[continued from part 1]
Besides being submerged in oil, the layshaft also has a form of forced oil
flow to the bearings. There are holes drilled in the front of the main
case, and channels cast into the front cover plate, such that oil can get
to the front end of the layshaft. The layshaft has an axial hole and one
or more radial holes to allow oil to flow into the bearing cavity inside of
the laygear. Rotational motion of the laygear causes some centrifugal
force (oops, sorry for the big word) in the oil between the laygear and the
ends of the main case. This force moves oil away from the layshaft at the
ends, effectively dragging oil through the bearing cavity inside the
laygear, and creating oil flow around those needle roller bearings.
Without this flow metal particles could accumulate inside of the laygear
and accellerate the wear on the layshaft. With this flow any particles
there will be flushed out and (hopefully) settle out in the bottom the the
gearcase.
And while we're on the subject, in light of this last statement, how many
of you know that you should change the oil in the gearbox and rear axle
about every second oil change? More inportantly, how many of you actually
do it? Better check the maintainance schedule in the shop manual. For
most people this may nearly match the interval for flushing the brake
fluid, another sorely neglected point.
Now there's one more bearing in the gearbox, the one at the far rear of the
mainshaft where the driveshaft connects. MGA 1500s have a bronze bushing
here with a sliding spline connector for the driveshaft yoke. Very late
MGA 1500s and all later MGA and all MGB gearboxes have a ball bearing here,
and a flanged coupling for the driveshaft. In the case of quick
acceleration a generous splash of oil may find its way back there. At
steady cruising this will not happen, but this action is not left to
chance. Along the inside of the rear case, about half way up on the right
side, is cast another oil catch trough. Whenever the output shaft is
turning oil is splashed up here by the speedo drive gears. This oil runs
to the back of the rear case where it can enter a cross drilled hole and
end up just in front of the rear bearing. There is a large flange in the
casting here surrounding the mainshaft, so as to encourage the oil to flow
farther back and to flow through the rear bearing. Behind the rear bearing
there is a groove inside the bottom of the case to allow oil to flow
forwards below the rear bearing and through a hole back into the forward
part of the rear case. As such, oil will flow here constantly and bathe
the rear bearing whenever the mainshaft is turning.
Now as to the question of whether you can tow one of these things with the
driveshaft connected without damage, the answer is a resounding "it
depends", and what it depends on is the type of gearbox. The real parts in
question here are the bronze bushings inside of 2nd and 3rd gears (and
sometimes 1st), and also the needle roller bearing at the front of the
mainshaft. At this time it is my opinion that the oil pump being driven on
the output shaft will NOT supply forced oil to the mainshaft while being
towed in the 3-syncro gearbox, but that it will work in the 4-synchro
gearbox. This is because the oil supply to the pump is from splash in the
top of the main gearcase.
In the case of the 4-syncro box, the large reverse gear on the mainshaft
will always be turning with the mainshaft, and being unshielded will throw
plenty of oil up into the catch trough on top. So if the oil is at the
correct level the entire gearbox would be properly lubricated by the pump
while towing. So as a challenge at this time, I would like to know if
anyone has ever towed a car with a 4-syncro gearbox with the driveshaft
connected and incurred any internal damage to the gearbox? I think this
would not be a problem, and 4-syncro units can be towed with the driveshaft
connected without worry.
In the 3-synchro box 1st gear (shared by reverse) is always turning with
the output shaft, but it looks to me that the shape of the 1-2 shift fork
shields the top of that gear and will prevent oil splash in the direction
of the catch trough up top. I do not know this for sure, but I may soon
install a clear plastic side cover on a 3-syncro box and spin the output
shaft in an attempt to find out. This feature does not however necessarily
preclude towing these cars with the driveshaft connected.
For starters, I don't think that would be a good idea for a 3-syncro car
that has been sitting for some years without running, as there may be
little or no liquid oil in the mainshaft bushings. It would be a great
advantage if the car could be driven for a short distance before towing to
distribute oil throughout the gearbox. I would guess that running the
engine and idling it in neutral would NOT do the trick.
If the 3-syncro car can be driven some first, then I think it would be no
problem towing it for a short to moderate distance with the driveshaft
connected. I myself have towed an MGA more than 100 miles on the rear
wheels without disconnecting the driveshaft, with no appearant problem, and
no noticable wear on the 2nd or 3rd gear bushings when disassembled many
years and many hard miles later. Once there is a proper oil film in the
mainshaft bushings it will remain there for quite a long time, as the gears
in question are simply idling stationary with their own weight on the
bushings as the mainshaft turns. However, if the 3-syncro car is towed for
more than a few hours with the driveshaft connected, the oil film in those
bearings may ooze out or break down and solidify from the heat of friction,
and this could cause some damage. So in this case, tavel at your own risk.
Any comments are welcome, as I think I will soon be posting this on my web
site. And once again, PLEASE do NOT copy this entire memo when responding
to one or two of the finer points, but just copy the pertinent parts you
wish to mention.
Cheers,
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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