[dscott]

Jim, It is normal to run a 28oz balancer and flywheel on the windsors. Some of the builders will use the 50oz to cut the cost of the heavy metal needed to balance the crank. You really shouldn't have much in the way of vibration. If the engine was not balanced it should have been. Most of the 393 strokers use the 400 crank and after machining they do need to be balanced. The builder should have done a rotating balance on the crank, flywheel and balancer. The rods and pistons should have had a static balance. Normally if an engine is out of balance you will feel the vibration at certain rpm's (1800,3600,4800). If it is a constant vibration sometimes it will come from the sidepipe mounting. If the pipes are not isolated from the rear mounting point you will feel the engine vibration through the car. The thing that bothers me is that the rear main seal is out after so few miles. If the engine is out of balance the first thing to go is the seal. The vibration will beat the bearings to the point of failure. If you are going to attempt the rear seal replacement I would pull the rear main cap and check the bearing for wear. I would check with the builder and make sure he balanced everything. I hope the news isn't to bad but at this point I would hate to see you lose a motor.
Don