for I suppose.  I just wanted to ring out as much of the inherent imbalance as I can.       

Hugh

Edited October 4, 2018 by Hubert_25-25 (see edit history)

[Hubert_25-25]

Buick Friends, 

I just finished documenting how I balanced my connecting rods.  This is a link to the posting.   Now I get to start assembling the block.  Thanks as always for your help.    Hugh

http://forums.aaca.org/topic/316231-balancing-connecting-rods/