Hi

Thing is, i have lm7 block, LS1 stock crank and set of 4 gen 5.3 rods and pistons. Stock ls1 pistons and rods (with rings, bolts and bearings) is 1293-1295g. 4gen 5.3 pistons and rods (with rings etc. as well) is 1301-1303g. So overall weight difference is not that much, but because ls1 has heavier pistons and lighter rods than 5.3 vice versa, do i still have problem with balance?

 

That falls fairly close to factory balancing tolerances.

 

Total weight and the bobweight that a crank is balanced to is two different things. The rod weight of the rod is split between the big end and small end. The big end and the rod bearing weights are counted twice when calculating the bobweight. The big end is where the majority of the weight difference is between an Gen 3 and Gen 4 rod.

 

That’s a hard one. Honestly the “correct” thing to do would be to balance it. However. Knowing what I know now, I essentially wasted $200 but I guess I paid $200 for “peace of mind”. You see, I just built a 5.3 with H beam rods and forged pistons. The weights were off just a little like yours. Ended up being so close that when he balanced it, all he did was lightly tap the crank counter weight with a drill bit in two spots. Said after, as long as I’m turning under 7500-8000rpm that it would be just fine and was within factory spec.

I did an experiment this past weekend. I built a big CC atv drag engine. I originally had a titanium wrist pin in it. Started making some noise, pulled it apart and one thing I didn’t like was the slack that titanium wrist pin had. So I put a regular heavy duty unit in. 20 grams heavier. Bike is easy to work on so it was worth trying and wouldn’t take long to pull back apart if it vibrated. It didn’t. Matter of fact, the owner I built it for said the engine felt smoother. FWIW.

 

You need some factory balancing.

 

And how do you get that after the engine leaves the factory?? Stupid statement....

 

must be trying to get posts up so he can do some spamming

 

I agree. After surfing around I've noticed that all his posts are made today, some with incredibly obvious statements.

 

Just 10g may not sound like much, but that's a 52 lb difference in inertia at 6000 rpm. Your difference is likely more than that since the equation is bobweight = (piston + rings + pin + small end) + 2 x (big end + bearings + oil).

If it were an old farm engine that will spend much of it's life putting around at <2000 rpm, then I'd say not a big deal. For performance use though, definitely balance it.

 

Thanks for the great answers, i will get it balanced and let you know what they did to it and how much.

 

Is that on the compression stroke or the exhaust stroke ?