Over the last few months, My neighbor and I have each been working on
our A's. Mike is doing a much more thorough job than I. His car will be a
top notch driver when he is done. Anyway, last month we were putting
Mike's engine together. We decided that the timing gears looked a little
rough and new ones were in order. Also, removing the head bolts
caused some marks on the surface so the thought was "better safe than
sorry" and new ones were ordered up.
Well, the motor started to go back together last week. The bottom end
was buttoned up and the head was going on when SNAP- one of the
new Moss head bolts decided that 40 ft-lbs (on the way to 50 ft-lbs) was
too much to take. Mike borrowed my torque wrench to check his and
found out that both read about the same so it was not a question of over
torquing. Mike said that the other long head bolts felt a little "spongy"
when tightening them as well. Upon inspection of the head bolts we
figured out that the shorter studs had a dot on their tops. According to
some sources, head bolts with a dot or the number "22" on top can be
torqued to 60 ft-lbs instead of the standard 40 ft-lbs. The long head bolts
had no dot or no. 22. The bad head bolt suffered a ductile tension
fracture about 1/3 of the way up the threaded portion at the top- just
below the nut I would guess. This is not where I would have expected
an undamaged stud to fail- it would more likely have failed at the course
threaded end due to the smaller cross section. I am guessing that the
stud had an inclusion or some other defect. In any case, the long studs
were replaced with the original long studs which took the 50 ft-lbs with
no problem. The shorter studs felt firmer when they were tightened so
Mike left them in. Maybe ARP head bolts are a good deal after all.
The second issue we ran into concerned the cam gear and thrust plate.
Mike put in a new thrust plate and cam gear, again from Moss. After
tightening everything up, the motor was jammed solid. When the cam
gear was loosened the motor freed up again. For those of you unfamiliar
with this part of a B series motor, the cam thrust plat fits into a slot
between the front face of the front cam bearing journal and the back face
of a shoulder on the back of the cam gear. The depth of the shoulder on
the gear must be deeper than the thrust plate is thick or it binds when it is
tightened. Anyway, comparing the old parts to the new showed that the
old parts started with about .002" clearance- the shoulder was about
.142 deep and the thrust plate was about .140 thick. Wear had opened
that clearance to about .005" with most of the wear occurring on the
back of the thrust plate. The shoulder on the new cam gear was only
.100 deep- a full .040 (1 mm) shallower than the original. Furthermore,
the new thrust plate was only about .130 thick. So, the new parts would
not work together. Just as bad, neither of the new parts were right for
replacing the old parts. After a considerable amount of time considering
the alternatives, the motor went back together with the old cam gear and
thrust plate. The play did not seam excessive for a street engine. The
new crankshaft timing gear fit fine by the way and, since the old crank
gear was the one that was worn the most, the new one was used.
Has anyone else experienced something like this lately? Maybe Mike just
got the two bum parts in Moss' warehouse. If you are thinking about
replacing these parts in the near future, take the time to measure the
clearances to make sure what you put in is better than what you took out.
New is not necessarily better.
Regards,
Bill Eastman
61 MGA still the daily driver since the other car has been at the body
shop for 5 weeks now.
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