Rebuilding the Q-ship; a 1964 Harley Davidson Sportster

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Part 5 - Pistons; Axtell and Dytch Vintage pistons

When you flip the piston over, you can see how different the pin bosses look and how the “heat line” is different. In this case, the dark brown is limited to the center of the crown and becomes progressively lighter as we move down towards the oil ring. However, unlike the stock piston, it is clear high temperature oil isn’t just escaping – it is traveling down the skirt and cooling the whole thing. Good. Take a look at the oil holes . . . notice how they are drilled.

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Part 5 -- Pistons; Dytch and Axtell

Here’s the fun part; these pistons and their pins weight 492.5 grams. That is almost 10 grams lighter than a stock piston – which is ¼” smaller in diameter! Part of what makes them so light is that they are hollow over the pin bosses. Most Dytch pistons are exactly the same.

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Part 5 -- Pistons; Vintage Forged Piston sectional

We’ll jump to the side for a moment and look at early forged stroker pistons. These came in two variety, 2 ring and 3 ring pistons. Three ring pistons were for stock to short strokers and two rings were for long strokes. The major difference as seen in these sections is the compression height. The pin is moved towards the crown to make up for the extra stroke length. This allows you to run minimal or no stroker plates depending on the application and build parameters. These pistons are light and tough – and have been unavailable for some 40 years. However, you can copy them if ordering custom pistons.

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Part 5 - Pistons; S&S Stroker Pistons

Next up is the venerable S&S stroker piston. At one time, S&S made these for 900s and 1000s. Today, only 1000s are available and in only one compression height. This particular piston is a TRW forging and when you flip it over, you can again see a very distinctive heat pattern on the crown.

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Like the stock piston we looked at, it is clear the heat path terminates at the oil ring. When you look at the oil ring land, you’ll see it is slotted, not drilled. Current S&S pistons come drilled to avoid cracking at the end of those slots.

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Part 5 -- Pistons; S&S stroker piston

When put on a common wrist pin, the difference in compression height becomes clear. These pistons are also quite light. Together with their wrist pin, but without rings, they weigh 446.1 grams – or a whopping 55 grams less than a stock piston. That is a substantial amount of weight savings.

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Stock 900 Left; S&S Right

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stock 900 left, Axtell in the middle, S&S on the right

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S&S foreground, Axtell in the middle, and stock 900 in the back

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Part 5 -- Pistons; JE 900 standard bore stoker piston

We also happen to have acquired a set of JE stroker pistons for a 900. These are .050 overbores and have a compression height sized for 4-5/8 strokes. These pistons were run for a short period and so there is not a tell-tale heat mark to read. When we examine the total piston, some traits become clear.

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S&S stroker piston left and JE 900 stroker piston right

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Stock 900 left; Axtell 3.25 middle left; S&S stroker middle right; and JE 900 stroker right

This is a 3 ring piston and rather than using a notched pin button to support the oil ring; the JE piston uses short pins held in place with circlips. This means a high quality, flexible oil ring must be used to avoid issues. The oil ring lands are drilled and the top ring land is substantial.

When placed on a common pin, you can easily see how much “shorter” the piston is compared to stock. In essence, the top ring on the stroker piston is at the same level as the oil ring on a stock piston. These pistons are lovely, but heavy. They weigh in at 481.8 grams with pin, but without rings. This is almost 20 grams lighter than stock – but the weight is almost all in the thick crown. This means the piston will rock quite a bit in the bore and wear rings/ring lands more quickly than a lighter piston. Remember what we said about compromises?

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Part 5 -- Pistons; Venolia Custom

For comparison, here is a custom Venolia piston for a 3-3/8 bore and what appears to be a 5 inch stroke. These are brand new and never run. Notice how “light” they appear on the underside compared to the JE pistons. They also have drilled oil ring lands and use a captured pin without an oil ring support.

They feel great in hand because they are well balanced with even weight distribution from crown to skirt. In total, they weigh 509.2 grams with pin, but without rings. This is 8 grams more than our standard piston – despite this Venolia being 3/8th of an inch larger in diameter.

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Stock 900 left; Venolia Right

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JE 900 stroker piston left; Venolia Right

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Part 5 -- Pistons; Ring choices

While we are talking stroker pistons, let’s take a quick second to talk about piston rings. Because of the sheer abuse these pistons will take, we really don’t want to mess around with too many exotic things. Chrome faced rings, moly coated rings, and unusual profiles are not going to buy us anything but a lighter wallet. Instead, we want one of two types of compression rings: Hastings “square” profiled rings – which for all intents and purposes are “stock” rings or total seal rings. Both are plain rings and work very well.

Oil rings present more choices and challenges. Strokers already pass more oil than a stock bike. If you want a bike that uses no oil, don’t build a stroker. We want flexible oil control rings that can deal with extreme piston rocking, elevated temperatures, and a heavy handed rider.


HD shipped ironheads with one-piece “ladder” style oil control rings clear into the late 1970s. These rings are next to worthless. They will polish a bore nearly smooth and pass more oil than just about any other style of ring. They also are not terribly flexible and can fracture with heavy piston rock. In short – don’t use them for anything other than decoration.


