Matt Kulka wrote:
> He went on to say that English crankshafts are "heat-treated" and from
> his tone of voice, this didn't sound like an easy issue to deal with.
> He went on to say that he didn't know anyone in the area who works on
> them. Although he said he'd check around and call me back, I can't
> escape this feeling of dread. (He also used the term "Nitriting" -
> evidently another name for the same thing.)
First off, you want a crank where all the journals a) are on the exact same
center line and b) provide just the right amount of clearence to the
bearings. When you buy new bearings, they are the standard size and then a
little bigger in increments of 0.010 (10 thousands of an inch. The machinist
must check for staightness, out of around, and normal wear to see how far
out of spec the crank is. Then he must determine which bearing size to grind
the crank journals. That is, if the largest error, either out of round,
worn, or off center, is 15 thousands, then he buys the 20 thousandths
oversize bearings and grinds all the crank journals down to match that size
(plus the normal clearance).
Alternatively, if the crank is rare and already been cut. Then first a weld
bead is applied to the worst of the crank journals and then machining is
done to grind back down to stock. This usually requires some form of heat
treating and tempering to get uniform hardness back into the crank.
I wasn't aware that 'B' crankshafts were nitrided -- at least from the
factory. Those folks more knowledgable than I can certainly step in and set
us all straight.
The goal was to add some surface hardness to the crank. The method was to
heat the crank in an oven to something less then 1000 degrees and then
introduce nitrogen in the form of ammonia gas. The heat releases the
nitrogen out of the ammonia (NH3) and the nitrogen becomes embedded in the
surface metal of the crank to a thickness of around 0.025 (25 thousandths).
Thus the surface of the crank becomes a 'nitrogen alloy' which is
significantly harder than the normal base metal.
The heat and nitriding process will swell the crank about 0.0005 (5 ten
thousandths) and this should be accounted for in the final maching process.
It will also make a certain amount of out-of-round in the area of 0.0002
which is sometimes just ignored.
So who cares? Well, if the crank has not been machined before, and it was
nitrided, then it can be cut down 10 thousandths to achieve roundness and
the nitride surface has not been eaten away. On the other hand, it is damn
difficult to tell the history of an old crank.
I ain't a machinst/chemist but what I think is the hot setup is, if the
crank is going to be machined, is to measure the new bearings, machine the
crank journals such that it is too big by 0.005. Then send the crank out for
nitriding. When it comes back, have the final machining done to trim the
crank down to size. This final maching will be very slight and will not cut
down into the surface hardening very much.
The 'in the know' guys will look at a crank and proclaim it to be nitrided
because the non-machined surfaces, even after cleaning, with be black as
coal -- characteristic of the ammonia gas bath.
Chevy called the process "Tufftrided" when they nitrided the cranks on 327
Corvettes back in the 60's. They used a different (and now EPA banned)
process to introduce the nitrogen and those cranks looked green. Few
manufacturers nitride cranks today which means you may not need to worry
about it (for a gentle, occasional driver) and your machinist is just trying
to impress you with his vast knowledge.
My 2 cents worth has been depleted.
--
Bob Allen, Kansas City, '69CGT, '75TR6, '61Elva(?)
"The qualities of team players are most highly regarded by incompetent
coaches."
|