Thread: different connecting rods?

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Hi all of you,
I have a question.
I can't get my engine to stop vibrating. I just rebuilt it, it vibrated before and I rebuilt it completely. Top end is new, heads fine, pistons new, the only old parts are: Connecting Rods, Crank, Block. It doesn't idle very well, but I guess that' still a setting issue. But it vibrates slightly all the time, and when it gathers RPM it keeps on vibrating in a sort of slow but strong VVVRRRRMMMM....VVVRRRMMMM.... way, getting faster with engine speed. I made double sure all clearances and everything were right.
BUT: I have two different types of conrods. 5 of one type, 3 of the other. They all weigh the same (+/- 1 gramm) but the three have heavier big ends. Can that cause vibration that strong? It's only about 20 gramms more on the big ends... Didn't think that would matter with a big heavy crank like in an SBC...
Could it really be the con rods? Or would a not balanced crank do that? All in all, I have no idea where those parts came from, they were in the engine, which had been rebuilt before.

Hope somebody knows about the effects of incorrect balancing...
Thanks, Max.

PS: It vibrates even without TC and Fan assembly attached, balancer is new.

Hi Denny,
I know how to lighten rods, I just didn't think I would need it, robably silly of me... It's not easy getting proper rods at a decent price here in Germany. The damper is new, the flexplate is straight. The new damper didn't change anything, and it's a 350, so it's internally balanced. If Damper and Flexplate are balanced in themselves that should be OK then, I guess. But I might try pulling the front pulley off and running it completely without any outside connections, just to make sure it isn't just a bent pulley or something before I pull out the motor (for the second time in a yerar )
Any other ideas where the vibration could come from?
Thanks a lot,
Max

My buddies motor started vibrating one day and it ended up being the fan. There was something wrong with the clutching or something? He changed it and it cured it. Just something to think about.

Originally Posted by DennyW

You should have got a hold of a matched set of rods.

Did you double check to make sure you have the right balancer on there ? How about the flywheel ?

The rods should all have been weighed. Find the lightest one, and match the rest to that. You weigh the tops of the rods, and match, then the bottoms, and match them. Then, you weigh the total weight again.

I think Denny is on the right track.
If your rods vary in weight a little you would never even know it.

So would that mean just a couple of gramms more in the big end of three rods would cause such strong vibration? As I say, it can't be the fan or the TC, because I took them off to check whether it was them and it didn't make a difference...
The last turning thing outside the engine I haven't checked yet is the lower pulley. I'll have to get some kind of auxiliary cooling pump before I can check that, though.
Thanks for the input,
Max

Originally Posted by MadMax

So would that mean just a couple of gramms more in the big end of three rods would cause such strong vibration? As I say, it can't be the fan or the TC, because I took them off to check whether it was them and it didn't make a difference...
The last turning thing outside the engine I haven't checked yet is the lower pulley. I'll have to get some kind of auxiliary cooling pump before I can check that, though.
Thanks for the input,
Max

The oil splashing around from the pan up to the rods will make it vary more than a few grams.
Are you sure you have the right balancer on there?

Hi Erik,
well, I'm not completey sure. There are many kinds of balancers, mine is for internally balanced 350s. I just bought the same size I had on before so that my timing tab would fit, the new one is slightly lighter. But I didn't think it would make much difference in a not-race application? What balancer would you recommend?
Thanks for helping, anything before I pull that engine out again...
Cheers, Mad

Originally Posted by MadMax

Hi Erik,
well, I'm not completey sure. There are many kinds of balancers, mine is for internally balanced 350s. I just bought the same size I had on before so that my timing tab would fit, the new one is slightly lighter. But I didn't think it would make much difference in a not-race application? What balancer would you recommend?
Thanks for helping, anything before I pull that engine out again...
Cheers, Mad

Any of the internal balancers will work.
Make sure it does not have a metal piece on the inside of it.
It will be about 4 inches long and about 1/2 to 3/4 an inch thick and in a shape of an arch.
If it has this on there it is a balancer for a ext. balanced engine.

Hi Again,
I've now double checked to make sure it's the right balancer and the flexplate is also correct and straight. But my bottom pulley is slightly bent. However: I've traced the real problem to a completely different source: The ignition. I just thought I might try and fiddle with it, and hey presto, it did it! The vibrtion isn't completey gone, I can still feel it at highway speeds, but nowhere near as bad as it used to be. Seems to be my vac-canister, which is very sensitive to even small differences in vacuum. I always set my ignition the same, 8° BTDC static and left the rest to the dizzy. But somehow that didn't do it. As soon as I have time I'll tune up my ignition properly.
I still have that vibration issue, but I don't think it will cause any further problems, it's just very slight. Somehow my bad ignition seems to have amplified the problem...
Thanks for all the help, I'll keep you posted as soon as I find the actual source of the vibration.
CU,
Mad

Mad Max, I believe www.summitracing.com does international orders. A set of Summit Stage 1 connecting rods is $230.