anyone know the stock rod weights of the 5.7 rods from an 87 camaro and the stock rods used in the 79 350s?

 

Sinthetik,

Plain old stock rods (cast billet, forged afterwward - NOT "pink or PM rods) usually come in at just under 700 grams or thereabouts. I've seen some way over, and some as little as 660g. "Balance" apparently isn't a major issue for a stock engine.

EDIT: Some of the really ugly looking stock rods go WAY higher than that before balancing efforts. Those were just numbers I've pulled from spreadsheets from builds, and compared the beginning masses of parts. Some are way off the scale, but can be cleaned up to match those 700g rods to make respectable "sets".

 

305 rods are even heavier... lot of meat on the to compensate for the smaller piston... the rest ther hacked out of the crank...

zroc

 

305 rods are substantially lighter than 350 rods. The entire length of their beam is narrower than 350 rods, except for a few 305s such as L69s that came with X rods. If you lay a typical 305 rod next to a typical 350 rod, it's pretty obvious what the difference is; the scales will tell you the same thing.

Rods are not made to "compensate" for smaller pistons in any way. It's not necessary. Pistons, and the small end of the rod, are reciprocating mass, not rotating; therefore as long as all 8 are the same, that's all that matters. The actual weight itself, whether heavy or light, makes no difference as long as they're all the same.

If the rotating portion of the rod mass is made smaller, then the crank counterweights have to be made smaller, to retain equal masses on either side of the axis of the crank. Since a good portion of the beam of the rod appears as rotating mass, it affects engine balance; pistons weight, and small end weight, don't. The crank is counterbalanced differently in most 305s to compensate for the the lesser rod weight. That's why even though 305 cranks and 350 cranks are usually made from the same casting (casting # 442), they don't exactly interchange; the counterweights on the 305 one have been ground down farther. You can swap a 305 crank and 305 rods together into a 350 and it will work fine, or vice-versa, since the pistons don't figure into the part of engine balance that the crank has to deal with; but you can't mix the 2.

Most stock 350 rods come in at around 720-740 grams, untouched. 305 rods are about 30 grams (roughly an ounce) lighter on average.

The way the factory builds motors (or at least used to) was to weight the rods as they come out of the final machining process, and put them into a "n-dimensional array" of big end weight, small end weight, total weight, and length; and when one box got full enough to build a motor with, they would match a crank to that weight. That's why factory rods are all over the place, yet the motors run smooth: the factory doesn't "balance" them the same way we do. If you take a bunch of rods from different motors, the weights will be essentially random, and will look like the factory pays no attention to "balance" whatsoever; but if you get a set of factory rods out of a single engine, they will be closely matched.

Misinformation and "old wive's tales" don't do anyone any good. Please don't make up bogus stuff and post it as an answer to a technical question. This is a site for facts and information, not hearsay and made-up guesses.

 

haha funny phrases

 

RB,

Are you talking about rod/cap/bolt assemblies, or the bare con rod? I've seen rod caps from 141-149 grams alone. The caps are a good place to trim "big end" mass, too, since they can be poorly finished, with lots of opportunities for reduction.

I'm guessing you're right about the factory assembly techniques, since that is basically the same method used to grade springs to make "pairs" or "sets" Nothing gets wasted, eventually.

As for the rod mass alone, I suppose the factory method produces acceptably matched sets, but I've seen variation over 23g in the same engine "set". It's a lot more rewarding to "waste" time getting the variation down to a couple grams or less.

Not to try to cover for anyone, but maybe ZROC was looking at a truck 305. I've not seen inside any of those that I know of, so they may be heavier ???

 

RB, that was a great post. I learned a thing or two from it myself. Explains a lot. I always kept rod sets together based on nothing but instinct. If you asked me why I did it, I probably couldn't have given a very good answer. Now I know why that little voice in the back of my head was telling me to do it!

Would you do everyone a favor and please put that in it's own separate post and submit it? I'm sure a lot of people would like to see it. People who may not have tuned into this thread.

 

Regarding "truck" parts- 305 or otherwise. Truck motors USUALLY get the heavy-duty parts. If a truck 305 was to come with the heavier 350 rods I wouldn't be surprised. The number of times I've been called on the carpet when I told somebody something "doesn't exist" (when it actually did) is directly related to how may times the example in question was a truck motor. Never say never. GM did a billion little changes over the years that nobody can account for with blanket statements.

80% of the time I make mistakes about stuff has to do with whether we're taling about either:

1. A truck vs. a passenger car
2. Freakin' Canada. They do everything different up there.

 

atcually... they were out of an 84 305 caprice... they were nearly exactly the same as a L69 rod... lot of meat on them... the 350 rods I was comparing them to were 70's truck... it was years ago... but they definately were heavier... the wieght diff between the rods was hillarious... so now what??? posibly a variance from different years???

wasn't hear say... just my experiece with them... I spent a lot of time matching the weight of the rods on a calibrated scale...

you got my brain tickin' RB... its all good though...

zroc

 

Any you guys know the weight of the ZZ4 rods. I am getting new ones and wanna get some lighter or about the same. Was looking at the 4130 eagle or scat rods.

 

Either way, you'll want to check their set balance match first, then end-to-end balance, then have the rotating unit balanced as an assembly. If you're lucky enough to have a "V" wheel stand, you can do it yourself if you can find the correct weights. I like to use the magnetic strip tapes that can be curled/stuck around the rod journals, and trimmed with scissors. Unfortunately, I no longer have access to a stand.