Model Forum / General / Models / November 2008

I can however, merely for amusement you understand, relate a short
story from my distant past with some "new" apprentices. The task was
to time a small 2 stroke with basic (we used to call it field)
equipment, at least enough to get it running. One bright light
established TDC with a long screwdriver through the plug hole, even
explaining how he had averaged the "dead" crank movement at TDC to get
an accurate flywheel TDC mark. For some reason he then took off the
exhaust pipe, SCRATCHED the piston with his scriber moved the piston
down 1/16" and set the timing. His "other" mistake was that he moved
the crank in the direction of rotation not against it :-) I suspect he
went through life with a good understanding that scratching pistons
was not the best thing to do. <Vbg>

Don't get me started on apprentices. I've had a few 'helpers' over the years
in various capacities although none of them employed for money as such. Last
one was mid 40s doing a degree in motorsport engineering as part of some
kind of mid life complete change of career. Started him on stripping down
and cleaning stuff. He got a Peugeot flywheel off the crank ok by himself
and then I said he could take a crossflow one off an engine that needed
stripping later. He comes back a bit later saying he can't get any of the
bolts out of this one, they just keep rounding off. I go and have a look and
he's still using the 13mm socket that was correct for the Peugeot engine.

"WTF are you doing?" I say or words to that effect.
"I thought flywheel bolts were 13mm." he replies.
"Well they are on f**ing Peugeot 205 engines but not on every other engine
known to man. Ford Crossflow's are imperial and the flywheel bolts are 1/2".
Can't you even tell when a socket isn't the right fit for the bolt?"

Grrr. I might expect that from a teenager who's never used a socket set
before but how can someone in his mid 40s who's stripped and built umpteen
of his own engines not be able to use tools properly without help?

He spent the next few months breaking various other things including the DRO
on my mill at which point we decided this was not going to work out.

There was one lad in his early 20s many years ago who was brilliant though.
Point him at anything and he'd think it through for himself and then do it
right without having to come running asking silly questions but those types
are few and far between.

To be fair to him, two-strokes are traditionally set up by measuring
piston position, not crank angle. An old Villliers engine I've owned
was set up this way, as were a couple of relatively modern Japanese
two-strokes.

One theory I saw (IIRC it was in a book about two-stroke tuning) was
that flywheels might not be perfectly aligned, especially as the
cranks on a two-stroke are often pressed together. So long as the
crank throw is accurate though, piston position is a sufficiently sure
way of ensuring that the timing is correct, even when measuring near
TDC.