Bill,
Seems to me that it would make sense to exclude the pressure plate from
the assembly to be balanced and perhaps get this balanced separately? It
is, in any case, so light that I can't think it would make much
difference.
Re the 3rd bearing. I think the answer is that this was really only
intended for use with the "bent wire" cranks. It shouldn't be necessary
with a fully counterbalanced crank.
The following extract from an article by Mike Hawke, which will be
published with the February Triple-M notes in "Safety Fast!" may help:
"The most significant design modification to the installation came late
in the day and was the provision of the outrigger ball race at the front
end of the crank. This was fitted from engine 2501AJ on. Remember, that,
by then, the J2 was the only small-cam model in production. J1, J3 and
J4 were all finished and the large-cam K and L-types were well into
their lives. The days of the J2 must have been numbered, so why make
this change then?
The extra ball race does not strengthen the crank so much as change its
stiffness and hence its natural period of vibration. The rear main
bearing is held along the length of the bearing (provided it is in good
condition) and may be considered encastre. The front end, held by a ball
race may be considered to be simply supported. Adding the second ball
race makes the front end encastre too and doubles the stiffness of the
crank, putting up the natural frequency of vibration by a factor of the
square root of two. (e.g., from, say, 3,000 rpm to 4,200 rpm). What had
suddenly made M.G. all vibration conscious?
One possibility is that they had changed the clutch cover plate from
cast iron to aluminium (equivalent to taking a few pounds off the
flywheel). Had they reduced the damping effect thereby? Or had they
changed the natural period of vibration of the crankshaft assembly to
one where it became intrusive? To make changes like this when the model
was "on its way out" does seem odd but the outrigger ball bearing was
subsequently retro-fitted to many earlier cars."
The alloy clutch cover plate does, of course, save some weight (helping
to partly compensate for the much heavier counterbalanced crank).
If you would like to contact me separately, I have a small supply of the
proper later cover plate (i.e. cast from an original alloy cover plate
rather than cast from the earlier cast iron plate - there is a
difference!) at a lower price than Sports and Vintage.
John James, Keynsham, Bristol, ENGLAND
-----Original Message-----
From: owner-mg-mmm@autox.team.net [mailto:owner-mg-mmm@autox.team.net]
On Behalf Of Bill Putnam
Sent: 09 January 2003 17:32
To: mg-mmm@autox.team.net
Subject: J2 engine Questions
Greetings,
Does anyone have any comment on how to best balance the J2
crank/flywheel/clutch
assembly? The pressure plate has some slop where it rides in the
pin guides, so I would expect that this could easily move a little
bit within the flywheel, throwing off the balance.
Is there a general consensus on how useful the additional 3rd bearing
that can be fitted to the front of the engine is? I will be using
an Allen billet crank and rod set 73 mm stroke in the engine.
Does the alloy clutch cover plate as supplied by Sports & Vintage
motors provide a noticable improvement in throttle response? My
original steel clutch cover is in good condition but the flywheel
and clutch assembly seems awfully massive for an engine of this size.
I will be putting a blower on this engine and racing the car in VSCDA
so smoothness and durability at high rpms is desired (or is that
an oxymoron with a 2 bearing crank :-) ).
Thank you,
Bill Putnam
J2 3488
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