Cylinder Construction


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 something.

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 cylinder.

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 design.


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 structural integrity?

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 straightness 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 ring seal.

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 camper finally.


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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