ENGINE REPAIR NOTES REGARDING CHINESE-MADE ENGINES
We receive numerous calls from other dealers and repair centers throughout the nation regarding something that seems perplexing to the majority of customers and mechanics not familiar with the Chinese-made engines and one question that repeatedly comes up is the difference between what is called the "small head" and "large head" 125cc engines. So in the following we attempt to provide all with information on this issue.
CCs, or cubic centimeters, has nothing to do with the actual piston size or head size, or even engine size in the physical sense. Measuring CCs is the volume measurement of air/fuel displacement within the cylinder. It is measured by knowing the bore and the stroke of the piston and not just the diameter (width) of a piston. Once the bore and stroke is determined, then a simple mathematical equation can be applied which is v=πr2h ÷ 1000. To perform this measurement as close as I can to layman terms is as follows--
First to determine the stroke measure the distance of the piston’s travel within the cylinder in millimeters to the Top Dead Center and the Bottom Dead Center positions. On our 3125R Models it will be 57 (you drop the mm until the final solution). This is the stroke represented by “h” in the equation.
Second, finding “r2“(radius of piston squared), in the 3125R Model ATV the diameter (bore) = 52.4mm. To find “r” you divide 52.4 by 2 that equals to 26.2mm becoming “r”. “r2“= 26.2 times 26.2 which is 686.44.
Third, now by knowing the bore at “r2“and the stroke at “h”, the volume (CCs) can now be determined (“x”below denotes multiplication) (π represents Pi at 3.141592):
v= πr2h ÷ 1000
v= π x r2 x h ÷ 1000
v= 3.141592 x 686.44x 57 ÷ 1000
v= 122921.32 ÷ 1000
v= 122.921 rounded to 125cc (which is the common custom in engine manufacturing.)
Now in the case of some Chinese-made 110cc, the bore is the same but the stroke is shorter. In the case of QIYE ATVs, the bore is generally 52.4 whereas the stroke is 49.5. If you plug that into the formula you get 106.748 which rounds to a 110cc.
The confusing aspect that many “Certified ATV mechanics” or other individuals who are ignorant of this fact and knowledge is they see a 110cc piston and assume straight off that they have been taken by a dealer which is not the case at all. This is especially important when ordering parts to repair Chinese-made engines for those who have worked mainly on other brands, i.e. Honda, Yamaha, and so forth, they are accustom to seeing pistons, heads, jugs, etc. in different sizes. It is true and common knowledge for Chinese ATV Service Centers, such as ours, to know that the Chinese do produce large and small head 125ccs, which the same head of a 110cc and piston of a 110cc will be incorporated into a longer stroke jug thereby making it a 125cc. So it is an accepted fact in the industry that the determination of CCs have very little to do with just the piston and head size, however, the jug is different. The jug is what is important, as it is where the cylinder is housed where the volume is determined, the volume being CCs as described and depicted above.
With our experience, we have found that large head 125cc engines to present many problems. The clutches seem to be a problem along with valves. Our experience has found that the Small Head 125cc is much more reliable and cost-effective to maintain over the course of time. There are many benefits with a Small Head 125cc over a Large Head 125. The first notable benefit is the cooling aspect. Having a longer jug with the small head allows for more cooling surface over and through the fins. This is probably one reason that they last longer as a given. Another difference is that the "Small Heads" burn fuel more efficiently over the entire stroke. The horsepower is comparative in both with no noticeable differences. It is important to note when ordering parts online from various parts houses you need to know which one you have, a "small head" or "large head". The reason being, many of the Chinese manufacturers of these engines made both styles, and likewise, the distributor may sell both styles so when you need 125cc parts, make sure you specify to them because even some of these individuals that you speak with while ordering parts may not know the difference. Now you will be able to explain the difference to them. So for a "Small Head" 125cc you would typically order 110cc piston, rings, and head gasket. And next to that, there may be more differences within particular brands so it is wise to seek the assistance of an experienced repair center. This expertise doesn't come from book knowledge, rather hands-on experience to know the differences and the applied science behind the mechanics.
