At 09:32 AM 11/13/2000 -0600, Rick Ehlert wrote:
>.... I down-shifted to third to round a curve and when I did, third gear
made this whirring gear noise! It only does it when de-cellerating in
third, ....
>....
>1966 MGB Mk1
>3 syncro w/ OD
Okay, time for another gearbox tech session. The possibilities are not
exactly endless, but there are alot of them. Bottom line first is that it
will probably not incur immediate catastrophic failure, but you should plan
on a gearbox rebuild in the not too distant future. If from this time
forth the noise should get significantly louder in a short period of time,
that would be a good indication that the end may be very close at hand.
Disinterrested parties should delete now. Otherwise, it's school hour.
The gear and shifter parts I will be mentioning herein can be clearly seen
in this picture:
http://www.chicagolandmgclub.com/photos/gearbox_reb/gbx_261.jpg
Please ignore the broken tooth on 3rd gear. And I'm sorry but you can
never see the mainshaft front thrust washer in the fully assembled gerabox.
For that you need a section drawing of the assembled gearbox as you would
find in the factory workshop manual, but I will do my best to explain its
function (the thrust washer, not the book).
Because of the angle of the helical teeth on the gears, when you
decellerate it pushes rearwards on the laygear and forwards on the gears on
the mainshaft (in 2nd and 3rd gears only with the 3-syncro gearbox).
Because your noise only happens with 3rd gear I think it is not a problem
with the laygear or layshaft or the laygear thrust washers, and also not a
problem with the rear main bearing which takes the thrust load of the main
shaft in both directions (back of main housing, not the tail bearing). We
are searching for something that is unique to 3rd gear.
The foward thrust load on 3rd gear is taken by a locking thrust washer on
the mainshaft just ahead of the (3rd) gear. This part is keyed to slide
over the splines on the mainshaft, then rotates into a circumferencial
groove in the shaft where it can transfer the thrust load to the shaft.
This thrust washer is then held in place (prevented from rotation) by a
spring loaded pin which pops up out of a radial hole in the shaft to engage
the splines inside of the thrust washer. The thrust washer is a steel part
with a slightly ecentric oil grove in the flat face, and it bears directly
against the steel front face of the (3rd) gear. This is a steel on steel
thrust joint and takes a fair amount of thrust load on overrun, especially
at higher speeds. This is similar to the thrust washers at either end of
the laygear, but the relative rotational speed between this mainshaft front
thrust washer and the (3rd) gear is slower than the speed of the layshaft,
and the parts are splashed or trickle fed with oil, not submerged in it.
When engaged in 3rd gear there is no relative motion at this joint, because
3rd gear is locked to the main shaft. However, when engaged in 2nd gear,
with 2nd gear locked to the mainshaft there is a similar thrust load here
(at the front thrust washer) on overrun as 2nd gear is pressing forward
against a bronze thrust washer which in turn presses forward against 3rd
gear. And at the same time there is a relative rotation here (at the front
steel thrust washer) due to the difference in rotational speed of 2nd and
3rd gears, when the 3rd gear is turning faster than the mainshaft. So on
overrun in 2nd gear it can incur wear on the front thrust washer and the
front face of 3rd gear.
In the case that the front thrust washer might become dislodged from it's
proper location, then 3rd gear can be pushed forward on the mainshaft on
overrun until the thrust washer bears against the back face of the 3rd/4th
sliding hub (no relative rotational motion), and the sliding hub then bears
against the back face of the input gear. Under these conditions there is
relative rotational motion between the input gear and the inside sliding
hub, and these metal to metal surfaces were not designed to be in thrust
contact, so you may be getting noise at this point.
However, the same condition should also occur on overrun in 2nd gear, and
appearantly it is not, as you are not reporting the same noise in 2nd gear
(yet). And this condition can also make it pop out of gear on overrun in
2nd (BTDT) as the (2nd) gear drifts forward against a bronze thrust washer
which pushes forward against 3rd gear (and the 3rd gear bronze bushing),
etc, and when 2nd gear moves far enough forward it will disengage the drive
cogs and pop out of gear, but appearantly it is not doing this either (or
at least not yet).
Another condition to consider. If the front thrust washer is still in the
proper location but is badly worn (and probably also the front face of the
3rd gear), then the (3rd) gear can move forward just a little with overrun
in either 3rd or 2nd gear. Again, in 3rd gear there is no relative
rotational motion between the thrust washer and the (3rd) gear, but there
is motion there when engaged in 2nd. In this case any noise emmanating
from the front thrust washer should be heard in 2nd gear, not 3rd, so I
also give this a negative nod.
With the same worn front thrust washer and overrun while engaged in 3rd
gear the (3rd) gear can move forward slightly from it's intended location,
in which case it may be touching the back face of the 3rd/4th outer sliding
hub. There is still no relative rotation between these two parts either,
but they were not originally intended to touch at this point. With these
parts in very close proximity, or touchung, a certain amount of fore and
aft vibration in either of these parts could produce a hammering effect
that might sound similar to a gear whine at the same frequency.
And one more possibility that comes to mind. When 3rd gear is engaged the
outer sliding hub comes very close to the front face of the driving 3rd
gear on the laygear. If the 3rd/4th shifting fork is out of place (for any
of a number of reasons), then the outer sliding hub might actually be
touching the front face of the cluster gear. These parts are on different
shafts rotating in opposite directions, so any incidental contact at this
point could generate a gear whine type of noise as the teeth on the laygear
rub against the side of the outer sliding hub. Another thing that could
allow this condition is a mismatch or disposition of the front thrust
washer on the layshaft.
Now I think I've run out of ideas, but as fate would have it there's always
the possibility of something unusual going wrong that could easily defy
logic. The point of all this is that whatever the problem is, it resides
inside of the gearbox, and it won't be fixing itself, and the only way to
get at it for repairs is to remove the gearbox from the car. Now please
don't shoot the messenger for the bad news.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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