3400 Crankshaft

[GrandAmGTTT]

I just got a 3400 engine today and tore it down to the bare block. However, when i pulled the crank it said 3100 on it? Remind you this is out of a 2000GAGT. Are the cranks 3100 and 3400 the same?

[bszopi]

Yes, they are the same. They are also the same as the crank in the 3.1 MPFI engine from the late 80s/early 90s. All share the same stroke, so why change up the crank?

[GrandAmGTTT]

i'm not changing the crank, i was just curious if they were the same just to make sure before i build

[Superdave]

Same crank was used in the gen2 3.1, 3.4 DOHC, 3100, 3400 and the RWD Iron head 3.4

[bszopi]

My comment was not about you changing the crank, but GM changing it.

[SwiftDrift]

I hate to post on a vintage thread... But i am very interested in engine balance, and seeing that they all use the same crank with different pistons makes me start to think, different piston weight and the same crank would throw things out of balance right?

[3400-95-Modified]

GM never balanced the engine... so that should answer your question... At least that's what I've been told, so someone correct me if I'm wrong.

Only other thing I can think of is they were very close to the same weights and any variation was simply accounted for by drilling the crank to proper weight but I don't even know if that was done.

[SwiftDrift]

thats crude. is there anyway to buy a crank that does not have the drilling, so someone can balance it themselves?

[geoffinbc]

No. But if you want to fill the holes you can weld it up. It is nodular iron so it wont crack. You can also have a good machine shop install mallory metal weights to increase the balance if needed.

GM most likely did a mass balance where all cranks were cast and balanced with the same weight and the pistons and rods were all within a certian weight spec.

[Superdave]

I pulled a crank out of a 3.1 and put it in a 3400 during a low budget rebuild back in '09. It ran great, nothing odd and he's put like 30K miles on it.

[SwiftDrift]

I just dont like the idea, how much does it cost to balance an engine? I want my engines as balanced as possible. They say v6s have an end to end imbalance, i wonder if lighter outer pistons could reduce this. If so it would have to be done just right. Maybe i should make a post specifically about balance.

[caffeine]

You shouldn't need to worry much about balance with a stock bottom end. With solid motor mounts in my Firebird I hardly feel any vibration at all. No interior rattling or anything like that.

[SwiftDrift]

even at idle?? :0

[caffeine]

Even at idle yes it's perfectly smooth.
V8s are more naturally imbalanced than V6s. Heck 4 cylinders are way more imbalanced than V6s as well. That's why most modern 4 cylinders have balance shafts while most modern 60*V6s do not.

Balance shafts are more common in 90*V6s. I'm not really versed on the details but the V angle is best at about the same # of degrees a between power strokes. The most ideal angle for a V6 is 120* but that would make for a very wide engine so 60* is used as a compromise.

[SwiftDrift]

yeah i am not all about four cylinders... i love the idea of 60* v6s, but why did gm take the step of going to 60* for better balance without even balancing the rotating/reciprocating assembly? lol. you wouldnt happen to have the weight of your pistons and rods would you?

edit: also balance shafts can smooth some imbalance, but they also add other vibrations in different directions