Three-piece oil control rings come in a couple of different varieties. While they “look” the same on casual observation, there are subtle differences in the overall thickness of the rings and the style of expander. One style uses relatively thick rings and a rigid expander. The other uses thinner rings with a more flexible expander that utilizes two expander rings. This style ring conforms to the bore better and is more tolerant of flex/rocking. Whenever possible, we want the most flexible style oil rings in a stroker.

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Well, the Q ship more or less got mocked up last night. Just have to bolt on the handlebars, add the wheels, and figure out my side stand angle. Then, I calculate a few spacer and bushing things . . . and break it all down for paint, powder, plating and parkerizing.

Unusually, almost all the chrome I can clean up -- save the fork sliders. Those need a full replate, which is surprisingly inexpensive. And, other chrome things like the chain guard will be stripped and refinished in black.

Mrs. Chuck was surprised at how fast this one came together. It's often amazing how quickly things assemble when you have the hardware organized and use mostly OEM parts -- not repops.

Now, let's make sure we are honest -- for those of you staring at this bike -- it is NOT 100% factory correct. Big deviations include: repo front fender that isn't quite right, smooth sided rear fender struts, early K model swing arm, a generic, long chain guard, pretty much all the cad plated looking hardware is stainless steel, and there is that generic repo tail lamp/license holder. If you were restoring this bike for judging -- all of this would likely need addressing.

Keep in mind, I am NOT an AMCA judge and my statements should not be taken as even a hint of gospel if you are interested in restoring to that level of detail and skill. Seek those people out if a winners circle award is your goal and follow their advice.

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Cylinder Musings

At this point, we’ve decided on our pistons and we have a choice of cylinders from stock to exotic. So, let’s talk briefly about big bore cylinders.

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The photo above is of three different big bore cylinders we are going to look at in this thread. But, before we get there, let's review some information.

Stock cylinders were cast by the Motor Castings Company (MOCASCO) for Harley. This is the triangular mark that looks vaguely like the old AMA logo, found on the cylinder base. They are made from 30,000 psi grey iron and give a very good service life. Currently, brand new cylinders for 900 sportster cost less than $125-150 per cylinder. Most aftermarket cylinders do not carry the MOCASCO mark. For strokes up to 4.5 inches, it is generally not necessary to lower the oil return holes. Stock cylinders measure 5.330 inches from the base gasket to the head gasket surface, though many aftermarket cylinders like the India-cast cylinder pictured below are slightly over-length.

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Dytch was among the very first companies to offer big bore cylinders, starting around 1960. The first Dytch cylinders were cast by MOCASCO and carry many of the same markings as OE (original equipment) cylinders. The challenge is that the cylinders were not “beefed up” and so they were already pretty thin even at 3-3/16. These cylinders are easy to identify by the presence of a “45” cast into the cylinder base – signifying they are 45,000 psi grey iron. They also are generally marked “front” and “rear.” These cylinders will work if in good condition – but they are more fragile and sleeves are more difficult to install

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The "45" is circled in the above picture to make it stand out.

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More cylinder musings

In the early 60s, Dytch upped their game and introduced 60,000 psi grey iron cylinders. The overall castings are heavier than stock 900 castings, but not so much so that they are immediately noticeable. These cylinders were also cast by MOCASCO and are easy to spot by the “60” cast on them – and usually because they have thicker fins and cylinder bases. These cylinders were available in 3-3/16 or 3-1/4 with the latter being the most popular option. Many of these cylinders also have 3/8 instead of 7/16 head bolts. This allows for slightly more clamping surface at the head gasket to help prevent blown gaskets.

Cylinder lengths were customizable and it is not unusual to find cylinders that are over-length. This allows the cylinder to be mounted directly without a stroker plate in some applications. These cylinders are also .060 longer than stock at 5.390 inches. This means that we may not need to use stroker plates depending on our stroke length. You can also see the major difference in the fire ring’s outer diameter. This means the cylinder heads must be counter-bored to suit.

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Above: notice the smaller head bolts . . .

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Above: measuring the OD of the fire ring on a brand new cylinder

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Above: measuring the OD of the Dytch big bore cylinder -- notice the difference?

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Above: to accomodate the big bore cylinders; both the heads and the engine cases must be suitably bored. Please ignore the "egging" of the head bolt holes -- we'll be fixing that poor piece of machine work later in the series.

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Above: the "over length" of the cylinder -- this extra length means we should not need stroker plates for the motor Chuck is planning.

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Even more cylinder musings

The last set of cylinders we are examining are late Dytch/early Axtell cylinders. Long story made short, Axtell took over Dytch. They continued offering the earlier products, but also introduced even bigger bores. These are also cast in 60,000 psi grey iron by MOCASCO.

Notice however, these cylinders have much thicker cylinder bases than stock, no fire ring, and 3/8 head bolts. These cylinders can be bored/sleeved with relative ease. They are also much longer than stock at 5.490 inches – meaning they can accommodate strokes approaching 5 inches. These are truly life-time buy cylinders and you are likely to outlive them.

