Most of the engines powering scooters 200cc and under today are GY6 type engines. First, read this short article from Wikipedia about the GY6 engine:<a href="
http://en.wikipedia.org/wiki/Honda_GY6_Engine">WIKIPEDIA ARTICLE</a>
Here, we will discuss the GY6 coding stamped on the lower left engine case near the mount, where the cylinder bolts to the crankcase. There is some variation on the format of this identifier but some things remain constant. The basic code is six digits and anything extra is usually manufacturer specific information. Let's take a common code and break it down: 157QMJ
1 = the number of cylinders
57 = the bore in millimeters of the cylinder
Q = a GY6 type engine
MJ= a numerical code for the displacement in cc's, in this case 150cc
The last two letters bear further explanation. The M is constant but the next letter ascends alphabetically with each larger step in displacement. MB = 50cc, MC = 60cc, MD = 70cc, MH = 110cc, MI = 125cc, and MJ = 150cc. It is assumed the newer 170cc's coming out would be stamped MK. Scooter Doc member Runningslow has confirmed that his 170cc scooter is stamped 161QMK which would translate: one cylinder with a 61mm bore, and a GY6 type engine with 170cc displacement (168cc to be specific in this case). 200cc GY6 type engines are soon to hit the market and we will have to see how they are designated when they come out, Scooter Doc guesses ML.
To further complicate this matter, specific manufacturers such as Quinjiang will put their own identifier in the code. An example of this would be QJ139QMB, a 50cc engine.
A "P" after the 1 indicates a head type on the 150cc engines that is sometimes not compatible with parts for the 157QMJ engine, so make sure with your parts supplier when ordering. Sprocket informs me that the "P" head is a taller and that engine develops about 2 HP more than the non "P". Further, many of the new scooters (2009-2010) are using "'P" type engines and there is only one big bore kit for them. If you plan to do extensive mods then you may want to go with a non-P engine, for which there are many more kits.
An "E" in the code indictes a two stroke which is not a GY6; also a 250cc engine is not a GY6.
Another distinction is the long case vs. the short case engines and the CVT belt they use. With a few exceptions, the 125/150 GY6 has two belt sizes: 743 for the short case, and 842 for the long. If you have a 10 inch rim on the rear with a tire that is 3.50 wide or less, it's a short case. The belt cover will also measure 16 inches from end to end. If you have a 10" rim with a 4.00 wide, or a 12 or 13" rim, you have a long case. The belt cover will measure 17 1/8" long. Another easy way to tell is simply to count the number of retaining bolts for the belt cover, the short case has 8, the long one has 10.
