How interchangeable are 318 harmonic balancers?

ProjectBazza said
Based on my TDC tool it appears it has slipped a little under 10 degrees. Of course it could be "user error", as my eyes ain't what they used to be, and I'm doing this hunched over the engine bay with a bad back, but....

But the engine is original, and I've been told by the owner more than a few times that in the interest of originality, no one has messed with anything w/o their knowledge or consent (same family since new).
Click to expand...
With a piston stop there is very little user error. Rotate one way until it hits the stop, mark zero. Rotate the other way till it hits the stop, mark zero. Halfway between will be TDC. Slipped is slipped wether 10 or 100 degrees. You’ll need a new one.