We hope that this may clarify parts and repair issues for all of our customers, other dealers and repair centers throughout the nation encountering this facet that they may not be fully aware. As a given, we do not sell the large head 125cc but we do stock parts for them and can provide service on them as needed.
CCs, or cubic centimeters, has nothing to do with the actual piston size or head size, or even engine size in the physical sense. Measuring CCs is the volume measurement of air/fuel displacement within the cylinder. It is measured by knowing the bore and the stroke of the piston and not just the diameter (width) of a piston. Once the bore and stroke is determined, then a simple mathematical equation can be applied which is v=πr2h ÷ 1000. To perform this measurement as close as I can to layman terms is as follows--
First to determine the stroke measure the distance of the piston’s travel within the cylinder in millimeters to the Top Dead Center and the Bottom Dead Center positions. On our 3125R Models it will be 57 (you drop the mm until the final solution). This is the stroke represented by “h” in the equation.
Second, finding “r2“(radius of piston squared), in the 3125R Model ATV the diameter (bore) = 52.4mm. To find “r” you divide 52.4 by 2 that equals to 26.2mm becoming “r”. “r2“= 26.2 times 26.2 which is 686.44.
Third, now by knowing the bore at “r2“and the stroke at “h”, the volume (CCs) can now be determined (“x”below denotes multiplication) (π represents Pi at 3.141592):
v= πr2h ÷ 1000
v= π x r2 x h ÷ 1000
v= 3.141592 x 686.44x 57 ÷ 1000
v= 122921.32 ÷ 1000
v= 122.921 rounded to 125cc (which is the common custom in engine manufacturing.)
Now in the case of some Chinese-made 110cc, the bore is the same but the stroke is shorter. In the case of QIYE ATVs, the bore is generally 52.4 whereas the stroke is 49.5. If you plug that into the formula you get 106.748 which rounds to a 110cc.
The confusing aspect that many “Certified ATV mechanics” or other individuals who are ignorant of this fact and knowledge is they see a 110cc piston and assume straight off that they have been taken by a dealer which is not the case at all. This is especially important when ordering parts to repair Chinese-made engines for those who have worked mainly on other brands, i.e. Honda, Yamaha, and so forth, they are accustom to seeing pistons, heads, jugs, etc. in different sizes. It is true and common knowledge for Chinese ATV Service Centers, such as ours, to know that the Chinese do produce large and small head 125ccs, which the same head of a 110cc and piston of a 110cc will be incorporated into a longer stroke jug thereby making it a 125cc. So it is an accepted fact in the industry that the determination of CCs have very little to do with just the piston and head size, however, the jug is different. The jug is what is important, as it is where the cylinder is housed where the volume is determined, the volume being CCs as described and depicted above.
With our experience, we have found that large head 125cc engines to present many problems. The clutches seem to be a problem along with valves. Our experience has found that the Small Head 125cc is much more reliable and cost-effective to maintain over the course of time. There are many benefits with a Small Head 125cc over a Large Head 125. The first notable benefit is the cooling aspect. Having a longer jug with the small head allows for more cooling surface over and through the fins. This is probably one reason that they last longer as a given. Another difference is that the "Small Heads" burn fuel more efficiently over the entire stroke. The horsepower is comparative in both with no noticeable differences. It is important to note when ordering parts online from various parts houses you need to know which one you have, a "small head" or "large head". The reason being, many of the Chinese manufacturers of these engines made both styles, and likewise, the distributor may sell both styles so when you need 125cc parts, make sure you specify to them because even some of these individuals that you speak with while ordering parts may not know the difference. Now you will be able to explain the difference to them. So for a "Small Head" 125cc you would typically order 110cc piston, rings, and head gasket. And next to that, there may be more differences within particular brands so it is wise to seek the assistance of an experienced repair center. This expertise doesn't come from book knowledge, rather hands-on experience to know the differences and the applied science behind the mechanics.
We hope that this may clarify parts and repair issues for all of our customers, other dealers and repair centers throughout the nation encountering this facet that they may not be fully aware. As a given, we do not sell the large head 125cc but we do stock parts for them and can provide service on them as needed.
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