WHY DOES SQUISH MATTER?

A very important parameter of cylinder head design is the squish clearance. Let's use the yz250 for example. Because of manufacturing tolerances, the stock yz head has a pretty large squish clearance to begin with at .070-.080”. The point of the squish band is to shield and cool end gasses from the flame front to prevent detonation and to add turbulence to aid mixing and make for more efficient combustion. By having the squish clearance too big, like on the stock head, all the gas in that space is never forced to the combustion chamber and isn’t burned until too late of a crank angle to do effective work. This costs power, fuel efficiency, and makes jetting finicky. Also the flame front can creep into this space and actually cause detonation. The yz250x head is even worse. To add to the volume of it (lower compression) they merely made the squish even bigger, around .105-.110”, making these problems even worse. There are people all over forums with stock yz250x’s that need far higher octane than a normal stock yz to not detonate.

Our head is designed to have a nominal squish clearance of .050” which gives a great mix of reliability, safety, and performance. We then tuned the compression ratios, squish areas, and msv’s of each head profile to match the type of riding they’re intended for. The msv is the speed which the fuel air mix moves across the squish area into the combustion chamber and is another very important tuning parameter. To high and the engine will be too peaky and risk detonation and too low and the engine will need excessive advance to run right and will be slow in general. The msv, compression ratio, squish area, and squish clearance all affect each other so a lot goes into getting the right combination.

Our sx head has the highest compression, slightly higher than stock and has the highest msv that we’d advise running. This gives a very hard hitting powerful motor with broad snappy power. The mx has slightly lower compression than stock but an msv that’s higher than stock but lower than the sx to let the bike rev out as freely as possible and give maximum overrev. The xc has a unique design that focuses the flame front for a smooth torquey powerband. It’s compression is similar to the 250x but the msv is between the stock X and the stock yz.

We can modify the oem head to any of these specifications but the billet head offers additional cooling because it has more coolant volume and has been designed for maximum heat transfer which lets the bike run cooler no matter how hard you ride it. The sx requires fresh 93 octane or better, and the mx and xc have done fine on 91 but we always recommend fresh 93 octane non ethanol fuel.

The principles described here are applicable to two strokes of all displacements, and real benefits can be seen on any two stroke whether it be for motocross, road race, enduro, watercraft, or snowmobiles.

ARE THESE JUST ANOTHER HIGH COMPRRESSION HEAD?

These heads are much more than a high compression head. In fact, many of our domes are lower compression than the stock heads they replace. The goal with these heads is to increase combustion efficiency to get an all around healthier running engine. There are several aspects that we optimize, these include the squish clearance, squish velocity, compression ratio, the overall shape of the combustion chamber, etc. The end result is a quick, complete deflagration that uses as much of the fuel available without giving it a chance to detonate.

WHAT BENEFITS SHOULD I EXPECT?

While more power is certainly a benefit of these heads, it goes beyond that. Carbed models benefit form much more consistent jetting, 6000ft elevation gains and 60℉ temperature swings without changing jets are a huge plus with these setups. Usable power is also key. We noticed particularly with TPI models the throttle response does not necessarily feel connected to the response you get from the engine; there's a bit of hesitation off idle at low throttle openings, then the torque comes in as the engine builds some momentum. Our XC dome adds to the smoothness of the stock bike by adding torque in this area and giving a more linear response to throttle inputs. More control, more traction, and less stalling are what the user would gain. More detonation resistance is another benefit, along with most users reporting increases in fuel economy, particularly on carbed models. Oh, you want arm ripping power? We can get you that too, and everything in between!

HOW DO THE DIFFERENT DOMES CHANGE THE POWER CHARACTERISTICS?

We change several aspects of the combustion chamber design to give the unique power characteristics of each dome.

