Wednesday, February 2, 2011

Common Questions 2stroke pistons 12mm vs 10mm Jog vs Franco Morini what are the differences?

Common Questions 2stroke pistons 12mm vs 10mm Jog vs Franco Morini what are the differences? What's the most common 4 stroke?

We get these questions every single day about pistons and different piston sizes for the 50cc units. I'm going to try to answer them as best as I can without going into too much tech detail, but should make it easy to know the names of the brands. The most common 50cc units are Yamaha based, after that the QMB, and then the Franco Morini / Suzuki / Hyosung 2stroke.

Minarelli JOG are known to come in 40mm and 41mm depending on the company the norm is 70cc = 47mm, 72mm = 48mm, 52mm = 90cc, 54mm = 100cc give or take depending on the piston company. Athena, Polini, SN all have small changes but stay true to this formula.

MRP also carries OEM Liquid Cooled Minarelli Cylinder Kits for the F-12 / F-15 Liquid Cooled, the Beta, Benelli LC, Italjet Dragster 50cc LC, as well as the Aprilia SR50 LC. We have both the 50cc and 70cc versions in MRP, OEM Malaguti/Aprilia, and Polini.

Qingqi QM50QT-B2 50cc 2-stroke scooters have a 10mm piston pin and are smaller than most pistons. These are usually found on what we call disposable older scooters that companies like Aaron rentals used to sell in the US. Very low end stuff.

Franco Morini / Hyosung / TGB / Kasea / Xpeed UM (post 2005) all Suzuki Address based units use the standard 2stroke Franco Morini and this needs to be checked before ordering.

MRP also carries the rare but still sought after Liquid Cooled Dragster 50cc Franco Morini cylinder. This is the only US model to use the Liquid Cooled Suzuki style cylinder and we have it in stock as well. Same as the Franco Morini cylinder we sell but in a 70cc LC version.

MRP also carries pistons for older Derbi engines such as the Atlantis / Predator which have Derbi branded engines. Later models post 2003/2004 in the USA use the Piaggio engines before Derbi stopped importing. We have these Piaggio 2stroke pistons as well in both racing and OEM from Polini, MRP, and SN.

The QMB139 usually has a 13mm diameter, we have seen some at 13.25
also some Chinese models say 50cc and are really 80cc out of the box. Instead of 39mm or 41mm they come in 50mm designs right out of the unit. Almost every private label no name importer of Chinese units uses this engine. If you never heard of the brand and they don't advertise chances are they import this bike. It has a history of the most false EPA labels in the history of US importations so more than likely if you have a 50cc it's what people call GY6 Honda 50cc or QMB139.

Chinese Clones of the 2T JOG Minarelli Yamaha Engine
The most common issue is also the most popular Yamaha clone engine and thats the confusion at stores on what to stock. We suggest stores should ALWAYS and I mean ALWAYS have 2stroke 10mm and 12mm pistons in stock for stock cylinders and a big bore piston. Sometimes its just cheaper to go big bore or to repair a cylinder by going one or two mm bigger than destroying a perfectly good Jug. Just our opinion, but if you don't have a boring machine then always stock both 40mm/41mm and 47mm pistons in both pin sizes. Let me explain why.

Our experience started back in the early days with Kasea, Derbi, Honda, and the Yamaha lineup. 90% of the Yamaha units all used the Minarelli - Jog 50cc engine. Until mid 2004 all the factories using clone Yamaha engines had the 10mm piston pin. Later on companies in China switched (started with Vento and Wangye in 2005) soon after the Taiwanese brands like Eton and CPI switched as well since most of their pistons were coming from the mainland. These units eventually arrived in the US as 12mm pistons. So dealers repairing units be it a Vento or an older CPI made Muz (the Daytona Florida based company sold over 20,000 scooters in the USA from Taiwan) would constantly cause confusion at the dealer level because you never knew what you were going to get when someone brought their scooter in for repairs. So it became the norm to always have both piston sizes in stock at all times. Especially if you had a rental shop on the beach.

For European brands that made it to the US all of them are 10mm, be it a Malaguti F10, F12, F-15, Aprilia SR50 or even an Italjet unit. Except the Franco Morini Italjet units or the Aprilia SR50s with Piaggio or Ditech engines. Benellis and Betas made in Italy were 10mm the new re-launched Benelli brand made by QJ ( same as Diamo, VENTO, TNG, CPI ) were all 12mm on their carburated models.

Lesser known Taiwanese brands like Adly scooters and ATVs were at first 10mm and eventually turned in 12mm as well.

One interesting note is United Motors of America which sold the most popular 50cc in the Southeast called the X-Speed. This unit was originally sourced from CPI, then later Wangye, and eventually in 2005 was no longer a JOG engine but a Hyosung ( Suzuki - Franco Morini based engine same as the TGB lineup of 2 strokes ) so only pre-2005 X-Speed units use the 12mm JOG pin, anything older than 2004 is 10mm.

As you can see in the video measure the pin of the piston before installing. This is how thick the pin is. If you haven't removed the piston and pin then measure the diameter of your front CVT pulley aka Variator. If it is 21mm, your wrist pin is 12mm. If you have an 18mm version then its a 10mm unit.

Counting the full number of outer variator splines is another way to determine this, but beware some Chinese OEMs will use JOG 90 parts on a 50cc unit. This also happens on some Apex, Dinli, Eton, ATR ATVs. These older units interchanged a lot of variators. The smaller 18/10mm crank has 17 splines, the larger 21/12mm crank has 15. This should help you when checking to see which JOG engine you have in your possession. For more info visit the OEM parts charts on our website at we also have OEM repair PDF manuals that show you how get the OEM part number for these pistons and cylinders. Use the OEM manuals if you must absolutely have the OEM part and don't wish to use a generic part in your repairs.

Remember a lot of dealers will try to sell you a Chinese piston for an Aprilia or a lesser quality cylinder for a Malaguti, we strongly recommend using the OEM ones that MRP sells. We get these from the sources plus we have Italian brands such as Athena, Polini, and Malossi should you want a more professional upgrade for your scooter. For more info visit us at for a list of cylinders, pistons, and upgrades check out our new online PDF and E-Catalog.

To order click on places to buy.

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