After two days of trying, this wasn't making it through the list server.
It was probably being ignored as being a bit too large, so I broke it into
two parts. Sorry for the delay.
At 02:58 PM 12/15/98 -0600, Robert Allen wrote:
>.... I'd be curious to hear from those who have successfully rebuilt the
manual transmission of a 'B' or Midget and who took the time to figure out
the lubrication mechanism of said transmission, then describe for us
slow-witted doomsayers how it would be perfectly okay to _not_ disconnect
the driveshaft when towing. ....
Okay, I accept the challenge (but maybe not the conclusion). I have yet to
do a 'B' gearbox, but I have done a Midget 1275 gearbox, and MGA gearboxes
(several). The MGA units are very similar to the early MGB units, and all
of the above share a similar system for lubrication. And especially for
the "slow-witted", I will be using lots of small words, so bear with me.
And 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.
Before going any farther I want to state soundly that the 2nd and 3rd gear
bronze bushings (and 1st gear in the later 4-syncro boxes) are not just
splash lubricated, but are in fact pressure fed (low pressure) with a flow
of oil through a drilling in the mainshaft. The source of the forced oil
flow is a helical screw oil pump on the mainshaft, just behind the rear
main bearing and in front of the speedo drive gear. This part is a little
illusive, because the catalogs usually just call it "spacer" or "distance
piece". Please remember that this part is mounted on the output shaft, so
it will turn whenever the propshaft rotates, whether the engine is running
or not. So here's how it works.
In the front gearbox housing, just inside of the side cover, cast into the
inside top of the case, is a trough that will catch oil being splashed up
by the gears on the mainshaft. Oil runs to the back of this trough,
through a hole the back of the front case, and into a passage cast in the
front face of the rear case, where it runs down (by gravity) to a point
just below the rear main bearing. From here there is a passage cast into
the rear housing leading back and up to a point in the bottom of the close
fitting bore just under the helical screw "spacer" (which I will hereafter
refer to as the "pump". Since the origin of this oil flow is at the top of
the case, well above the mainshaft, the resulting small pressure head
causes the oil to flow readily upward into the close fitting chamber around
the pump.
When the mainshaft turns in a clockwise direction (vehicle traveling
forward), the helical groove in the pump draws oil forward. At the front
end of this helical groove is a radial hole that conducts the oil flow
inward to the inside of the pump, which has a hollowed out cavity around
the mainshaft. From here the oil flows into a radial hole in the mainshaft
to get into an axial drilled hole in the center of the shaft, where it then
flows forward towards the front of the mainshaft.
In the front end of the mainshaft there is a restrictor orifice. This
allows a little oil to flow out the front to lubricate the needle roller
bearing between the input shaft and the output shaft. The function of said
restrictor is also to retain a little oil pressure inside of the mainshaft
to induce the oil to flow to other places. Also in the mainshaft there are
more radial holes leading to the bronze bearings inside of the 2nd and 3rd
gears (and 1st gear in the 4-syncro MGB boxes). Assuming the unit is
assembled correctly, there are also aligning holes in the bronze bushings
that allow this oil to flow out to the small space between the bushing and
the steel gear to create an oil film bearing, similar to the journal
bearings on the engine crankshaft.
The bronze bearings inside of the 2nd and 3rd gears (and sometimes 1st) are
normally idling, and the only load on these bearings is the weight of the
gear itself. This is true when the engine is running in any mode other
than 2nd or 3rd gear (or 1st for the later units), and also when the car is
being towed without the engine running. When you actually engage one of
these gears while driving, that gear is locked to the mainshaft to provide
the output rotation, and as such there is no relative rotation between the
gear and the shaft while it is transmitting the driving load. As such, if
there is any oil at all in the gearbox, these bronze bearings seem to last
indefinitely. The only one I have ever had to replace was one 3rd gear
bushing that was damaged on one end when the third gear thrust washer came
loose from the mainshaft.
Now there are other bearings in these units. The oil level, when properly
adjusted, lies about at the top lf the laygear hub, so the layshaft and its
needle roller bearings and most of the laygear and reverse gears are
submerged in oil. Whenever the input shaft is turning lots of oil is
transfered from the laygear to the gears on the mainshaft, and oil is being
splashed all over inside of the housing. The large ball bearings on the
input shaft and on the main shaft are just dipping into the oil so the ball
races are liberally bathed in oil while turning.
[continued in part 2]
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