Setting Your Squish Clearance



Something we emphasize a lot is the value of getting your squish clearance right. A proper squish clearance makes the motor both more efficient and less prone to detonation. When you're assembling your high performance motor project, this little detail is well worth the small amount of time and effort it takes to get it right. You'll have more power, better fuel mileage, and your motor is less likely to "ping" on you.


In the ideal world, your squish clearance should be set to about .030". Tighter than that risks piston to head contact; looser starts reducing the chamber turbulence effects that you're going for. Unfortunately, I don't think I've ever pulled apart a factory assembled engine that had a squish clearance of less than .050"! So you can see what a huge opportunity this is to improve on your motor.


So let's talk a little about what squish clearance is and how to adjust it. Put simply, a squish band is the area or areas where the piston comes in close proximity to the chamber as it passes through top dead center (TDC). Fuel trapped in this space gets squeezed out at high velocity and generates turbulence in the chamber. Turbulence in the chamber promotes better atomization and less separation of the fuel and air, encouraging it to burn.



Shown above is an angled squish band, i.e. the squish band is at an angle from the horizontal. More common in stock HD motors is a flat or zero degree squish band, which is the same thing except that the fuel is squeezed out horizontally. Here are some factory heads with flat squish bands:


Evolution Big Twin heads have a zero degree squish band on one side

of the chamber only.



Buell XB and 2004-up XL1200 heads have a pear shaped chamber with

flat squish bands on each side. This is commonly referred to as a "bathtub"



Twin Cam heads also have a zero degree squish angle in a

bathtub chamber, although it's oval shaped.


So squish bands are common in HD/Buell motors. In fact, the only modern HD models that don't have squish bands designed in are the 86-03 Sportster models, pre-Thunderstorm Buells, and all 86-present XL883's.


In any event, the idea is the same whether the squish band is angled or flat: the piston comes close to the squish squish shelf in the head as it passes through top dead center, squeezing fuel out and generating turbulence, and that improves the efficiency of the burn while helping resist detonation. And regardless of which HD or Buell model you have, unless you've optimized the squish clearance, it's probably got a lot more clearance than it should have, just due to the way the motors are mass produced. HD has to design things such that in a worst case tolerance stack-up situation, the clearance is still adequate to prevent piston to head contact, and that means the typical motor will have much more clearance than you'd like.


So now that you know what the squish band is all about, how do you go about measuring and setting it? It's really not hard at all.  Since none of us are small enough to climb down into the chamber and measure it directly, we have to do it indirectly. The easiest and most straightforward way to do this is with some small strips of .065" diameter solder placed strategically around the piston:


.065" diameter solder placed around the squish area of a Twin Cam piston.

A small dab of grease works well for holding the solder in place. Some

people use tape instead, just be sure to tape it in an area outside the

squish area so the tape doesn't change the measurement.


Simply place some small pieces at strategic places around the piston dome as shown above, Turn the motor backwards slightly to pull the piston down the bore a little, put your gasket in place making sure the rivets (if so equipped) aren't getting sandwiched in between the head and cylinder (grind off any that do), torque the head into place, rotate the piston through TDC, and remove the head.



With any luck, your individual pieces of solder will look like this. Now simply measure with a caliper:




Squish clearance is then modified to the desired value by adjusting the height of the cylinder, the thickness of the head and base gaskets, using a piston with a larger compression height, or some combination thereof.


Both head and base gaskets are available in different thicknesses, select as needed

to achieve a .030 to .035 squish clearance. See our gaskets page for the various sizes.




As often as not, we don't use a base gasket! In many cases the proper squish can only

be achieved if we leave it out. Three-Bond 1104 works great in it's place. Squeeze it out around the base of the spigot and then spread it around the base surface with your fingers. The excess can be easily wiped off with a rag soaked with lacquer thinner. In the future, when the cylinder is removed, the old 1104 is also easily cleaned off with lacquer thinner.


There are a couple of other things to think about when you tighten the squish. Number one, since tightening the squish lowers the height of the engine, you need to be aware that your lifters are being preloaded more and you’re reducing the valve-to-piston clearance, as well as changing the way the intake manifold fits. Generally, at the .020” to .030” you’ll typically be dropping the top end when you optimize squish, these things aren’t a concern. Measure them if you’re not sure. If you’re lowering things beyond .050” or so (not likely unless you’re specifically milling the heads for more compression), you need to pay attention to these details.


Second, tightening the squish does cause an increase in compression. Often it does so without any introducing any pinging because the more effective squish band has detonation reducing effects. But be aware of it. At NRHS, we measure all chambers and adjust the size to give the desired compression ratio at .030" of squish clearance. So if you're buying a matched set from us, you're good to go.


One more thing, if you're inside your motor doing a hop-up project, consider angling your squish bands. Zero degree squish bands are easy to mass produce and they work reasonably well, but an angled squish band offers a couple of advantages. First, the fuel being squeezed out of the squish band is aimed more directly at the flame front. Second, an angled squish band reduces the shelf that the air must go around as it enters or exits the chamber, thus improving breathing.


 There's only one factory motor that came with an angled squish band:


Buell Thunderstorm heads came from the factory with a cast-in

15 degree angled squish shelf. A matching 15 degree domed piston

was used in these motors as well.


 Buell Thunderstorm heads and matching pistons are set up this way, with a squish angle of 15 degrees. However, since the Thunderstorm squish band is cast-in, it's not very accurate. The squish clearance will typically vary around it's perimeter and it can even have fuel traps. We frequently machine the Thunderstorm squish shelf accurately to match the piston:


A Thunderstorm head that's had it's squish band machined out to 30 degrees,

shown with a 30 degree domed piston


Unfortunately, the Thunderstorm head has quite a bit of excess deck overhang around the perimeter of the chamber, and as a result, we almost always have to mill at least .040" off the head to remachine the squish band at stock diameter. Often it takes .050", it just depends on the specific heads.


"Lightning" heads, as used on 96-03 XL1200S models, Buell Blasts, and pre-Thunderstorm Buells, also have a squish band cast into them, 10 degrees in this case. But they were paired with flat top pistons so they don't have an effective squish band as delivered. The Buell Blast actually comes with a 15 degree Thunderstorm piston under it's 10 degree squish shelf, so it has a mismatch from the factory, and it's in a bad direction that creates a fuel trap.


We often machine out the flat squish bands on other factory heads as well:


A 2004 XL1200 head, which started life with a pear-shaped bathtub chamber

using zero degree squish bands, after machining the chamber for 30 degree

squish bands.


Common squish band angles you'll see are 10, 15, 20, and 30 degrees.


There's a downside to angling squish bands, though. An angled squish band gives up squish band surface area for a given dome height. The steeper the angle, the more squish area you give up, unless you make the dome taller. For this reason, steeper angles such as 30 degrees (which approximates the valve angles) are usually only seen in conjunction with higher domes and thus higher compression ratios. Shallower angles, down to the zero degrees found on many factory Harleys, are more common on street motors. For most street applications, we recommend an angle of 15 degrees, as this provides a good balance of squish area and angle without an excessively tall dome. For most race applications, we use a 30 degree angle.


In summary, adjusting squish is really not a lot of trouble at all, particularly if you’re in there anyway. It takes very little extra effort to get this right. And it pays back triple. You can make the engine run better, reduce pinging, and improve your gas mileage all at the same time.



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