How to Balance Connecting Rods

[vairmech]

This spawned from another thread but I copied the quote from there:

Good comments Jim.

I'll add having a bunch of rods lets you "cherry pick" them for closest weights when doing an engine balance. Better than trying to grind rod for balance. I had one engine I bought were half the rods were so ground down there were unreliable.

Note in early 65 Chevy used up left over 64 cylinders by putting a band around the top for the bigger gasket O.D. on 65 and newer engines. I have a NOS 65 cylinder with the band. Good paperweight

There are 2 weights to a rod if you are going to weigh it. There is the big end weight and the overall weight.
I have to ask why not grind on a rod? You just have to do it in the correct spots and there are lots of them!
This rod is the extreme where I took off 10 grams of weight. This rod went into a race engine pushing 7000rpm! I have to add that just because a rod overall weight is the same the big end weight that is really the important part can be off enough to cause problems.



So, how do you at least check the balance of the rods? Below is the rig I made, the rod needs to be level for this. The bearings are dry with no lube so there is no drag, I clean and blow them out before each use. The small end needs to be hanging down with no push or pull on it, that changes the readings. I have marked on my scale where the big end stand needs to sit for repeatability and repeatability is usually with in .1 grams! BTW, 25.6 grams to the ounce!!!



Below you can see 2 rods with one on the balance stand with the same big end weights. You can also see I polished off some of the forging pinch material and the other rods in the background. That pinch material does not add to the strength and possibly could contribute to a rod breaking in the extreme cases because it causes a stress riser.



Once the big end is balanced then you need to do the overall weight, below you can see 2 rods, look at the small end, that little square pad on top is almost gone! This was an extreme set of rods I did for the race car. Not all rods need to have that much taken off. When you do the overall weight that pad is the ONLY place to remove material, anyplace beyond the centerline of the piston pin hole changes the big end weight.



I try to keep within a couple of tenths when I do the overall weight. You ask, why do I place the rod in this manner? The balance point is just about the center of the pad. YES, it makes a difference how you place the rod so do it the same every time.



This is just a brief summary of what you can do and not really a how to do this thread as there are a lot more details than you think.

[66vairguy]

Good numbers. Yes you have to measure both ends.

Although a 3 gram delta is desirable in a street engine according to Nash, he said street engines are fine at 8 grams that don't exceed redline and 3 grams is desirable for around 6,000 RPM and 1 gram is the goal for an over 7,000 RPM race engine. Not my words ----

After cherry picking rods, and having those rebuilt (always some material taken off big end) I had a 6 gram total rod weight delta. After just a little grinding in non critical areas my delta was 1 gram for total rod weight with a 6 gram delta between the different big ends and different small ends. Good enough for a street engine that does not exceed 5,000 RPM.

After a lifetime of dealing high RPM causing premature alternator and cooling bearing failure, not to mention engine and transaxle wear --- I'm happy with not exceeding 5,000 RPM.

Just me

[rcavictor]

Ken, how do you start the process? Pick the lightest rod in the set… remove the casting stuff and then make all the others match the desired weight?

This is cool, I always wondered what some of the ‘best practices’ were to accomplish this. Thanks for documenting - and looking forward to the upcoming YouTube videos of your shop work ;)

[vairmech]

I start with the big ends, I weigh them all marking on the rod what the weight is. Then I start with the lightest one and set it off to the side using that number as a reference. I then proceed to match the other rod big ends to that. After the big ends are done, I then do overall weight taking the lightest and setting that aside again for reference and remove the weight from the top of the rod to match.

If I were to do videos some of them would be so long it would get boring to watch the complete process, I would almost have to do stop action to condense everything!

[Wagon Master]

What is your go to area on the big end for removing weight?

[vairmech]

What is your go to area on the big end for removing weight?

I generally start around the rod bolts, there is a lot of sharp flash there that does not need to be there. I take care to leave
the numbers though. Then I will take a pass at the rod cap removing enough to take the cast in mark from the middle, not so much on the ribs. Then I look for the thick area and start there following the contour of the shape already there. If you look at the pictures think you can see all of this.

[American Mel]

These bots are really getting tiresome.

[Scott H]

These bots are really getting tiresome.

The IP addresses for them have been banned. I'm still going through and removing posts.

[martyscarr]

Thank you Scott

[pgvair]

Ken, where did you get your scale?

Wade Lanning