• The SX dome will have the most aggressive power. Hard hitting low to mid with the most peak power and torque. It generally works well for supercross, arenacross, aggressive woods riding, general motocross, and high altitude.
• The MX, or MXC in the case of the 300, is designed for linear power with an emphasis on top end and overrev. It is smoother than the SX but more aggressive than the XC, and revs the fastest. It generally works well for desert riding, general motocross, flowy woods, and play riding.
• The XC dome is designed for smooth, linear power with an emphasis on low end luggability and traction. It is best suited towards enduro, single track, technical hill climbs, slick conditions, or for anyone wanting a smooth, torquey response.

HOW DO I TAKE A SQUISH SAMPLE?

take a piece of acid core or rosin solder (use 2-3mm diameter solder when testing an oem 250/300cc head, and 1.5-2mm diameter solder when testing a oem 125/150 head or any of our billet heads) and bend it into a U shape as shown below. Insert the solder transversely in the cylinder and hold the solder to the crown of the piston with a dab of grease. Be sure that the solder is directly above the piston wrist pin and does not scrape the cylinder walls. Install the new cylinder head (o-rings are not necessary for this part), add the copper washers, and tighten the nuts to the OEM recommended torque specs, tightening gradually in a star pattern. Using the flywheel of the motorcycle, gently rock the piston past top dead center several times by hand. Remove the cylinder head and measure the squished portion of the solder with calipers or a micrometer. A tape measure is not suitable for this application.

On our 250/300cc billet heads, the ideal clearance is 0.050in (1.27mm). Values under 0.045in (1.14mm) should not be run unless its a race gas dome. Values under .045in or over .060in (1.52mm) can be corrected with different thickness base gaskets for some models. For clearance under 0.45in use a thicker gasket, and a thinner gasket if the clearance is over 0.60in. Please contact us if your squish in not in this range and we will advise.

On our 125/150cc billet heads, the ideal clearance is 0.035in (.9 mm). Values under 0.030in (.76mm) should not be run unless its a race gas dome. Values under .030in or over .045in (1.14mm) can be corrected with different thickness base gaskets for some models. For clearance under 0.30in use a thicker gasket, and a thinner gasket if the clearance is over 0.45in. Please contact us if your squish in not in this range and we will advise.

For our modded oem heads, squish clearance should be within a few thousandths of an inch from the ideal values listed above.

WARNING: This procedure should be repeated for every top end and bottom end rebuild, as part tolerances can vary by manufacturer.

WHY ISN'T THE DOME FLUSH WITH THE OUTER HEAD?

The dome is designed to protrude between 0.002” and 0.010” from the surface of the outer head. This is to ensure that there is a tight seal on the head gaskets and is totally normal. The clamping force of the nuts of the head, when torqued properly, will fully seal both the dome and outer head. If the dome protrudes from the head by less than 0.002” contact us.

ARE THE DOMES PRESS FIT TO THE OUTER HEAD? HOW DO I SWAP THEM?

The domes are close slide fit to the outer head and can be easily removed and interchanged. This is not a press fit and can be done by hand. With the head off the bike and the spark plug removed, press around the spark plug hole with your thumb until the dome comes out of the bottom of the outer head. The dome is sealed and held into the outer head with an oring (sold here), and when installed on the cylinder, clamping force will keep it from rotating. One o-ring is included with each dome.

WILL MY BIKE RUN ON PUMP GAS WITH AN APEX BILLET HEAD?

The short answer is YES! If it did before, it will run better on pump gas now. All of our real world testing has been done with local pump gas to ensure real world reliability. We pride ourselves on getting more power and cleaner running engines on pump gas than people thought was possible

That said, every bike is slightly different and some bikes may require higher octane dues to other modifications or external factors. Also some gas stations just have bad gas. With that in mind try to only use non ethanol 93 octane ((R+M)/2 method) as it does not degrade as quickly as E10. If you have to use E10, we recommend putting it in your weedeater or chainsaw if you don't finish your tank within a 2 or so weeks. Another word of advice is if you’re filling up with a pump that also dispenses 87, make sure to clear the line out by putting at least a half gallon in your car/truck. It might not seem like much, but if you're only getting a gallon it's going to end up being 89, not 93. Additionally, try to go to gas stations that does a lot of business, that way the gas you’re getting hasn’t been sitting in their tanks for an extended period of time.

