Compression is within spec on all 8 cylinders and the spark plugs are good. I will get it in my shop and up on a lift tonight and try to find the source of the vibration

 

What spark plugs ya got in there??

 

I still havent been able to find the source of the vibration in the engine. It still only does it at about 2300 rpm, but is smooth at all other rpm so i know its a balance issue. Ive just been driving it the way it is, but now the vibration has caused the rear main seal and the transmission front pump seal to start leaking so i need to dive into it again to try to find the source. The engine is a 360 so its externally balanced, and i have a new weighted harmonic balancer and a B&M weighted flexplate on the engine. Ive checked the timing, compression, spark plugs and distributor and have no problems there. The part that stumps me is that ive checked everything on the engine, yet i still have a mystery vibration that is destroying the rear main seal and the front transmission seal and i have no idea what could be causing this.

 

Think I would take the flexplate to the parts store, and compare it to a stock unit.... maybe the holes were drilled wrong..... (thus, putting the balance weight in the wrong spot.)

Also, On the older trucks, I though that the balance weights (for the engine) were on the torque converter? If you have weight on the converter, and the plate, yeah, that's gonna be a problem.

 

When i rebuilt the trans, i got a new torque converter from transtar without weights on it so only the flexplate is weighted. Since i have to pull the trans to replace the front pump seal, I think I'll send out the torque converter to have weights put on it and then install an unbalanced flexplate so that the balance is transferred from the flexplate to the torque converter and hopefully that fixes it

 

LA 360s are balanced different than Magnums. Magnums had the weights on the flex plate. LAs had them on the TC. You need a balanced TC, with a neutral flex plate. Even the harmonic balancers are weighted different. They changed the Pistons in the Magnum engines so they are weighted different from LA to Magnum. Double check and make sure that B&M flex plate is for the LA, and not a Magnum. Same with the balancer. LA TC bolt pattern is A symmetrical, only bolt up one way. Magnums are not offset. Easy way to tell, but I think B&M makes the flex plate with both bolt patterns if I'm not mistaken. I went through all of this when I did a 360 Magnum swap in my 91. I smoked the pump seal, and trans in the first mile of the first test drive. Another $3,000 later for a rebuilt trans, and TC, I was happily melting tires lol.