I am working on is a 2003 G3 5.7 Hemi block that I put a stock 2008 6.1 Hemi crankshaft in. The bob weight for the 6.1 crankshaft is unchanged from the factory and is a bit different than what the 5.7 crankshaft would have been. My dilemma is using aftermarket connecting rods and stock replacement 5.7 pistons and keeping close to the 6.1's bob weight. Here is what I have versus what factory is...

O.E. 6.1 Hemi:
Connecting rods = 653. grams average
Pistons = 435. grams average
Wrist pins/locks = 154. grams average
Total = 1,242. grams average

What I have:
Total = 1,266. grams average

That means I am 24. grams over on every rod/piston/wrist pin set. Since my wrist pins are 156. grams each I was thinking I can save some weight there. Trend has a set of H-13 pins with the correct dimensions that are 126. grams each. That would put me under by 6. grams per rod/pistons/wrist pin set...

Does that sound like a workable combo? I read it is better to be slightly under with the weights than over on weights. Any advice is welcome...

I like it

Do you have an opinion on H-13 wrist pins for the street? I will drive this quite a bit and that was my only hesitation about the H-13 material...

I'd call Trend and talk to tech. If he says go for it, I would. I think you will be fine but they are open tomorrow and a phone call away.

That's the problem. I did call, and that's where I got the information. He seemed really young and a bit lost in the conversation. I asked about different materials and he kept reading off the screen for the H-13 pins he found with the correct dimensions. He did not instill any confidence into me, unfortunately...

G3 pistons are very light to start with. I can't see an issue using the wrist pin that you want to use. I used a stock '03 bottom end with Eagle heads to increase compression and air flow. I can tell you that there isn't a lot of room for bigger cams with that combo, so new pistons might be in order anyway. I ended up using a '07 SRT cam.

I think where the weight difference is on the rod matters, rotating verses reciprocating. Maybe I'm misunderstanding the post.

How close is the end for end weight of the rods?

I've already got my pistons. Everything is pretty much set. I'm just thinking I need to lighten things up to match the stock 6.1's reciprocating assembly. This is just a street engine, but I want it to be as balanced as a high revving race engine...

My pistons are light at just under 415. grams a piece. They are weight matched to each other. My connecting rods are forged I-beams and are about 695. grams each and also weight matched to each other . All eight of my rod/piston sets are within 0.40 of a gram lightest to heaviest. I am just trying to get the set up as close to factory balanced as possible. Bearing life, smooth revs, and all that matters for a street engine to live a long time...

The big ends are approximately/close to 500. grams and the small ends are approximately 195. grams. My pistons are just under 415. grams a piece...

You are not understanding the term bobweight! It is not just the total weights of all the components.

Correctly, you weigh the big end with beam level and the small end supported, then you weigh the little end, beam level and big end supported.

Bobweight for one cylinder equals the weight of the big end plus rod bearing, plus one half of the weight of small end, pin, rings and piston.

As there are two rods per journal, the bobweight for a journal is 2 times the big end plus rod bearings, plus the weight of one rod small end, one piston, one pin and one set of rings. You can see that changing one gram on one end isn't the same as changing one gram on the other.

To answer your question about pins, I have no doubt that the lighter pin would be fine in your setup. Decades ago a friend was drag racing a 427 Chevy engine and used 0.088" wall thickness pins to offset some of the weight of the heavy TRW pistons. He used to twist that engine pretty tight, more than 8,000 rpm. There was never a problem with the pins. I believe some of the reason was the rigidity of the TRW clubs, but nonetheless that was quite a bit more severe than your pins are going to see.

R.

I understand how bob weight is calculated. I referenced my 6.1 Hemi crankshaft's bob weight only because I was asserting that it was stock and unchanged from factory. Thus, I am matching my rods/pistons/etc, overall, to the factory weights as to keep it all balanced together properly. My throwing out grams of this and that is only to show I have done the research and math regarding the parts I have verses the parts I am replacing. Keeping it all aligned as best I can...

I'd run it. A better wrist pin is not going to hurt a thing. I'd call Trend and ask for a different guy. Ask the person that pics up the phone for the engineer

I think you are right. It'd probably be just fine. I do need to call Trend again. I was thrown off by the comments I read about H-13 maybe not being the best for endurance applications. I am 400-425 horsepower, with a light'ish piston, in what will be a daily driver. I'd think H-13 would live forever in that situation. I don't rightly know firsthand though...

I do think bobweight is what you should be matching. And with that, where the rod's extra 40 grams are matters.

Assuming that the rings are the same weight, and making a reasonable assumption about the small end/big end weight distribution of the OEM rod, your best case is that you match the OEM bobweight perfectly, worst case, your bob weight will be about 35 grams heavier than OEM.

It is unlikely to be perfect, and unlikely to be 35 grams off.

Someone smarter than me will need to recommend an out of balance amount that is generally acceptable, and you'll need to decide how much it matters to you, but never really knowing for sure.

My guess is you'll be withing 10 to 15 grams and be fine.

Or you could weight both ends of both rods.

I made a call back and had a very specific conversation with a different tech and he said without hesitation that the H13 pins will have no issues in my application and to run them. It's the answer you and I both anticipated. Just makes me feel better hearing it from a seemingly more competent individual, than the first guy, at the manufacturer I guess...

That's good news. I hope it works out good for you.

"I want it to be as balanced as a high revving race engine..."

Get it balanced. OEM balance tolerances are, at best, sketchy. It's apart. Do it right and take it to a reputable machine shop to be balanced.
Also, here's a formula:
Rotating:
2x big end rod
2x rod bearings
oil (approx 5 grams)

Reciprocating:
2x small end
2x wristpin
2x piston
4x (or 8x if dual spirolox) wrist pin retainers
2x set of rings (include support rail if applicable)
multiply sum of reciprocating by 50% (51 or 52% work if you are consistently over 7k rpm)
add that number to the rotating sum and you have your bobweight.

Now get it balanced.

I was kinda wondering that myself, both shops I use only hit me for 175 for a balance.

I recently had three of the same type oem cranks spun against spec and there was a 32 gram spread. Factory balance sucks.