We are unable to manufacture any more at this time.

74 CU. IN. OHV
SINCE 1992

D. R. Miller
PO Box 4884
Springfield, MO 65808

(417) 890-8636 or 887-8919


All of our products are made in the U.S.A.

Made in America

Sturgis Ralley

PAIR $550.00

Knucklehead Cylinders

Replacement cylinders for the 74 cu. in. Harley-Davidson "knucklehead" are made from various materials. Harley-Davidson made theirs, from 1936 to 1947, out of gray cast iron. That's what we use.

There are a number of reasons that gray cast iron is the only suitable material for these engines. Those reasons are better cooling, vibraton damping, and elasticity.


Let's start with better cooling. You may think that aluminum would give you better cooling, but it doesn't. The minute you install a steel or cast iron sleeve in an aluminum cyclinder, you are going to have two problems.

The first problem is that heat simply doesn't transfer well between dissimilar metals. The second problem is that, even if you could overcome the heat transfer problem, different metals expand and contract at different rates. The number of problems that can arise when this happens is considerable.

This same problem arises with the cast iron alloy cylinders imported from Taiwan.

Cast iron alloy is not a dimensionally stable material with heat. Cast iron alloy expands and contracts at different rates in different parts of the cylinder. An example of the unreliability of cast iron alloy was related to us by one of our customers in Florida. He was riding his knucklehead when he felt something hit his leg. He looked down and saw half of one of his cylinders blown out below the bottom fins. He could see his piston going up and down.

Since then this fellow has bought all his knucklehead cylinders from us--one pair of cylinders for every knucklehead he restores, averaging a pair every six months.

There are several reasons the Taiwanese use cast iron alloy, as opposed to gray cast iron. Less attention to detail is required, cheaper tooling can be used, and overall production costs are much lower. Cast iron alloy quite often consists of old bicycles, 1953 Chevrolets, and landing craft sunk at Iwo Jima.

Gray cast iron requires more attention to detail. This detail consists of precise control over the temperature of the molten metal to be poured, careful placement of cores, and exact chemical makeup of the material.

High quality tooling. Tooling that would work for a cast iron alloy part will not necessarily work for gray cast iron. The long thin fins are the problem. Fins are made to cool the cylinder. The problem is that they also cool when the cylinder is being poured. If they cool too much the mold will not fill completely. This is called a cold run. A cold run results in portions of the fins missing. If they are poured too hot, the molten metal actually burns away the sand in the mold and leaves big chunks of metal between the fins. This is called burn-in. Gating and risers positioned properly usually help eliminate this problem. Sometimes this is not enough and shell molds, which are substantially more expensive, have to be used.

Higher production costs. Our raw unmachined cylinder castings cost more than an American distributor pays for a set of completely machined set of Taiwanese cylinders.

Vibration Damping

Some of you older bikers may remember when everybody and his dog were making springer front ends back in the late 1960s. The "big thing" was making springers out of 4140 aircraft steel. Harley made theirs out of 1020 mild steel. The difference was that the 4140, though a stronger steel, could not handle the vibration the 1020 could. The Harley springers lasted, the 4140 cracked.

Gray cast iron handles vibration better than any other metal. Compare the figures in this chart:

Relative Damping Capacity

8 X 10-4 *
White Iron
Malleable Iron
Ductile Iron
Gray Iron, Fine Flake
Gray Iron, Coarse Flake
Eutectoid Steel
Armco Iron
* Natural Log of the Ratio of Successive Amplitude
From Iron Castings Handbook, p. 256 (1981).

What we use is coarse flake gray cast iron. "Coarse flake" refers to the graphite in the cast iron.

The graphite in our cylinders is much more evenly distributed than the graphite in the originals manufactured in 1947. Metallurgy has improved a lot since 1947. Our cylinders were featured in the May 1994 Hot Bike magazine, "Finally, Somebody Does It Right." Our cylinders are .200" thicker than the originals between the mounting flange and the bottom fins. The originals had a tendency to crack in this area.

That cracking had two basic causes. The first cause is that the originals were too thin in this area. The second cause is that, because the graphite flakes were not evenly distributed throughout the metal, areas where there was a heavy concentratio of graphite flakes ran cooler than the areas in which the concentration of graphite flakes was much thinner. I.e., parts of the cylinder expanded and contracted at different rates.

