> Today I was reading an article about diffs in Spitfire & GT6
> Magazine. It said that one shouldn't use a GL5 oil in a Spitfire
> diff because it will eat away the copper bits. Presumably it meant
> brass and any other alloy containing copper. I've heard that GL5
> warning for gearbox oil but never before for a diff. True or false?
> Anyone care to comment? Randall Y?
What I know of the problem is very tangled, Jim. Basically there is an
additive used in both conventional GL-4 and conventional GL-5 gear oil, that
under heat and pressure, breaks down to form sulfuric acid. This is what
gives old gear oil that distinctive smell (vaguely like rotten eggs). The
acid attacks the surface of copper and brass, leaving a dark film that in
effect protects against further attack. (This process happens even in air,
which is why old pennies turn dark.) But in theory, on rubbing surfaces
like synchro rings and thrust washers, the film rubs away and the fresh
surface corrodes again. I say "in theory" because the only documented cases
I can find of this being a problem is in antique Ford differentials ... I've
not heard from _anyone_ who has seen the problem in a Triumph or similar
car.
At least some conventional GL-5 reportedly contains more of this additive
than at least most conventional GL-4 gear oils do; maybe even twice as much.
But it's my belief that most GL-5 oils will also pass the GL-4 standards ...
the API GL-5 standard includes a test for "yellow metal" corrosion that
seems tougher than the test for GL-4. In other words, it's not the
"GL-5-ness" of the oil that makes it a problem, but rather how much of this
additive the oil contains (which the oil makers don't like to tell you).
Sta-Lube used to sell a "GL-5/GL-4" oil, but AFAIK they don't anymore, due
to consumer confusion. I think AMSOIL still does.
The 'big' Triumph differentials do contain brass thrust washers, I assume
the 'little' ones do, too.
My solution to the problem is to run Valvoline full synthetic GL-5 gear oil,
which has much less of that particular additive (and hence lacks that "gear
oil" smell). That way, I get the superior gear tooth protection without the
corrosion concerns. I credit the synthetic for making my TR3A differential
(already well worn when I got the car in 1984) last through all the hard
miles I've put on it since then.
One other point, the API has also created a "MT-1" certification, that
covers only "thermal stability, high temperature cycle stability, and copper
compatibility." I don't know too much about this spec yet, but it seems
likely to me that any gear oil that will pass MT-1 and also either GL-4 or
GL-5 will be safe to use in a Triumph differential. All of the Valvoline
GL-5 gear oils also meet MT-1.
Randall
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