From: Ken Streeter <streeter@sanders.com>
Therefore, it has a very high sulfer and phospor content. Sulfur
and Phosphate reactions start at a lower temperature, and Gear Oil
has much more additive than motor oil. This additive is corrosive
to copper bearings and bronze synchro rings.
Along which lines, TR transmissions up to '73 had bronze gear
bushings. In '73 the TR6 box switched to steel, for this reason among
others. Unfortunately I don't know enough about Spit gbx's..
Gear mesh in Gears literally chops up and cuts apart the long polymer
chains of Viscosity Index improvers.
I was interested to see the recent comment about running 20W50 in a
gearbox, because I have also been told the above...
I think there are several things to keep in mind about this
discussion. In the TR2/3 era, both Triumph and Laycock specified SAE30
engine oil for the box, SAE50 in hot climates. At some point in the
TR4 era, the factory spec switched to gear oil. At the time, this
would have been GL4 oil, which has a less aggressive additive package
than today's GL5 oils. (Has anyone ever seen a Laycock publication
which spec'd gear oil in their overdrives?)
Three problems could show up with the switch to gear oil. The first is
damage to the bronze bushings. I think that 60's GL4 oil would be less
likely to cause this than today's high-additive stuff, but in any case
bronze components were phased out of the TR box in part to avoid it.
The second worry is problems with sludging if the additives come out
of suspension a bit. The weak spot here is the restrictor hole in the
O/D operating valve, which controls the rate of oil leaving the
actuating pistons when you disengage the O/D. It's -tiny-, a small
pinhole. If it clogs, the OD won't disengage right. There is no
remotely similar situation in the basic gbx, so it is much less
sensitive to this problem.
The third issue is slipping of the cone clutch in the O/D caused by
the extra lubricity of some gear oil additives. Things like teflon,
moly, graphite, etc. are right out. There are conflicting opinions
about EP additives in general, perhaps because there are different
additives. It -might- be true that any EP oil which claims to be safe
for limited-slip differentials would be OK in this regard, because
there is some similarity of mechanism. On the other hand it's safe
to say that current gear oil manufacturers are not particularly
worried about the needs of Laycock cone clutches..
All that considered, I'm in the engine-oil camp for older TR boxes. To
offer a single datapoint, I've been running a TR3 O/D box for 15 years
with Valvoline SAE30 or SAE50, depending on season. I bought the car
with unknown mileage and a cheesy "restoration" on it, and in that
time have added about 47K miles, roughly half highway and half local
tooling about. Recently, I tore this box apart because it was out of
the car anyway and I thought i'd see what it needed before parts
became unavailable. Both the box and the O/D look to be in pretty
good shape - the synchros are dead, but the gears and gear bushes are
still within spec, the layshaft isn't beaten to a pulp, and the O/D
bushings, gears, and clutch are great (the O/D hydraulics are grungy
and the O-rings are shot, tho.) I don't know if things would have been
better or worse with gear oil, but there's nothing to suggest that the
box was terribly under-lubricated. (TR4 boxes - there is not much
different; the 1st-speed synchro shouldn't matter, but the, er,
"minimal" layshaft bearings in some TR4's might conceivably benefit
from better oil..)
-john
--
John Wroclawski
jtw@lcs.mit.edu
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