On the Jackshaft modification:
By installing a stud into the jackshaft, you can provide
some extra support to the front "nose" of the jackshaft.
The double timing chain gears on the front of the jackshaft
will induce bending moments. This load, when extreme due to
high rpm, will induce a crack around the portion of the
jackshaft nose where it goes from a smaller diameter to a
larger one (the front bearing support).
By installing a stud, which will reach further into the
jackshaft, it will keep that front portion of the nose
under more compression, lessening the chance for a crack
to start at the portion.
Usually the only time I have heard of the Jackshaft
modification being needed is on high rpm engines. Ones
that are usually taken to 8,000 rpms. So I didn't recommend
such a fix for street engines. One roadster snapped the
jackshaft while waiting at a stop light! Really odd time
for it to fail!
So it is a worthwhile modification, after all! Still the
failures are fairly rare events, not one I loose sleep over.
Sincerely,
Tom Walter '67 2000
Austin, TX '68 2000
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