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Chuck will be using a set of Dytch 60,000 psi cylinders shown in the middle pictures. These are 3.25 bore and just long enough to allow fitment without plates. This particular set came with heads machined to match. The next step in the process is measuring the spigot diameter and length so Dr. Dick can bore the cases to suit. With all that done – we send the cases off and wait for their return. And, while we are waiting; we will get back to assembling a chassis.

Just for fun, we also pulled some stock pistons down off the shelf to show readers just how much bigger these are . . .and keep in mind even bigger bores are possible :-)

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Above: When working on strokers, a supply of cylinder plates/shims in various thicknesses is helpful. Please do not try to just use thick or doubled up base gaskets. The cylinder base ears are already highly stressed -- gasket distortion just makes it worse. Use metal shims and either thin .010 oem fibre gaskets or NO gaskets and your favorite non-hardening sealer (chuck is partial to yamabond).

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Above: Anyone remember Hap Jones? We had some sets of NOS 900 pistons on the shelf and pulled them down for the next few illustrative photos.

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One last set of hints and tips.

Often when you find big bore cylinders, they have what appear to be really weird bore measurements. As in so odd it often has caused people to think a cylinder was worn out when in fact it was ready to go.

Here's why -- the minimum piston clearance we are aiming for is .008. That is not a typo. Dytch began recommending this as a minimum clearance more than 50 years ago and it holds just as true today. Similarly, most stroker pistons were finished with this type of clearance in mind and many experienced builders aim for around .009/.010 piston to bore clearance as displacement goes up.

When you are tootling around swap meets, a cheap pair of digital calipers is fine for getting a rough idea of the bore. For a better idea; we need to pull out some tools.

The easiest way to get an accurate measurement is with an inside micrometer or bore gauge. In the picture below, Chuck is using a starret bore gauge.

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If you look closely, you will see this cylinder reads out at 3.249 inches -- right where it should be for this cylinder.
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We generally take several measurements down the length of the cylinder to look for taper and egg shaping. This cylinder was consistent top to bottom -- making it a perfect candidate for some light honing and piston fitting. The surface rust is just that -- on the surface and will disappear in the first few moments of honing.

If you don't have a bore gauge/internal mic -- don't fret -- we can always turn to another tried and true method; telescoping gauges and an outside micrometer like the ones pictured below.
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It does take more skill to use telescoping gauges and get a consistent, accurate result. It is really a matter of learning how to use the tools properly and practicing. "old hands" certainly have "feel."

So, what in the HECK does that even mean?

Most folks don't use telescoping gauges correctly. They simply plop them in, twist the tension knob and measure away. Let's refine this a bit.

To use the guage well, it must be handled with care. Start by feeling for when the tension knob fully locks down the arms. Do not be tempted to crank the knob -- just apply enough force to hold the arms in place. Loosen it just enough to compress the arms and insert it in the bore.

Once you have the gauge in the bore, move it around a bit and make the arms perpendicular to the bore. Now, rock the gauge 10-20 degrees to one side and tighten the tension knob. In the below photo, Chuck has initially rocked the gauge to the left.
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In one, smooth, gentle motion -- rock the gauge the opposite way. This "sets" the size of the bore as the gauge comes over center. Carefully remove the gauge. In the photo below, Chuck has rocked the gauge to the right and is about to remove it from the bore.
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Now that we have our telescoping gauge set -- we can use an outside micrometer to make our measurement. Remember, gently hold the mic and slowly "rock" the gauge between the anvils until a slight drag is felt. If you go fast or too hard, you run the risk of compressing the gauge and ruining your measurement.

Done correctly, this method should give you the same measurement as the bore gauge/inside mic. In the photo below, you can see the outside mic reads 3.249 -- same as our bore gauge.
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Don't expect to achieve the same result right away. It takes more time to build skill using telescoping gauges -- with holes below .500 giving the most trouble.

Finally, the reason we avoid using digital or vernier calipers for these measurements is because they oft induce error. This error is compounded by many cheap digital calipers such as the harbor freight special shown in these pictures. According to the literature supplied with the caliper; it has a factory tolerance of +/- .001 inches -- but long experience says this is really more like .003 and very apparent if you do many back to back measurements against gauge blocks. Right before Chuck snapped these pictures, he "calibrated" the calipers against a one inch gauge block. As in the past, this check showed these particular calipers read on average .004 small. So, we are dealing with .004 small to begin with -- coupled with a .001-.003 tolerance swing. That means we can be .003-.007 off in our measurements . . . which is not acceptable for motor work.

To illustrate -- see the below picture. That is the "best" measurement we could get with the cheap caliper -- note that at 3.242 it is a full .007 off the freshly calibrated, higher quality instruments.
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And, just to allay any fears; all measurements were taken at 73 degrees F and 52% humidity. The instruments, cylinders, and gauge blocks were normalized for 24 hours with one another before measuring. If you don't have gauge blocks; high quality 1-2-3 or 2-4-6 blocks can be used. Most of these are precision ground within .0005 of their target dimension -- which is below what these mics read.