I have a 97 F-150 with a 4.2L V-6, and about a year ago I had to have
the engine rebuilt. ( I have subsequently been told by various people
that the shop that did my overhaul has a reputation for shoddy work.)
Anyhow, there was a 20,000 mile warranty and after about 25,000 miles
the rebuilt engine started acting up, and finally died.  I couldn't
afford another $2000, so I decided to work on it myself this time.

Now I think that I have found the problem ( or at least *a* problem):
The crankshaft timing-sprocket key had sheared, and so the sprocket was
partially slipping on the shaft.

Here's my question:  The design of the keying arrangement seems so
incredibly poor, that I'm wondering if maybe the rebuilder used the
wrong parts or something.

The Woodruff key (half-moon shaped) is about 1.4 inches long by about
5/16 at the deepest point by 3/16 wide.  But the slot in the crankshaft
is positioned in such a way that only about 1/4 of an inch of the key
engages the sprocket, and the other 1 inch of the key engages the
vibration damper.  Furthermore, the slot in the sprocket hub is only
about 1/16 inch deep!  So it's no wonder that the thing sheared; the
amazing thing is that it lasted as long as it did.

It looks to me like the key slot in the crankshaft must have been
milled in the wrong place,  but that seems awfully hard to believe.
And I don't see any way to re-position the sprocket because it was
lined up correctly with the camshaft sprocket.  But I'm sure that if I
just go out and buy a new sprocket & key,  they will fail again in a
few months.

Any suggestions ???

TIA,   john w.   (  jwallacq AT hotmail DOT com )

P.S. >>  One other wierd thing, and I don't know if this could possibly
be related: The chain tensioner seems to be jammed in the fully
extended position. Even now that I have it removed from the engine and
out on the workbench, I can't get it to retract.  So it must have been
putting a lot of excessive tension on the chain.