General octane requirements (please see product listings for model specific requiremnts):

• SX: 93+ from 0-2000ft, 91+ from 2000+ ft
• SX/XC: 91+ all altitudes
• MX: 91+ all altitudes
• MXC: 91+ all altitudes
• XC: 91+ all altitudes

CAN I USE RACE GAS WITH AN APEX PUMP GAS DOME?

You can but it wont make more power. Octane ratings are a measure of a fuels detonation resistance, not how much energy they have. You can always use more octane than required, and there are two advantages to using race gas with a pump gas dome. The main advantage is you know you'll always be getting the same fuel. You don't have to worry about variations from pump to pump. The other is if your race gas is highly oxygenated, you can get slightly more power out of it, but its shelf life will be reduced and it will require noticeable jetting adjustments.

Long story short, if you use a highly leaded 110+ octane gas with a pump gas dome, you wont make any more power but will pay a lot more for it. If you always use race gas, contact us for a race gas specific dome.

DO I NEED A CUSTOM PISTON TO USE AN APEX HEAD?

Our heads are designed to work with all OEM parts so that no aftermarket parts are required for use. Quality OEM equivalent aftermarket parts are also acceptable. Please be advised that cheaper quality base gaskets are the most common cause of squish clearance with our heads being on the tight side. Also note that we advise against Wiseco pistons since they use slighly different piston crown dimensions than OEM/other aftermarket pistons. They will work, but are not as ideal.

CAN I USE AN APEX HEAD ON A BIG BORE?

That depends. We are able to make custom big bore domes for most models, however we may not be able to make then for all sizes of big bore.

• For 125cc models we can accomodate 134cc and 144cc big bores
• For 250cc models we can accomodate 265cc and 285cc big bores. Note that our heads are not compatable with 293/300 big bores

WARNING: IF YOUR BIKE IS A BIG BORE, WE NEED TO KNOW! A CUSTOM DOME WILL BE NEEDED TO FIT YOUR BIKE AND USING A STANDARD ONE CAN RESULT IN ENGINE DAMAGE! PLEASE CONTACT US IN ADVANCE!

WILL I NEED TO REJET MY BIKE WITH AN APEX HEAD?

To get the full benefits of the head you will likely need to rejet. The increased combustion efficiency of our heads generally prefers slightly leaner jetting than the OEM heads they replace. We have recommended jetting charts available for many models as images in the product listings. For models without jetting charts available, a good rule of thumbs is to start with stock and be prepared to go one main and one pilot size leaner.

Please be advised, jetting is to be done on an individual basis. Faster riders, dunes, atmospheric conditions, or other aftermarket parts can require richer jetting than our recommendations

• YZ250(X) 1999+
• YZ125 2005-21 & YZ125X 2020-22
• YZ125 2022+ & YZ125X 2023+

If you are looking to learn the ins and outs of jetting we have attached detailed instructions on how to jet a two stroke using the YZ250 as an example

APEX JETTING GUIDE

CAN I USE AN AFTERMARKET CARBURETOR WITH AN APEX HEAD?

Many of our customers use aftermarket carburetors with our heads. Please be advised that while these carburetors are marketed as jet-less, additional tuning is often required when combining them with any aftermarket parts. Please ensure your engine is getting adequate fuel as lean conditions will cause engine damage.

MY BIKE IS FUEL INJECTED, WILL I NEED A REFLASH?