All machine tools have gray cast iron bases for this reason. For example, if you have a Bridgeport milling machine, the base is made of gray cast iron in order to dampen the vibration of the machine.


Gray cast iron is elastic. That is, it will return to its original shape when it is compressed, heated, or bent. The opposite of elastic is called "plastic." "Plastic" means the tendency of a metal to change gradually under load at temprature, also called "creep."

For example, let's say you have a set of ductile iron cylinders on your Harley-Davision. Ductile iron is actually a stronger material than gray cast iron. The problem is, it is not elastic. Here is what can happen.

You take your bike out for a ride. The ductile iron cylinders heat up, perhaps the rear cylinder deforms .001" and then cools down. Since your cylinder metal is not elastic, the deformation stays with it. Ductile iron will deform, even if in microscopic amounts.

On your next ride your cylinder now deforms another .001". You now have a .002" deformation, an increase significant enough that, if you have a flat surface that you drag your fingers across with a .002" ridge, you can feel it.

At some point, when your cylinder has undergone a sufficient number of microscopic deformations, your piston will seize.

Some of the "big inch" cylinders (like 120 cu. in.) seem to do okay with ductile iron, even after 40,000 miles or so. The reason for this is that, the larger the bore, the less difference the same size deformation is going to make. The primary difference between a set of 74 cu. in. ductile iron cylinders and a 120 cu. in. set is that it will merely take longer for the 120 cu. in. set to seize a piston. Talk to any Harley mechanic who has worked on the "big inch" cylinders and he will verify that they do deform.

Ductile iron also runs hotter.

The thermal conductivity of gray cast iron is 46 W per meter. The thermal conductivity of ductile iron is 36 W per meter. Assuming the same fin size on two cylinders, one made of ductile iron and the other of gray cast iron, to be equal, the ductile iron cylinder is going to run much hotter.

Ductile iron was unknown until the 1950s. It obviously was not used in Harley-Davidsons manufactured prior to 1948.

The shape of graphite particles in ductile iron resemble ball bearings; that of gray cast iron resemble corn flakes. The conductivity is higher in gray cast iron because of the semi-continuous nature of the graphite flakes.

Why Do Some Use Ductile Iron?

The "ductile iron" cylinder business apparently started with drag racers. The fact is, ductile iron is a stronger metal than gray cast iron. Ductile iron will withestand higher cylinder head pressure than gray cast iron. If your traveling is limited to a quarter mile, you're going to use a much cooler-burning fuel (such as nitro-methane), and you're going to take your engine apart after every race, it is a better material.

For everyday use, it's an eventual disaster.

Test Your Cylinders

You can test the difference between ductile iron, Chinese cast iron alloy, and gray cast iron with a ball peen hammer. If you hit (preferably not on the fins) a gray cast iron cylinder, the sound you hear will be a "thunk." Tap a ductile iron or Chinese cast iron alloy cylinder in the same fashion and you will get a "ping" (from the vibration).

Our Track Record

We put the Fish carburetor back in production in 1981. Fuel injection caused us to cease production of this product in 1996.

In 1992 we produced our first run of 74 cu. in. gray cast iron cylinders. In ten years we have not had a single complaint about our cylinders. Many customers have reordered for other bikes.

In 1998 we began manufacturing steam engines.

It is our eventual goal to put every single part of the 1947 Harley-Davidson "knucklehead" engine back in production, made out of the correct materials. At present we are working on the tooling for the cylinder heads.

Current Inventory

At present we have sold out of knucklehead cylinders. These cylinders are approximately .010" undersize and need to be bored to the exact dimension you require. We are not boring them or honing them for the following reasons:
    1. Once bored, cylinders have to be honed.
    2. The hone you use is determined by the type of piston ring you use. You do not use the same hone for cast iron and chrome rings.

Coming Soon: Knucklehead Cylinder Heads

When You Sleeve Original Cylinders

View our photo album: Putting the Knucklehead into production

The Fish carburetor manufactured by the Brown Carburetor Company from 1981-1996

1 horsepower, one-cylinder horizontal steam engine manufactured by Mike Brown from 1998 to the present

3 horsepower, two-cylinder horizontal steam engine manufactured by Mike Brown from 1998 to the present

Home Page

Questions? Comments?


This page was updated on 28 May 2011