4.6 vibration at 2500+ RPM

[Thorwon]

I've built a 4.6L short rod stroker. I used a 1988 Comanche block, the light crankshaft and rods from a 4.2L.
SPEED PRO Hyper H802CP40 pistons.
The camshaft is a Crane Cam H-260-2, .456 intake .484 exhaust.
And a 7120 head.
I put a new Dorman 594-018 Harmonic Balancer on it.
It starts and runs great, but at about 2500 RPM there's a vibration and the higher the RPM the worse it gets. Idling it's as smooth as it should be.
Everything I can think of is right.
Anyone have any ideas what it could be?

[Cheromaniac]

A bad harmonic balancer? I wouldn't buy Dorman anything unless there was no alternative. Get an OEM balancer and see if that resolves the vibration.

[Thorwon]

A bad harmonic balancer? I wouldn't buy Dorman anything unless there was no alternative. Get an OEM balancer and see if that resolves the vibration.

That's going to be the next thing I'm trying. I've got a OEM on the way.

[6TIME]

FYI years ago I initially built up a stroker very similar to yours with the oem short snout 4cw crank and it had vibes/harmonics at 2500-3k on up. We swapped to the 12cw crank and it smoothed things out greatly. It’s still my daily driver today at 235k on the motor. Food for thought if the damper doesn’t change things for you.

[Cheromaniac]

You shouldn't need to change the crank to eliminate the vibration unless the crank was out of balance in the first place.

[Thorwon]

FYI years ago I initially built up a stroker very similar to yours with the oem short snout 4cw crank and it had vibes/harmonics at 2500-3k on up. We swapped to the 12cw crank and it smoothed things out greatly. It’s still my daily driver today at 235k on the motor. Food for thought if the damper doesn’t change things for you.

I hope the balancer I have coming tomorrow (Monday) will fix the vibration. If it doesn't then I'll have to put the spare 4.0 in the truck.
I need my truck running and don't have the money for another crank.

[Thorwon]

Well the balancer didn't help it.
I guess I'll be pulling the engine, so I can remove the crankshaft.
I've got to see if there's a shop local that can balance it.

[Cheromaniac]

Well the balancer didn't help it.
I guess I'll be pulling the engine, so I can remove the crankshaft.
I've got to see if there's a shop local that can balance it.

You might also want to check if the connecting rods are weight balanced.

[Thorwon]

You might also want to check if the connecting rods are weight balanced.

What I have found so far.
The rod/piston assemblies are 124gr different from the orignal 258 rod/piston assemblies.
The new rod/piston assemblies are lighter by 124gr.
All new rod/piston assemblies are less than 1gr different in weight.
This weight is including rod, piston, rings and bearings.

The local speed/machine shop wants $375 + any heavy metal needed to check/balance the crankshaft.

[Thorwon]

I'm going to build it back to a 4.0, all I need are pistons. I have the rest of the parts.