At NRHS, we offer three distinct & different types
of cylinders with our engine kits:
- all aluminum construction with nikasil plating
- full cast iron construction
- hybrid aluminum with iron liner construction
We're often asked about the relative merits of these
various types of cylinders, so I thought it might be
useful to give our view on this.
All of these types of construction have trade-offs;
if there were no trade-offs, all cylinders would be
made the same way. So it comes down to which
trade-offs are important to you.
One of the most important characteristics of a
cylinder, and one that doesn't get talked about
nearly enough in my opinion, is bore distortion.
Cylinders distort due to the loads placed on them,
and also due to non-uniformity in their thermal
expansion. They twist and distort when they get
clamped down, and they twist and distort as they
grow from the heat of the motor.
This distortion is a bad thing.
In particular, ring seal suffers. When you have an
out of round cylinder, you're asking the rings to
change shape as they move up and down the bore.
That's not going to happen to a very great extent at
all. Instead, they leak.
But in extreme
cases, I've seen it get worse than that. For
example, I've seen cylinders that distorted so badly
that they would literally
cause head bolts to break in two and cause cylinder studs
to be pulled right out
of cases when the motor was warmed up and/or run
hard. It doesn't happen because they grow; all
cylinders grow. It happens because they don't grow
evenly. So the loads on the head bolts change as the
motor heats up. Well, each of those head bolts is
only designed to hold a quarter of the total clamp
load on the head, and when you push one much farther
than that due to cylinder distortion, it breaks
Here's a true story that graphically illustrates
this. About 3 years back, we did a top end job for a
guy using all-aluminum cylinders, on an Evolution
Big Twin, using a brand that we don't carry anymore.
This was a new product at the time so this was
the first time we had done a project with this particular
Well, shortly after he took delivery, the bike
pulled a couple of cylinder studs right out of the
cases. We fixed it with heli-coils, which makes it
actually stronger than the original aluminum, and
not long later, we got a call from the owner,
stranded in Kansas, with the same issue, different
studs. He got it
fixed by a shop there, and sometime later, after he
was back in CO, it happened again.
The guy ended up with a whole new engine kit using a
cast iron cylinder. We never sold that particular
all-aluminum cylinder again, believe me, as the
whole ordeal cost us a pile of money. His bike has
been fine since the swap.
We had a similar issue with another set of cylinders
from the same company, except this was a special
application set for a set of S&S cases, and the
problems were actually worse. It'd literally snap
the head bolts in two every time we ran the bike
hard. Changed the cylinders to a different
brand and construction and the issue went away.
It's not all-aluminum construction per se' that
causes the issues, it's the design of the casting
itself. Some all-aluminum cylinders work fine,
others have problems inherently built in to the
The casting design also affects how the
cylinder behaves in the torque plate. Here's another true story
that illustrates this. Not long ago, a local shop brought us a pair
of aftermarket cylinders for boring and honing.
These cylinders were aluminum with cast iron
sleeves, and were hefty and appeared to be high
quality stuff. We did the job, fitted the pistons
precisely to .0020 clearance, and delivered them.
Well, the guy calls us up, mad as hell, tells us we
didn't get the fit right, the pistons won't even go
into the cylinders! He brings it all back over to
us, and he's right, they won't slide in. We put a
cylinder into a torque plate and did the
measurements again. Hmm, dead nuts perfect. We then
put the piston into the cylinder with the torque
plate still attached and it fell through!
That was just a very poorly designed cylinder, it
was moving more in and out of the torque plate than
the clearance spec for the piston. How the hell
you're supposed to make that work, I have no idea.
One of the really big problems with these bikes, and
something that definitely contributes to distortion
issues, is that metal bracket on the left (spark
plug) side that connects the front and rear heads.
That bracket is steel, and the cylinders and heads
are aluminum. Aluminum expands at almost twice the
rate of steel. Think for a minute about how that
bracket tries to twist the top of the motor as the
motor warms up.
So how do you evaluate cylinders for distortion and
One way is to make measurements in and out of the
torque plates and see how much it moves. Now with it
still in the torque plate, heat the cylinder, top
to bottom, and put your dial bore gauge back through
it and see what it's doing, not only for the
of the bore but also for roundness.
