Where should I remove mass from a 1970 con rod?
 

Rebuilding a 1970 E engine, I noticed one of my con rods is different - it's black. So I presume it's an S con rod with the nitriding.

Anyway, it weighs 10g more than the others, which I guess it would be best practice to resolve, since I'm hoping to make a relativelyhigh-revving engine. Four of the original five are machined around the big end journal whereas this one is not. I hypothesise that they were machined to closely weight-match with the lightest. Is this a normal place to remove weight? And is it a good idea for me to remove weight here from the heavier black one, to get them more uniform?

https://uploads.tapatalk-cdn.com/202...7b4eaaadbf.jpg

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When I balance a mismatched Carrillo rod recently, I was advised to take material off the sides of the big end, flanking where the bolt goes. Not off the cap where people sometimes do, as that can affect stiffness at a critical place. Basically round off the sharp outside corners to make them rounded.

The factory spec is 9 grams btw, but it could help to bring within a few grams of the others. You can also pair rods strategically according to weight, 1 and 4, 2 and 5, 3 and 6.

All the others are within 2g of each other so I imagine someone has balanced these in the past. Then the black one was added as a later replacement.

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10 grams is weight we wouldn't try to remedy.
The 9 gram tolerance is ridiculous. Our target match is .5 gram.
Overall weight is less important than end to end balance.
You could easily see 10+ grams end to end mismatch in an engine that was built by someone with some less than stellar technique. Simply put, your rods could have the same overall weight with an end to end mismatch of 10+.
Matching rods is a huge challenge given the variance from set to set.
2.2 rods are not an especially expensive rod set so I would recommend sourcing a matched set and balancing them to achieve the best balance available with your equipment.
The other challenge is quality rod bolts. ARP does not make a replacement so if we are required (by budget) to reuse the stock rods, we modify the rod body to accept the 2.4/2.7 ARP bolt.

Why wouldn't you just buy a correct rod?

The answer to that is in my previous answer. The rod groups are so numerous that finding a single rod that matches is a fools errand.
Obviously if the rods are mismatched in this engine, the last builder struggled to find a match.
I come here to provide the one and two time builders the best chance at success.
Searching for a single matching rods will cost far more than just buying a matching set.
Simple example: You spend 25 hrs looking at rods that don't match. You finally find one that is close and the seller charges you $ 100. Seems reasonable. Now you have to rebuild the set and balance to match.
Now the other scenario, you spend an hour to source a matching set and the seller charges you $240. Now your rebuild time costs should be less because you're starting with a match set.
I guess the question is, how much time are you willing to waste and possibly never find a single replacement rod?

EDIT: I just spent less than 2 minutes finding 6 sets of 2.2 rods on EBay for less than $300.

Why wouldn't you just weight-balance your existing rods?

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The answer is: It's impossible to remove 10 grams from a Porsche rod without challenging to integrity.
Next question, what are the end weights? Where is the difference? Do the other rods match end to end? Are you willing to risk an overhaul in order to save a few bucks?
Every engine builds has compromises but the best route to success when building these old engines is to keep the computerizes to a minimum.
Do you plan to ring the neck on the little hot rod? Give yourself the best possible chance at success.

A story from a past poster: He was having trouble making a rod and piston combination balance so he welded a bolt into the wrist pin to compensate for the imbalance. Would it work? sure but at some point the elegance of your craftsmanship will reflect on the entire project.

Thanks Henry. I'll check those end weights and I'll measure thickness at the spot I propose to remove material. If the heavier one is thicker there I'll be comfortable from a structural integrity point of view taking it down to the same thickness as the others. I think of these stumpy little rods as massively over-engineered for the job they do. I don't think I've ever heard of a 911 con rod failure, at least not in a weasly 180hp engine.

An interesting statement given the fact that this engine had a previous rod failure.

The number one cause of engine failure in the 3.2 Carrera is rod failure. Granted that is an entirely different animal with it's own unique design failures.

lol Good point Henry, although being precise I meant a "primary" rod failure whereas this engine looks to have had a primary oil starvation followed by seizing & overheating piston, and finally the rod replaced either due to mechanical failure or heat damage.

What I mean is simply that non-S 2.2 rods are cheap and easily available. It's probably a $25-$50 WTB ad over in the classifieds. Seems like a better route to take than modifying a non-matching rod.

So... this is how it went. I'll let you know if it lets go!

https://uploads.tapatalk-cdn.com/202...78d0e96038.jpg

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I have an 70 T set with 80k I took out of the original motor
PM works best.
Bruce

Hello Rob ,
I know Ollie's used to balance mismatched rod sets this way , so hopefully
no problems .

Do you have numbers that tell you all/most of the imbalance is on the large end of the rod?

I’m including this animation of rod motion to help make my points:

https://youtube.com/shorts/vL_FwC51sDI?si=ApNrSpcPgGKRNrW8

The pink dot is kind of hypothetical. It is at the center of the big end, but it shows that the big end is pretty much rotational. The blue dot is the piston pin and small end, which is pretty much linear motion. Nothing new here for most people.

So, balancing the small end, or the sides of the shank seem risky to me. There is not a lot of extra on the small end, and the shank is where the maximum bending load is. On the plus side, it is largely linear motion, so the influence of the small end on rotating balance is low.

The closer you get to the large end, the bigger the influence on rotational balance, plus the sides of the bolt bosses are relatively low stress, hopefully always a compression force! That is the reason for the popularity of that location, but 10 grams is a lot.