Scoot F.A.Q.

Q. Which big bore kit should I get?
( 139QMB / GY6 50 / QMB139 / 1P39QMB )

First you need to decide on the cylinder bore size. The stock bore is 39mm, coupled with a 41.4mm stroke to make 49.5cc. Big bore kits are commonly sold in 44mm (63cc), 47mm (72cc), 50mm (81cc), and 52mm (88cc). 47mm and 50mm kits are most popular and easiest to find at low prices. Many times sellers exaggerate the displacement of these kits, frequently labeling 47mm kits as 80cc and 50mm kits as 100cc. Be sure to check the actual bore size when ordering to be certain you're getting what you're looking for.


Generally as bore size increases both positive and negative effects increase. Engine life tends to be greatest with the smaller bores, with maximum longevity expected from a stock bore and the least hours or mileage anticipated for the largest bores. Torque and horsepower output works in the opposite way. The largest cylinder bores are usually capable of the highest output while the smallest bore diameters make the least power. Please bear in mind these are generalizations. Not every 50mm bored engine is more powerful than every 47mm bored engine, and not every 39mm bored engine will outlast every 47mm build. There are too many additional variables at work including part quality, which components each is paired with, installation, maintenance, and state of tune.

Larger bore sizes require larger pistons, piston rings, and often use longer wrist pins. All of that leads to heavier and heavier piston packages as bore size increases. The additional mass puts more stress on components in the bottom end. Considering that at 7,500RPM the piston changes directions 250 times every second it's not too difficult to get an idea of why more mass would create much more stress. Since the larger displacement from larger bores also tends to create more power, they subject the rest of the engine and CVT to more stress in that respect as well.

Pistons weighed in grams.
Weighed Pistons

47mm/72cc big bore kits are the best compromise of reliability and performance for the average rider in my opinion. They offer a significant increase in displacement and performance, but aren't as prone to dramatically shorten engine life as larger bores can be. 50mm and larger bores are best suited for those who put maximum performance as their priority with engine life not being a major concern. 50mm+ cylinder kits have been known to cause engine failures in hundreds to just a few thousand miles on Chinese scooters with stock cranks. The most common failure is the big end connecting rod bearing, that's the bearing where the rod meets the crank. I installed a brand new Chinese replacement crank with the last 50mm kit that I used and the big end bearing failed at 2,003 miles.

There are a couple of ways that you may be able to improve engine life when using 50mm and larger bores. The same methods should help smaller bores as well, though it's not usually deemed necessary with the average build since they don't often struggle with such low mileage failures as the bigger kits. The simplest of them is to "de-tune" the CVT transmission. The idea is to use heavier roller weights and/or a softer contra spring to limit RPM and reduce stresses on the stock bottom end. Some have reported this method to be effective. If you are preventing the engine from reaching it's peak output, you're losing performance. If you're losing performance, then perhaps you would be better off using a smaller bore and tuning it properly.

Another way to try to improve engine life is to use a higher quality crankshaft. Naraku makes a "heavy duty" (HD) crank that is said to include much better bearings as well as better construction and quality control compared to Chinese cranks. Some say that upgrading to the crankshaft from a Kymco scooter that uses the 139QMB engine, such as the Agility 50 4T, is another option. Kymco is made in Taiwan, as is Naraku, and generally regarded to be of higher quality than Chinese brands/parts. You could try to find a connecting rod and bearing kit from a quality source and replace what's used in a stock Chinese crank, as well as the main crank bearings that ride in the engine cases. That is a big job and not an easy one, and a quality rod and bearings will probably not be cheap enough to warrant all of the effort unless you're accustomed to and have the equipment for disassembling and assembling cranks.

Some prefer to buy a stroker crank if they upgrade their crankshaft. A stroker crank has a longer stroke to increase displacement, typically 44mm vs the stock 41.4mm stroke. A quality longer stroke crankshaft should have better bearings and construction, just as the better stock stroke cranks would. They can also increase engine output, but there is again some sacrifice in engine life as stroke length and power increases. Anything that makes more power is likely to add some level of increased stress on parts, as said with the larger bore kits. Longer strokes increase piston speed at any given RPM because the piston must travel farther up and down the bore. In addition most stroker crankshafts use the same length connecting rod as stock cranks. That means they have a lesser rod/stroke ratio (connecting rod length divided by stroke = rod/stroke ratio), which increases side loading and potentially wear. I think the negative effects won't be a major issue, but if maximum longevity were my goal I'd stick with the stock stroke.

Another factor in determining which bore size you go with could be the need for a different cylinder head, or cylinder head modifications. As displacement increases with larger bore sizes compression ratio tends to increase if using the same head. This is because you're asking the engine to compress a larger volume of trapped air/fuel mixture into essentially the same space if you stick with the stock head. In most cases a 47mm big bore kit can be used with a stock cylinder head, but if you aren't already using it you'll almost certainly need to switch to premium fuel. The 50mm and larger bores really should be used with heads that have larger combustion chambers, often called big bore heads, to keep compression at a reasonable level. A stock head can be modified to increase the volume of the combustion chamber, but with the cost of some aftermarket heads being low most find it easier to use a big bore kit that includes a head or buy one separately. You can also find big valve heads. These should offer a larger combustion chamber as well as larger valves and porting that can improve performance.

Some 50mm and larger cylinder kits may require removing material from the crankcases so the cylinder skirt will fit. I have not heard much about this issue lately so I believe the later model engines commonly have an increased bore for the skirts. Larger kits, such as 52mm, usually have thinner cylinder skirts and similar outside diameters as 50mm kits. 47mm big bore kits usually require no crankcase clearance work. If you do find that a cylinder kit won't fit into the cases they can be machined. The bottom end can be disassembled and the cases can be taken to a machine shop for precise boring. There are also specialty tools for boring cases yourself or the work can be done with a rotary tool and burrs. Try to remove only enough material to allow the cylinder to fit and keep the bore round. Some have chosen to leave the bottom end together while clearancing the cases with a rotary tool. You run the risk of getting metal shavings or particles into your cases and bearings this way. If you decide to try it, take precautions such as stuffing the crankcase with rags, covering oil holes, and/or even hanging or supporting the engine so the bore is facing down and metal particles are more likely to fall to the ground. At very least use compressed air when the process is finished to try and remove metal debris. Some choose to flush the cases with kerosene. Complete disassembly is the safest option though.

Cylinder Skirt
Stock 39mm Bore
47mm Big Bore
Cylinder Skirt
Stock 39mm Bore Cylinder Skirt
47mm Big Bore Cylinder
50mm Big Bore
Cylinder Skirt
52.4mm Big Bore
50mm Big Bore Cylinder
52.4mm Big Bore Cylinder

Once you've decided on the bore size and other details of your build, you'll need to pick a kit. In general the Taiwan made parts are considered as a step above Chinese parts. Hoca, NCY, and Naraku are a few examples of Taiwan made products. That being said, I've heard of many members using very cheap Chinese big bore kits and being satisfied. I would advise using a seller that you believe you can trust so that if there is a problem with a kit out of the box you have a good chance of them helping you.

Related Info :
Big Bore Kit Installation How-To
Big Bore Piston Weight Comparison
64mm vs 69mm Valve Lengths

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