71 D100 318 crankshaft/harmonic balancer problem?

[cdog530]

Hey, I am not new to this forum for great info but this is my first post...
I was doing my normal maintenance on my 71 Dodge d100 (oil, greasing....) and I noticed there was a faint metal on metal grinding noise only noticeable with the hood up. Upon further inspection I noticed that the harmonic balancer is starting to wear into the housing with the timing marks on it. Looking at the back side of the balancer I can see that it is slowly grinding it down. Is it possible the the balancer is loose causing this? Or could it be a problem with the crank? The motor runs great never had a hiccup or anything. I am not going to drive it until I figure this out, don't want to cause more damage. The oil is always good and it never uses any. Any suggestions would be much appreciated.
Thanks
-C

[elacruze]

If the balancer was loose, it would be walking off the front. The balancer seats against a shoulder on the crank so it can't move towards the rear. I can only think of two things that could do that; First, that your crankshaft thrust bearing is worn. You can check this if you're careful by removing the flywheel inspection cover and carefully prying back and forth on the flywheel, to move the crank fore and aft. If you can move it enough to say you can move it, it's worn. I don't remember the spec but seems like the service limit on that is about .025" play.
Be careful if you have an automatic transmission not to hamfist it and bend the flex plate.
Second could be that the rubber ring on your balancer is shot, allowing the outer ring to move rearward. This should be obvious on a visual inspection, if the sandwiched ring is protruding anywhere it's shot.