pulled apart by the previous owner, cleaned, inspected and re-assembled, no
other work had been done on the engine. I only drove the car for a short while,
then pulled the TVIS engine for a smallport swap.
When stripping RWD parts off the TVIS engine I found that the crank pulley bolt
was not much more than finger tight. The pulley was loose on the crankshaft
(basically fell off) and the key had eaten part of the keyway. After getting a new
keyway cut in the old pulley, I found another better one, and put this one aside.
More recently I wanted to use the TVIS engine to mockup an exhaust side alternator
setup. When I went to fit a different crank pulley the key was being awkward, so
I decided to remove the timing belt drive sprocket. This proved to be a bit of a
problem as that sprocket was ON THERE !!! No amount of levering with multiple
screwdrivers made any impression, and no 2 or 3 leg puller that I could find would
fit in the tight space, until I got this China made special
Out of the packet it wouldn't fit, but an hour or two whittling down the ends and
eventually I had it working. Previously I had cut the end off the key and punched
it down into the keyway, just to make sure it wasn't stopping the sprocket from
moving. You can see it here just after the sprocket started to shift.
And this is what was locking the sprocket onto the shaft... FRETTING CORROSION !!
That orange stuff is not your ordinary surface rust, but a special kind of corrosion
not caused by moisture. When two close-fitting iron or steel surfaces are subject to
cyclic loading, or repeated small back and forward movements, fretting corrosion
can occur.
In this case the fact that the crank pulley bolt was not tightened properly, allowed
the crank pulley to move fractionally backwards and forwards on the shaft with each
firing stroke. That movement caused fretting between the key and the keyway, and
allowed the pulley to flog on the shaft, wearing the bore of the pulley. The sprocket
has much lower inertia, so being loose allowed the sprocket to "roll" on the shaft as
the tension of the timing belt pulled it from side to side with each turn of the crank.
After a few million cycles of load on, load off, fretting corrosion formed between the
two surfaces, locking the sprocket to the shaft. In dry situations you will usually find
orange rust powder, and in this case, multiple applications of penetrating oil turned
the powder to orange grease. This is the same orange grease that you will find when
you finally get that hard to pull SOB axle out !!
The only way to avoid fretting corrosion is to make sure there is no possibility of
movement between the surfaces, With a crank pulley, the bolt must be torqued to
specification. Unfortunately, when a press fit, shrink fit or high clamping load bolted
joint is not possible, no amount of anti-seize or moly grease will prevent fretting
corrosion.
Cheers... jondee86 (Yeah... new camera... WOOT
)
