Copper color connecting rods

[John Hogan]

I've rebuilt more than a few big and small blocks and never seen copper color connecting rods. Tearing down my 1973 340 to go to the machine shop I found that #3,6 and 7 connecting rods are copper colored. The #3 and #7 rods have no markings on the machined pad while the #6 rod has what appears to be a + symbol in a circle. Since they are totally the same color and show no sign of bearing failure or overheating what the heck is the reason for the different color?

 

[Sahara]

Gotta follow this.

 

[MIKESPOLARA]

Needed more weight for balancing. Can't add weight back so copper dip does the trick

 

[69Bee]

When a connecting rod comes down the line, and is just a smidge out of spec, the bad end is dipped in a pot of molten copper. The layer of copper reduces the size of the hole for a proper fit. The small end bore is reduced in size for a better interference fit on the piston pin, and the big end bore is reduced when honed too far over the spec. I have seen just the small end bore dipped, big end bore dipped, and the complete rod dipped. When they cannot be fixed this way, they get sent back to be melted down and start all over.

 

[John Hogan]

Searched FABO and found lots of differing reasons, most of which match 69Bee's explanation. However I'm still curious about the + in the circle that's not on the other two rods. The consensus at FABO seems to be they are fine to use.

 

[RemCharger]

Pull a cap and see what the year is on the bearing.

 

[Bighouse]

Well I learned something new.

 

[493 Mike]

I found the copper plating on 2 different rebuilt crank/rod assemblies for the 2 different 1000 cc Rotax snowmobile engines I rebuilt. They came right from the dealer.
Mike

 

[toolmanmike]

Normal thing. I have seen many and even more photos of them. There has been discussions here about them.

 

[oldkimmer]

Chevy started that way Back in the 60s/70s as their quality control was horrible. It’s not uncommon to pull a Chevy apart and see all 8 rods coated. Nothing wrong with the rod now other than it bugs my azz. Kim

 

[Krooser]

The copper has nothing to do with oversize/undersize rods.

When the rods are forged they get a dip in the copper electroplating bath to get a protective layer of copper before they go to heat treat.

After heat treat they are supposed to all go to a caustic bath to remove that copper before they get machined...for whatever reason some don't get the bath...Friday/Monday work, they needed the parts or whatever.

No internet bs here... I used to haul automotive parts for Chrysler and GM in my expedited freight days. We had a heat treat outfit in Lansing, MI that was a customer. They did heat treated cranks/rods,R&P's etc. for GM, Lunati, and others.

Saw that on every visit.

 

[Krooser]

If you look at aftermarket billet steel cams many cores will have the copper plating still visible between the lobes.


Google Image Result for https://www.12bolt.com/uploads/6/0/9/9/60992641/s948739236638414765_p196_i4_w2560.jpeg

 

[Krooser]

While these are power sports rods the process is similar...except they do not remove the copper...

 

[j-c-c-62]

The copper has nothing to do with oversize/undersize rods.

When the rods are forged they get a dip in the copper electroplating bath to get a protective layer of copper before they go to heat treat.

I wish I could confirm or refute definitively here, but I don't know enough on the topic, except copper is poor material to upsize an item that is undersized and that is heavily loaded, repeatedly, like a con rod. Krooser appears to be on target IMO.
Interesting observation, nevertheless.

 

[toolmanmike]

It's for the heat treating process before carburizing. https://www.cp-carrillo.com/n-12112-carburized-rods.html

 

[padam]

It's for the heat treating process before carburizing. https://www.cp-carrillo.com/n-12112-carburized-rods.html

The process Carrillo uses appears to be something different than what would be used on factory rods.

 

[toolmanmike]

The process Carrillo uses appears to be something different than what would be used on factory rods.

But it has something to do with heat treat. Copper isolates the steel from the carbon during the heat treat. I understand the small and large ends are heat treated, then machined, (obviously) whereas the beam is not.

I would believe this over everybody saying the copper is to build up rods that are out of size. I have read it all.

 

[MCODECUDA]

When a connecting rod comes down the line, and is just a smidge out of spec, the bad end is dipped in a pot of molten copper. The layer of copper reduces the size of the hole for a proper fit. The small end bore is reduced in size for a better interference fit on the piston pin, and the big end bore is reduced when honed too far over the spec. I have seen just the small end bore dipped, big end bore dipped, and the complete rod dipped. When they cannot be fixed this way, they get sent back to be melted down and start all over.

They are not dipped they are electroplated.

 

[padam]

But it has something to do with heat treat. Copper isolates the steel from the carbon during the heat treat. I understand the small and large ends are heat treated, then machined, (obviously) whereas the beam is not.

I would believe this over everybody saying the copper is to build up rods that are out of size. I have read it all.

The pics show copper on the ends of some rod bolts (not all.)
This suggests the plating occurred as an assembly, and perhaps the nuts changed later.
Bolt installation would occur after heat treatment.

Just because 2 rods have a similar copper color doesn’t mean the same process was used to get that color.

What was done 60 years ago in an auto assembly plant may be completely different from current aftermarket motorcycle parts manufacturing.

 

[toolmanmike]

The pics show copper on the ends of some rod bolts (not all.)
This suggests the plating occurred as an assembly, and perhaps the nuts changed later.
Bolt installation would occur after heat treatment.

Just because 2 rods have a similar copper color doesn’t mean the same process was used to get that color.

What was done 60 years ago in an auto assembly plant may be completely different from current aftermarket motorcycle parts manufacturing.

I'm out. No one on the planet has the answer to this question.