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For many people doing conversions (883 to 1200 or
88ci to 95ci), one of the first decisions they have
to make is whether to purchase new cylinders or have
their old cylinders bored & honed. The cost is
comparable either way, and the downtime can often be
less with replacement of the cylinders with new OEM
units. But the simple truth is that the machining of
your cylinders offers a HUGE opportunity to improve
on job HD did when they built your bike. In most
cases, you can get significantly more power as well
as engine life by getting your existing cylinders
bored and honed by a shop that knows what they're
doing.
Here's an example. This is a new take-off XL1200
cylinder (actually Buell X1, but same thing),
properly clamped in a torque plate. We have not done
any machine work on this cylinder, it's
exactly as HD delivered it. We're taking
measurements in the thrust direction and 90 degrees
to the thrust direction to show the roundness of the
cylinder. The dial bore gauge shows a relative
measurement, don't try to figure out bore size from
it's reading. Each numeral on the dial is .001",
or one thousandth of an inch, which we normally just
call a "thou". It has ten little lines between each
numeral, each representing .0001", otherwise known
as a "tenth", because it's a tenth of a thou and
thousandths is always our default unit of measure.
 
One inch down the bore. Notice how the cylinder is
only a few tenths out of round. This isn't too bad.
 
Two inches down the bore. Now we're more like 8
tenths out of round. This is bad.
 
Three inches down the bore. The cylinder is three
thou out of round. Really, really, bad.
 
Four inches down the bore. The cylinder is literally
8 thou out of round. This is terrible.
Consider for a moment that the piston is designed to
operate with a clearance of around 1 thou (cast
piston) or 2 thou (forged piston). Where do you even
make the measurement to determine the fit? It's kind
of ridiculous.
I wish I could say this is an unusual result, but
unfortunately, it's not. We've seen worse. We've
pulled old motors apart and found areas where the
rings never even touched the cylinder walls.
Likewise, we've seen better. It's something of a
crap shoot when you take an off the shelf
mass-produced cylinder and put it in your bike. The
same applies to all HD cylinders, stock, conversion,
XL, Evo Big Twin, and Twin Cam.
By comparison, we machine your cylinders straight
and round to two-tenths.
It goes without saying that ring seal is critical to
performance and engine life. Some motors run good
for a very long time, others use oil and run poorly
from the get-go, and this is one of the major
factors behind the variations.
In all fairness to HD, some things need to be noted
here:
- The spigot is really hard to get straight. It
likes to distort because it's so poorly supported.
But no way should it be anywhere near 8 thou off. A
good production cylinder, it'll be under 1 thou.
- The other cylinder of this set was only about half
a thou (5 tenths) total out of round until the
spigot and then it was about a thou out. It was
definitely a better than average factory cylinder.
- The type of torque plates used and the way in
which it's tightened also have an effect. It's not a
totally repeatable process. We can make a cylinder
perfectly straight and round, take it out of the
torque plate and it'll be distorted as hell, put it
back in the torque plate and it'll be close to what
it was when we machined it, but not exactly the
same. A few tenths off here, a few tenths off there.
But this cylinder's distortion is way beyond
the repeatability of the measurement.
So anyway, the message here is that your bike is a
mass produced product, and like all manufacturers,
keeping costs down is king at the motor company. But
when you go to rebuild your motor, or raise it's
displacement by converting it to a 1200 or 95ci, you
have some huge opportunities to improve on it.
Precision machining of your cylinders is one of the
biggest. You'll be rewarded with improved power and
longevity.
Don't even think about measuring or machining a
factory cylinder without torque plates. It ain't
even worth your time to stick the bore gauge in it,
it'll change so much. If a shop tells you they can
measure or machine them without torque plates, run,
don't walk, away. A well designed, hefty aftermarket
cylinder, for example Axtell) will change only
slightly in and out of a torque plate (although
Axtell machines ALL of their cylinders in torque
plates, unlike some companies). A factory cylinder,
and some aftermarket cylinders will move a mile.
They're flexi-flyers. I'll do an article on this
when I get a chance.
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