In a message dated 11/08/98 2:58:12 AM Pacific Standard Time,
neil.cairns@virgin.net writes:
<< The head
carries two cast iron camshafts, with 20;50;50;20 valve timings, and
.375" lift. The inlet valves are 1.6" dia. and the sodium cooled exhaust
valves 1.44" dia. Both set at 45 degree angle in the head, with double
valve springs. >>
The valve timing actually varied, with the earliest engines having slightly
more aggressive timing. This was soon standardized in production.
The exhaust valves were never actually sodium cooled - it was a good but
expensive idea, toyed with in development, and perhaps used in EX 181 (I'd
have to look it up).
<<The cylinder head, with non-renewable steel valve seat inserts cast in,
was of cross-flow configuration, giving efficient hemispherical
combustion chambers,(hemi-head.)>>
The valve seats are a serviceable item (thankfully).
<<two huge SU H6, 1 3/4" carburetters,(
carberetors,) with pancake air filters and a cold air feed duct, on the
other side. Interestingly, one of the inlet manifold securing nuts lives
INSIDE the manifold.>>
They used H8 2" carbs for competition! That nut inside the manifold caused
many a loss of sleep by nervous owners racing their cars. A friend has his
Loctited, lock washered, and lockwired. When I ask him if he thinks they might
be working loose, he still blanches.
I don't have the problem - I use a pair of Webers.
<<To feed fuel to the two carbs, it has a high
capacity SU electric fuel pump.>>
A 'double ender' as used on the Jag XKs.
<<To hold the solid skirt aluminium alloy,
convexed head piston on, it had 'H' section forged steel connecting
rods, with stronger horizontally split big ends. ( The ohv had angled
split big ends, to permit the rods to be drawn up through the bore.)>>
The rods did _not_ have horizontal split big ends. For this you have to wait
for MGB's (late engines only) and MGCs.
<<The engine was a BC16GB in BMC engine numbering terms. All you
could see under the open bonnet was two massive, polished alloy rocker
covers, it must have been a swine to work on in-situ.>>
It was. After only a few cars, they made removable inner fender panels
standard so you actually had a shot at getting to the distributor, for example
- still had to jack it up and take a wheel off mind you.
<<Again the distributors had a vacuum advance from the
but this was soon deleted due to ignition timing
wander.....not a good thing on such a fussy, cammy engine, known to be
prone to melting pistons! >>
This was an early measure taken to try and address the piston problem. They
didn't realize at this point that fuel, not ignition was the culprit. By the
way, with the distributor drive at the front of the dummy cam,. only supported
at one side, the Twincam was a very poor candidate for anything that put a lot
of resistance on the distributor, like a magneto. It would wear the gears out
fairly quickly.
<<This was a long chain, and could
mis-behave if not tensioned/timed correctly. MG went to great lengths to
ensure all owners/mechanics knew this,>>
The tensioners were aluminum and the threads inevitably pulled from the
constant tugging. They didn't realize this during production, but many
stripped tensioners began appearing at higher mileages. You can either
helicoil the tensioner, or do as I did in my race engine, and have a new
tensioner cast up in steel.
<<( Today we know it
was half the fault of the oil itself,too thin when too hot. Progress has
improved oil beyond recognition.>>
The chrome rings with the wrong bore finish and usually insufficient break-in
were also factors. They could have saved themselves a lot of trouble by just
using cast rings.
They didn't figure that the bores needed the oil spray that comes from the
little hole in the side of the big end (bet you always wondered why that was
there) so they specified bearings without the hole - they also had a little
more clearance than pushrod cars.
<<The gearbox ran very hot, and if the gearbox breather blocked up, the
expansion of the oil could cause leaks.>>
True of all of the cars, OHC or not. A little known fact about Twincams is
that for some reason, they used front transmission casings from the Magnettes,
which had provision for mounting the clutch slave on the top of the case (ie
right behind the head. The slave was mounted normally, down below of course,
but the unmachined flat still appears on all or most Twinkie cases. Another MG
mystery that we will never understand. Perhaps they felt that with the thicker
wall, the case would be stronger?
Again, Neil, thanks for taking an interest in an obscure model (sorry Dave,
but at the races, the most frequent comment I hear is that the onlooker has
heard of but never seen a real Twincam). The second most frequent comment is
to allude to their reputation for bad reliability and ask why I would race
one. Interesting that the rep lasts so long, but then I guess you think Nader
when you hear Corvair, notwithstanding that the problems with those cars were
mostly in the early models.
The third comment is often to ask how the hell you get the engine into the car
(the factory put it in before the body went on - the rest of us are on our
own).
Bill
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