Rebuild/balancing question

Boosted185

New member
I’m about to a start on rebuild on my 2nd gen 3s with aftermarket pistons and rods. I’ve looked into what goes into the process of balancing but am still quite confused on one simple question. Will they be able to balance the rotating assembly if they have all the weights(journals,pistons/pins/ rods) and have already machined block and possibly crank, but with me assembling it, I know they can balance the crank and then balance the pistons/rods to themselves but that doesn’t technically balance the rotating assembly. newbie question but I can’t seem to find an answer other then how they balance it
 

Magroo

New member
While not specific to the 3sgte, jafromobile has an excellent series on the 4g63 he built for his Frankenstein Elantra.

About 3 minutes in he shows what he does with the pistons and rods. He had the crank ballanced separately in a different video.

If you havent watched his videos they are quite good. I would probably start at the beginning of the Elantra series and go from there.

https://youtu.be/Rcu799LPJng
 

dbucks2k

Member
I'm starting my rebuild too. And my understanding is that when they balance the crank they need to know the weight of the rods/wrist pins/pistons.
 

Boosted185

New member
Awesome thanks for the replies, I copy pasted a paragraph from another forum explaining why you shouldn’t do it that way , how ever I’m not sure how important it is to a motor not exceeding stock redline “assembly to actually be balanced when the engine is running.
Ok, there are tolerances on everything. The crank stroke tolerance is what is important here. Say the stroke is 87mm (+/- .05mm) and the crank is spin balanced without any bob weights. Let's say that #1 is on the high side of the tolerance (87.05mm) and the #4 journal is on the low side (86.95mm). The crank is balanced to it's self as the counter weights where ground down to accomadate the different swing weights of the journals. The crank is fully balance, and in your world, it's ready to go. Now, add on the rotating weight of the piston/rod assembly that was balanced seperately. It should be pretty obvious by now that the #4 journal is going to see less force on it due to the .1mm shorter stroke compared to the #1 journal. Now, factor in the idea that the forces are in a cubic relation and that and you can see how a .1mm stroke difference can have a substaintal difference at 9500 RPM. That is why they use bob weights, even on an internally balanced 4-cylinder.”
 

dbucks2k

Member
A 3sgte with the stock stroke of 86mm at 9500rpm would have a mean piston speed of 5300 feet per minute. That gives a high chance of breaking a rod no matter what tolerance is met with the rotating assembly.
 

dbucks2k

Member
And the crankshaft should be balanced with Bob weights. That's why you need the weights of the rod/piston setup.
 

Boosted185

New member
Thanks I’m starting to understand now, I’m not planning on running 9500rpm it was just an example I found online
 
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