Many years ago when I was an apprentice, my boss told me to always check true top dead centre before installing a vibration dampener. The advice finally paid off on this 1970 351 Cleveland. It had apparently never run properly, lacked power, high fuel consumption etc. It looks like the factory balancer indeed "slipped" about 10 degrees or may have been off to start with. The timing chain was just out of spec, 15 of the 16 cam lobes were undersized but I suspect this may have been the main problem. Thought I'd share.
That would cause a problem,I had a 302 ford come in for a nasty shake after a rebuild, when they put the engine back in they put the old flexplate and balancer back on, they could not figure it out, I got a hold of it,and when I pulled the pan, turns out the machine shop changed crank shafts,What I remember some older fords ran 50 oz crankshaft and some 28 oz,turns out wrong crank to match the balancer and flexplate, so if that balancer move 10 degrees, you will feel a vibration.
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If it has Tits,Tires or a Track,your gonna have a problem with it.
These issues do happen, the stock type balancers that are bonded ...outer and inner rings can move. I had this happen on a reputable aftermarket piece. I checked TDC when setting up a cam and noticed the markings didn’t make sense. So back to basics... and yes it was out 10 plus...
So another more expensive unit solved it ...
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gotta be young and dumb before you get old and wise
Didn,t Ford move the timing marks in the early to mid 70,s . Like from a pointer to cast in the t cover ?? . Could be just the wrong date balancer installed . But Yeah after you figure out its a spun balancer there is usually a bit of cursing involved .
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I,m as cool as Milner , but axeually a bit more like Beckwith