Oil

When it says put 20W 40 in
can I get away with 15W 40?
It's only for the gear box and clutch as its a stroker.

 

Linger says...

Yes, but it should be 2 stroke gearbox oil not normal car/bike oil IMHO.

 

Well I thought that and I have some from the CR250 75w
The manual says 20W 40

So that's what should go in?

 

Never heard of it. Is that light gear oil?

 

IIRC, the first number is the viscosity when cold, the second is when hot -
so 15W 40 will be slightly less viscous before it's warmed up - so as long
as you don't thrash it from cold, I'm guessing you'll be OK.

 

Cheers, I googled for that but could only find technobable.

 

I thought that a 75w gearbox oil was like a 20 or 30 weight motor oil.

 

Auto Transmission Fluid. Also used in power steering and other
hydraulic applications.

No idea what are its properties relative to engine oil, though.

 

Automatic Transmission Fluid

 

Yep, not a problem. A lot of people swear by ATF for stroker boxes, but
I've never tried it myself.

 

Any oil is thicker when cold than when hot. The 15W-40 will just thicken
up a bit less than the 20W-40.
Why do the US standards have to be so confusing ? Why not just state
the viscosity of the oil at different temperatures, and use the same scale
for both engine and gear oil ?

 

Automatic trans oil, like the others said. It comes in two flavours,
thin and thin-like-water, it's probably the thicker one (red ? type A ?)
for this use.
I guess it is a good enough gear and roller bearing lube but it sort
of made me nervous when I used it, but the clutch loved it.

 

Linger says...

If the manual says 20W40 then that's what would be best but 75w should
also be fine. A lighter oil should give a smoother gear change but it
may not be noticeable IRL.

The viscosity of oil destined for a gearbox isn't important as normal
engine oil as its not pumped about[1] as the gears and clutch sort of
splash around in a pool of oil. Also gearbox oil doesn't need to cope
with the same temperature range as engine oil does nor does it need
additives to cope with the byproducts of combustion.

What it does need is good shear characteristics to cope with the cutting
action of the gear teeth. Normal car oils aren't designed with this in
mind and bike oils are a compromise so an oil designed for gearboxes is
best.


[1] Unless your KTM has a gearbox pump for some strange reason.

 

I may try it in one the MZs; just to see what happens.

 

If the stuff can put up with being in an auto box, an MZ box should be
no problem at all in theory. I'd expect there to be less drag too, so a
little more speed, which is only a good thing on an MZ.

 

I'd expect there to be _more_ drag, given that it's the viscosity
that's required in torque converters and the like.

 

Its viscosity is less than gear oil - simply because its thinner and
pours more easily.

Torque converters don't rely on viscosity but 'uncompressibility' IYSWIM
and also require the ability to lubricate and deal with heat but that's a
side issue.

I use ATF in forks.

 

Just had to check. The viscosity of ATF (Dextron III spec) at 100 C
is about half of SAE 90 gear oil and 2/3 of what SEA 30 engine oil
has.
A viscosity index of close to 200 sounds very good to me but I had
nothing to compare with so correct me if I'm wrong. If I'm right it
would mean that it stays pretty thin at gearbox temperatures (and
it really feels thin at room temperature).

 

A high VI is good, but a high film strength is the one to look out for.
Not all oils have high film strength, but dead dino oils tend to have
the lowest, and synthetics the highest.

 

Having had a quick Google myself, the Ixion site has a reasonable
explanation: http://www.ixion.org.uk/faq/oil.html.