Long story short is these heads are designed to get the most out of the stock map without a reflash. The combustion efficiency allows the head to burn as much of the available fuel as possible, which pairs well with the generally lean stock map. We recommend making sure you have the most recent dealership updated OEM map (Apex head or not). These are KM557EU2125102 or KM557EU2125103. For those interested in having ECU’s reflashed, we are in the process of developing our own maps.

IGNITION TIMING

The ignition timing of a motor is a very effective way to fine tune the power characteristics of your two stroke motorcycle. This is especially useful if you find yourself wanting power somewhere in between the domes that you’ve tried, or if you’re just trying to fine tune your stock motor. On two stroke motors, retarding timing will actually increase the amount of overrev a bike will have. This is because by retarding the timing, the exhaust gas temperature will be slightly higher, speeding up the speed of the sound waves in the exhaust. Conversely, advancing the timing gives peak cylinder pressure at an earlier crank angle, giving more torque in the mid range. The takeaway here is if you want to have a more aggressive powerband with a big midrange hit, advancing your timing is the way to go. If you want a smoother mid range and more overrev, than retarding your timing is the way to go. However, do not blindly adjust you ignition timing. This is a precise task that needs to be done with a dial indicator. Most instruction manuals will have instructions on how to set to the oem specifications. Adjustments from there should be done in 1 degree increments. If you are running one of our sx inserts, do not advance your timing at all unless you plan on running race gas. Similarly, for our mx and xc domes, we do not advise advancing more than one degree from stock timing, unless running race gas. Even with race gas we do not recommend advancing more than three degrees from stock timing. Excessively retarding timing will not harm your engine, however retarding past five degrees will greatly reduce the engines power. For those with a 1999-2019 YZ250, this link will be helpful.

WARNING: Do not trust the markings on the cases! These are often inaccurate! With YZ250 stator set of the case markings, your timing will often be 1 degree advanced or more!

Additionally, the stock YZ250X ignition curve is the same as a stock Y250 ignition retarded by two degrees.

YZ250X POWERVALVE GOVERNOR MOD

If you're a YZ250X owner, you've probably heard of the powervalve governor mod. It is a free mod and one that we highly recommend. It essentially involves removing a preload spacer in the governor assembly so that the powervalve begins opening sooner. This pairs well with our heads, and helps eliminate a flat spot in the power curve. There are several variations of this mod and the differences in end results between them are subtle. The simplest version and the one we recommend can be found at this link.

MY BIKE IS RUNNING LEANER THAN NORMAL OR OVERHEATING

If the weather is colder, the air will be more dense and make the bike run leaner. Going to a lower altitude than you normally ride will cause the same issue. Putting larger jets in the carburetor will solve this issue. Having a corrected squish will make the bike less sensitive to jetting changes, but they will still be necessary for large temperature or altitude differences.

Your bike could have an air leak. A leak down test is the only way to determine if there are any air leaks on the engine. There are many resources available online that show how to make a testing apparatus, as they vary from bike to bike. A good rule of thumb is that the engine should be able to maintain 5 psi for 5 mins. Do not pressurize by more than 7 psi or you will risk pushing your crank seals out. If your engine does have a leak, common leak points are the ignition side crank seal, clutch side crank seal, base gasket, reed cage gasket, and the carburetor joint.

If switching fuels, the desired fuel:air ratio may change as well as its volatility which will affect your jetting

Your spark plug may be too hot, see the spark plug section within the rich jetting issue.

MY BIKE IS RUNNING RICHER THAN NORMAL OR FOULING SPARK PLUGS…

If the weather is hotter, the air will be less dense and make the bike run richer. Going to a higher altitude than you normally ride will cause the same issue. Putting smaller jets in the carburetor will solve this issue. Having a corrected squish will make the bike less sensitive to jetting changes, but they will still be necessary for large temperature or altitude differences.

Often, when installing our cylinder heads the bike will run richer because it is burning more of the fuel in the combustion chamber that was previously wasted. Leaner jetting will solve this issue. This varies from bike to bike, so care should be taken to ensure the jetting is correct. Always feel free to contact us for advice on jetting your motorcycle.

