Con rod weight difference

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APRRSV

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I purchased a pair of connecting rods from Norvil a few months ago. They are part # 06-4896. I just weighed them today after reading in another thread about weight differences in pistons. My two rods differ in weight by 13 g; 418 v 405.
What weight difference is considered typical for off the shelf rods and what is considered excessive, i.e. needs attention?

Incidently, I also purchased a pair of GPM pistons from them. They were within 1-2 g of one another. So, I was pleased with that.

Thanks,
Ed
 
APRRSV said:
I purchased a pair of connecting rods from Norvil a few months ago. They are part # 06-4896. I just weighed them today after reading in another thread about weight differences in pistons. My two rods differ in weight by 13 g; 418 v 405.
What weight difference is considered typical for off the shelf rods and what is considered excessive, i.e. needs attention?

Incidently, I also purchased a pair of GPM pistons from them. They were within 1-2 g of one another. So, I was pleased with that.

Thanks,
Ed
Click to expand...

That is typical. They should be matched within a couple grams. Jim
 
You have to match them end for end, not just on the total weight.
Lots of info on the net, for example https://www.youtube.com/watch?v=QLpiF0E0EJU
Please note though that he is working with steel rods in this video. Alloy ones need more gentle handling :-) but the principal is the same.
I would probably use a file rather than a belt sander for example, and polish the rods carefully afterwards.

/Steve.
 
13g is a lot in finished forged alloy rods, could be a sign of insufficient forging fill. Sadly you'll not know if the difference is extra weight or 'missing' weight unless you can measure the rest of the batch.
 
At first I would say just send them back but as you bought them from norvil I doubt you will get very far !! If you hadn't weighed them you would never have known and I doubt it would be a problem unless you're racing etc hopefully most of the weight can be removed from the big end cap let us know how you get on cheers.
 
APRRSV said:
Thanks Jim. Where is the best place(s) to remove material?

Ed
Click to expand...

To balance them you would have to hang them and weight and match both ends. Not much point unless you are doing a dynamic balance on the crank also.

Of course they were never balanced from the factory -so the only time they were right was by accident. Jim
 
Madnorton, Baz and Jim, I just weighed the alloy upper and steel caps separately. The alloy uppers are identical in weight at 193.0 g. The entire difference in overall weight is in the steel end caps.
Given this info, does anyone have concerns that the weight difference could indicate a serious fault only to be discovered by catastrophe at a later date?

Thanks,
Ed
 
APRRSV said:
Madnorton, Baz and Jim, I just weighed the alloy upper and steel caps separately. The alloy uppers are identical in weight at 193.0 g. The entire difference in overall weight is in the steel end caps.
Given this info, does anyone have concerns that the weight difference could indicate a serious fault only to be discovered by catastrophe at a later date?

Thanks,
Ed
Click to expand...

That's good news. It's easy to remove 13 grams from a rod cap. I will post a picture Jim

Start here

Con rod weight difference


Arc over to here.

Con rod weight difference


Do not remove much material from the center so you don't reduce the bolt hole wall thickness.
do not grind so far that you get into the re-enforcing ridges that run from one side to the other.

Or use it as is, I don't think there is likely to be a structural problem -just made from a different batch. Jim
 
Also, check the thickness where they are spot faced to the alloy rod mating face - it may have not been taken down far enough, may not seem much but it will help. The forged caps are most probably over engineered for what they do.

At the rate these were pumped out when the bikes were made, there would have been many forging tools in use and I doubt they lasted many strikes before they were replaced, 20,000 caps a year would most likely have consumed 4 - 5 tools, so may explain the mismatch as explained by Comnoz.
 
Hi Mad, I'm not sure that I understand what you are referring to. Any chance you can post a diagram or pic?

Thanks,
Ed
 
Sure, the portion of the cap the bolt shank passes through, ie when the nuts do up they do up further on one cap than the other.
 
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