How important is the type of damper you choose if the 390 is internally balanced? Is a stock one just fine?

 

Stock one off a 360 is what I'm using. Should be ok.

 

410/428 were external and will not work on 390. I bought a perfect original damper from a 76 360 off ebay for $10. It works great.

 

428/410 were externally balanced at the flywheel, not the crank dampener. All FE balancers should be neutral balanced. Any dampener will work. I am using a 390 truck balancer with the 428 CJ 1UB crankshaft in my 416. You can use any FE balancer with a factory internally balanced 390.
Paul G.

 

I am confused about "balance" - what is it and what is "internal" and "external". How can a machine shop balance a rotating mass if they do not spin it?

 

They do spin it. They have a specialized machine specifically for balancing.

External balance means there is not enough room in the crankcase for all the counterweights needed, so some weight is used on the flywheel and sometimes harmonic balancer to offset this and balance things up.

Internal balance means there IS enough room for the necessary counterweights to fully balance the engine. No weight is needed on the flywheel or in the harmonic balancer.

An internally balanced engine, like the 360, 390, and 427, is inherently more stable than an externally balanced engine, like the 428, and most other ford engines. The reason is that external balancing creates unusual forces on the crank which can cause problems.

 

On the 410 motors were they balance the flywheel end externally i've had all my motors internally balanced. Slugging the crank isn't cheap but it's nice to run motors over 400,000 smooth miles.This makes a smooth running engine and and easy swap from auto to stick or mixing up of parts. All the parts that i'm changing around have been zero balanced ahead of time. Beware of the outer damper ring slipping on the rubber, make a scribe mark across both parts for a refference point. Carl.........o&o>.........

 

Thanks, Rusty - the last peice of the puzzle.

I found some info through google - the idea is to machine the pistons, rods, and wristpins such as that they all weigh exactly the same, and then put the crank into a special machine with special bob weights installed on each rod throw - these weights are equal to the combined weight of the rod, wristpin, and piston.

The crank is spun and a computer tells the operator where to remove weight on the crank to make everything balance - this must be the drill holes I see on some cranks.

I have rebuilt several engines without balancing them and never had a problem, but then I always went back with stock parts from that engine. Since I have been reading the proper way of doing things here, I now call them a re-ring and valve job rather than a rebuild.

If you are changing the rotating components I can sure see the need to balance. I am, however, still a bit skeptical in that what we are talking about can only be a few ounces, and, in real life, our cylinders will receive different amounts of the air fuel mixture, and even different A/F mixtures, thus presenting different loads to the crank.

It would seem the load presented by compression and power strokes would far exceed the stress presented by a few ounces out of balance.

What am I missing?

 

*****,
If any component in your engine is "a few ounces" out of balance you are going to have big problems. When rebuilding I always balance the rotating components (rods and pistons) to within a gram of each other. An engine that is significantly out of balance will cause the crank to "push" against the bearings and shorten engine life. My FE is a fairly mild performance build. All the components are balanced to one gram and then the entire rotaing asembly (balancer, pulleys, crank and flexplate) were spin balanced as Rusty described. If you are doing a stock rebuild it may not be necessary, but it is always a good idea. It is relatively cheap insurance, and it will ensure that your rotating assembly is as efficient as it can be, which will deliver more HP and MPH.
Paul G.

 

Paul, your right on on the balance, is that why i run motors over 400K, yup!. Whats a ounce or more at 6,500 rpm i'd bet 40 pounds beating the bearings. External balance the block feels smooth but everything inside is shaking. Go cheap and pay later. I've seen many FE motors that were factory virgins that were shakers, Ford was sloppy on their balance jobs, that production thing again. 410", want a 484". Carl........o&o>...........