If switching fuels, the desired fuel:air ratio may change as well as its volatility which will affect your jetting

It is possible that your clutch side crank seal is leaking and your bike is oil fouling spark plugs. If this is the case, your bike will have excessive amounts of blue smoke, will be hard to start, and when changing transmission oil, there will be less oil coming out than you are putting in. This will also show up in a leakdown test.

It is possible that the wrong oil ratio is being used. Too much oil will oil foul plugs, but using less oil without changing jetting will cause the bike to run rich. We always recommend starting with the manufacturer recommended oil ratio and brand. From there one can experiment if necessary.

Also oil type can be a factor. For example, castor oil is popular and provides great engine protection at high temperatures, however it burns dirty. It is best used in older, air cooled bikes, or when racing mostly at high rpms. For woods racing it might cause increased plug fouling and one should either switch to a synthetic, ashless oil or run it at a more oil lean ratio.

Spark plug type is also important. Make sure to use the manufacturer recommended spark plug or equivalent. Please see our spark plug selection FAQ.

Spark Plug Selection

All of our heads were tested with the OEM specified spark plug for each model. There is no need to use a special spark plug with our cylinder heads. We advise addressing jetting issues with jetting, however this information may serve as a usful resource to aid in tuning.

Most 80-100cc motocross bikes use NGK BR9EG or BR10EG

Most 125cc motocross bikes use NGK BR9EG

Most 250cc motocross bikes use NGK BR8EG

Most 300cc + motocross bikes use NGK BR7EG

The number indicates the heat range of the spark plug. Higher numbers are colder and larger numbers are hotter. Hotter than recommended spark plugs have better resistance to fouling but can risk overheating the engine. Colder than recommended spark plugs carry less risk of overheating, but foul more easily. Be careful if experimenting with different spark plug heat ranges. This should not be a substitute for properly jetting the carburetor.

The EG at the end of the NGK identification code denotes that the plug is for racing purposes. They can sometimes be substituted for the same plug but ending in ES or EIX. ES means its a standard plug, which will be slightly cheaper but will not last as long. IEX means the plug has an iridium electrode and will have better fouling resistance, but will cost more.

MY BIKE FEELS LIKE IT IS DETONATING…

Detonation occurs when high cylinder temperatures and pressures cause parts of the fuel air mixture to self ignite. Normal combustion is a deflagration, or an expanding subsonic flame front. Detonation on the other hand is supersonic in nature and often occurs at to early of a crank angle to do effective work. It also causes severe engine damage if left unchecked. It ruins crank and rod bearings, melts holes in the tops of pistons, and causes spark plug failures, to name a few problems. Early signs of detonation are a slight pinging or knocking sound under acceleration. It’s often described as the sound of a tiny ball peen hammer hitting a piece of metal inside of the engine. More severe detonation will cause loss of power during acceleration. The engine will also run very hot. Detonation will often cause small metallic flakes to be seen on the spark plug during spark plug reading.

Possible causes and solutions:

Ignition timing: too advanced ignition timing will cause detonation. Ensure that your ignition timing is set to the factory recommended setting, and see our “Ignition timing” FAQ for more info.

Squish clearance: we design our heads to have a nominal squish of 0.050” for 250cc bikes and 0.035” for 125cc bikes. However, due to variations in manufacturing tolerances from bike to bike, your squish may fall out of the desired ranges listed in our “How do I take a squish sample” FAQ. If this is the case, you can use different thickness base gaskets to correct for this on some models or you can contact us for assistance.

Lean jetting or an air leak: see the lean jetting section above.

Fuel octane: octane is a measure of a fuel’s resistance to detonation. Therefore if detonation persists, a higher octane fuel is required. That said all of our standard setups are designed to run on pump fuel. If you think fuel quality may be an issue, see our “will my bike run on pump gas” section of our FAQs page.