The cylinder that moves the least and stays the
roundest is the one that's going to give superior
The hands down winner when you do these types of
tests, no question, is the full cast iron Axtell.
It's a hefty, strong cylinder that expands and
distorts much less than any aluminum cylinder. It's
not even close. This is exactly why well-made cast
iron cylinders typically give the best ring seal
available. The downside, of course, is the
extra weight. They also don't dissipate heat as
well, but properly tuned, this doesn't typically
cause a problem. The vast majority of the heat goes
out the cylinder head.
Some sleeved cylinders move a LOT, in and out of
torque plates and also with heat. The dissimilar
metals expand at different rates and induce
distortions. I've literally pulled apart motors and
you could see areas on the cylinder wall where the
rings never even touched it.
Other sleeved cylinders work very well, however, and
provide an excellent balance between structural
integrity and heat dissipation. Particularly those
with iron sleeves made as thick as possible, such as
the Axtell product. The Axtell Twin cam iron liner
for example is 280% thicker than the iron liner in
the factory cylinder.
A well designed all-aluminum cylinder will grow
uniformly with minimal twisting and distortion, and
will offer excellent heat dissipation and long life. A
bad one, though, like what I described before, will
be troublesome. All-aluminum construction offers the
lightest possible weight and the best heat
dissipation as well.
Speaking of all-aluminum cylinders, without looking
it up I bet we've sold over a thousand of them over
the years. We were the biggest distributor for one
popular brand for many years, we were moving
more of them than anyone. Then we picked up the
Axtell brand when they came out with their version
in 2004, and after selling both for awhile, we
dropped the other brand. We're now the biggest
dealer of the Axtell product. So this is something
we have a lot of experience with, with two major
brands. I'd wager we have more experience with both
brands than anybody.
One thing we've learned in all that experience is
that all plated aluminum cylinders are not created
equal. There are real and measurable differences in
quality and design that ultimately directly affect
the satisfaction of the customer. And not just
differences in bore distortion and ring seal,
either. How well the manufacturer controls his
plating process has a huge amount to do with the
longevity of the plating:
We used to see this failure mode entirely too often.
Some of it was installation related: nikasil plated
cylinders are especially sensitive to the ring
gapping job, the cleanliness of the build, the
lubrication, and the break-in. But often we'd see
this happen even when those things were done right,
and it tended to come in batches, indicating
something had gone amiss in the plating process.
Here's an extreme example of how much grief this can
cause: A customer bought an engine
kit from us in early 2004 and had in installed by a shop
local to him. 3,000 miles later
the bike started using oil like crazy. But by then
we had dropped that brand of cylinder and were only
carrying Axtell's because we were seeing so many fewer problems
with them. So the shop that did the work tried to get
warranty replacements directly from the cylinder
manufacturer. Unfortunately, it took him 6 months to get replacements.
The warranty replacement set they finally got was the wrong
color, but the customer rode the bike anyway while
waiting again for the right ones. During that time,
he discovered that the engine was still
guzzling oil. They finally got a whole new kit of
the proper color out of the manufacturer and
installed it last fall. Well, by November he was on
the phone with me, frustrated as can be, because
that kit too was using excessive amounts of oil. The
"lifetime warranty" on those cylinders
wasn't doing him a bit of good.
The story had a happy ending, though. By then he had
moved closer to me, and I got him to bring me the
bike. I replaced the kit with one based on much
better cylinders. It ran great, not a hint of
oil usage, it twisted the dyno at
104hp, so I delivered the bike. He's a happy
From my point of view, as an independent dealer of
these things, I simply cannot afford that kind of an
issue. It costs me money and reputation both. If
that happens even 5% of the time it'll wipe me out.
So I make it a point to carry only the best products
I can find. I'm well aware that in some cases, the
cylinders we carry are a little more expensive, but
the old adage holds true, you get what you pay for.
Nothing low cost at all about a less expensive
product if it fails on you.
So anyway, hopefully this clears up some of the
misconceptions floating around out there about
cylinders. There are very real and important
differences between the various offerings on the
market, both in terms of construction type and
quality. Our choice of what products to sell is
not random by any stretch of the imagination, it's
the result of a LOT of experience with them. We
carry the best stuff we can find and we stand behind
them of course.
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