I'm having to replace 2x rods due to a minor fail, the replacement rods are 6 grms heavier than the originals on the big end. The machinist has suggested I fit washers to the original rods to make up the weight. Do you think there's an issue adding washers? Rod washers from ARP are 0.061 thick, thats 0.061 less thread engagement with the rod bolt. Am I overthinking this or is this acceptable?

Not an expert hear, but i thought of it this way. If you put a replacement rod with the extra gram weight in cylinders one and eight, the extra weight is out where the least transferred stress would occur. When you consider you are only adding 12 grams to the sum total of all four journals, ( bob weight 2200 x 4) or 12 grams added to 8800, it comes out to . 0013 of the total engine balance . Since a lot of the total engine balance weight is on the ends of the crank on a crank with no center counter weights, that miniscual one eighth of one percent becomes very insignificant in my mind. I'd run it without adding washers.

What connecting rods do you have?
Aluminum, steel, I-beam, H-beam, etc. Can you post a picture of the big end?

Can you grind the weight off of the heavy rods?

Or don't worry about it, Not enough to throw the balance.

I've had many stock forged steel rods in BB Mopar engine builds, both RB, B and SB motors as well as some street hemi 426 rods, come back from the balancers with a lot of the small end and big end rods ground off for balancing
BTW, weigh 6 grams of oil and see how much that is

I wouldn't worry until about 12g

rebel,
Looking at some of your posts this year, do you have steel Molnar rods?
If so, call Molnar and ask if there is an area on the big end of their rods that your machine shop (balance) can grind off 6 grams.
EDIT:

When I grind h-beams (belt sand) I always do it with the rib not across it. Unless your really particular, not worth it. 6 g's will be a pretty good carve out.

Yeah 6g is close to 1% of the rods total weight

I've seen recommended to take some off under the bolt head area, -I think- my Molinars sheet said to take it from the outside sharp corners of where the threads are, will try to check tomorrow, but im fairly sure they were not talking that much weight, just for "manufacturing tolerances"

Im a little dubious of where 440Jim has said as that shape is pretty specific and most likely FEA'd to the Nth to keep the weight down.

Have you compared the fasteners? they didn't just use a slightly different/longer bolt or anything silly?

Email/call Molinar?

Prob nothing wrong with what Gregsdart said either

Yes, 6 grams difference on the big end does sound like a lot.
Last year I looked at 4 boxes of Molnar 7.100" rods and these are the weights listed on the box matched sets:
Total _ BE wt _ PE wt
827.30 565.60 261.70
827.00 565.70 261.30
825.80 564.00 261.80
822.90 563.70 259.20

ok, my rods are steel H beam Molnars, I did look at changing the rod bolts & getting them cut down to match, but molnar rod bolts are rather small physically compared to Eagle, Scat, etc...The heads are smaller, the waisted shank is smaller, Molnar seem to use a very special bolt on their rods. To cut down a set of std ARP bolts to match will have them shorter than the bolts I'm already using. I sent Tom @ Molnar an e-mail last week, but as yet, I have had no reply. I did run the idea of swapping the rods around so the heavier ones were #1 & 8, but my machinist wasn't keen on that idea as I pull 8k between gears, & told me his balance job is why my engines last with minimal wear. He also wasn't going to grind the caps as he felt 6 grams was a lot to grind off these caps, maybe if they were Eagles which are physically chunkier, but not Molnars. When I first got my rods from Molnar, every single rod was identical, big & small ends weighed the same for the complete set of 8, which was impressive compared to the Eagles & Manleys on my other motors. I guess I'll just have to wait til Tom replies & get his impression of my predicament. I did send him the weights of my current rods so he could match them, maybe this was as close as he could get?

I would reduce the the weight on a rod bolt at the top of the head to remove 3 g see what it takes. You can get new bolts easily from Molnar I would think, if needed. Reducing the bolt at that dimension should be fine. It would be best to machine it off, if ground be careful about heat.

At 7.8 grams of steel per cubic centemeter, the 6 grams would represent a chunk of metal roughly . 4 by . 4 by . 3 devide that by two and that tells me there won't be much left of the bolt head for a socket to grip.
The machinist that is so worried about 6 grams extra out on the ends of the crank, i have to ask how he rationalizes using standard, non center weighted cranks? Or worse yet, running converters, flywheels and dampers that act as counter balancing?

Might be best to sell what you have and buy a new set of rods.
35 yrs ago I was a mechanic in a Shell station. Had a customer that spun a bearing in a Porsche 944 S.
I took it all apart, had the crank cut and got one brand new rod. I went back and forth to the local deli
and had the rod weighed on the cold cut scale. I ground a little at a time from the cap until it matched the original.
When I was done I remember thinking it looked a lot stronger before I touched it. I put it all back together and it ran great.
Not sure how long it lasted

Never do anything that will give you less thread engagement. I would just bite the bullet and buy some new rods. Scat makes some really nice I beam rods or go with BM aluminum rods. Just don't do anything to sacrifice the strength of the rod or the bolt. Just kidding about the BM rods But if you could afford them, best on the market.

When you say BM do you mean Bill Mitchell Engineering in Carson, NV ?

6 grams heavier? That's a little more than the weight of a quarter at 5.67grms. I can't image it making a bit of difference, but if you must then how about shaving the width of the small end of the connecting rods, or a little thinner on both ends. Leave the cap alone.

Tom Molnar told me to never remove metal from his rod from the cap, he to remove metal on the H beam sides above the rod big end

Finally got a reply from Tom. I was asking his impressions of adding washers, but it looks like thats not the go....
here's his reply
[bRight now with the shortage of parts everyone is seeing, these were the closed weights that we had. You are correct, putting washers on the bolts will reduce the thread engagement. By my calculations, you would need washers with the equivalent thickness of about .130". This will reduce the thread engagement by over 2.5 threads. To prevent threads from pulling off from bolts that are into steel, you need one times diameter of engagement which means you should have 7/16" of threads engaged. Our bolts have 1/2" of thread on them so shortening them by .130" will give you just about 3/8" of engagement. This is not enough engagement to prevent a problem. Even if you add weight to the other 6 rods, the crank should be balanced. It would be better to use the rods as they are and put them both on the same rod journal and spin the crank on the balance machine. In the worst case, would require a tiny amount of heavy metal added to one counterweight. Another option is to put the new rods on the #2 and #3 rod pins and just run it. Putting them on these two rod pins will counteract each other.

A couple of things I have to caution you about. First off, the bolts in our rods are not an off the shelf ARP bolt and actually have several features that make them an upgrade over what the others sell as an upgrade. Second, do not let anyone grind on the ribs on the rod caps. Doing this will knock the big ends out of round and cause potential bearing spinning or even rod failure.
][/b]