Get a caliper to measure it as many are 47.6 mm bore and may can use a piston kit and will need the gaskets too from here
www.scrappydogscooters.com/50cc_2stroke_Performance.html Note the 2Ts come in 10mm and 12mm wristpin sizes but since you do not have a piston to measure read this
MINARELLI 2STROKE PISTON PIN SIZE DIFFERENCE , 10MM vs 12MM
and variator size difference
Hey MO! How the heck do I know if my Minarelli based engine has a 12mm or 10mm wrist pin, without taking it all apart first? - Confused in Chicago
Our knowledge database is growing, but we know a few things. Adly's and Hammerheads are 10, so are the Italian Minarelli engines like Malaguti, Italjet, Beta, and Aprillia. The 12mm pin is found on later CDI's, chinese JOG-2T, Vento Triton. These engines are poplular in 2-stroke full size scooters and ATVs such as Malaguti, Yamati, United Motors X-Speed, Aprilla, CPI, Benelli, Vento, Eton, Adly and many more. Italian made units tend to use 10mm pins (Chinese JOG 2T clones tend to use 12mm) please measure and check before ordering. Certain brands such as CPI / United Motors / Muz older models used 10mm newer post 2004 use 12mm. All Italian made models use the 10mm such as Aprilia / Malaguti / Italjet that are Yamaha Minarelli based.
Probably the most accurate, without complete disassembly, is to measure the inside diameter of your variator pulley. If it is 21mm, your wrist pin is 12mm. If it is 18mm, your wrist pin is 10mm. You can also count the number of splines on the outer variator face where it mates with the crank. The smaller 18/10mm crank has 17 splines, the larger 21/12mm crank has 15. The larger crank was originally designed for the JOG90 engine, but comes in 50cc too. Also, don't confuse your Minarelli 2stroke with the Franco Morini type two stroke found in TGB R5 Laser, Key West, 101S, the Suzuki Address, and some Italjets. These kits are also available from Scrappydogs, as they are authorized dealers for TGB products.
The arrow on top of the piston should be pointed towards the carburetor . Have some snag ring pliers with tiny tips and compression ring pliers . The wristpins are a lil awkward to put in and do so at a slight angle instead parallel . I put in one and coat the wristpin and bearing in plenty of 2T oil . Its much easier to in a wristpin with the piston in your hand instead of doing both with the piston on the crank . The compression ring pliers make putting on rings super easy . I have broken some when doing it by hand . I bought the compression ring pliers at Oreillys for 7 dollars . Wet the piston and inside the cylinder with lots of 2T oil . It is not that hard to compress a ring and GENTLY get it started into the cylinder and move to the second ring and repeat . Once its in there I used a rubber mallet to gently tap the cylinder all the way down . Forgot to say make sure a new base gasket is on before that step . The head simply line it up and tighten the 4 nuts onto the studs . I started mine up to test it instead of having to remove the body panels if I would have needed to if it did not work . I only let it idle for a few to get warm some and gave it just a lil gas and do not rev it up more than just enough to rotate the tire while on the center stand . . Shut it down and go over the head nuts to make sure they are torqued . Then you can replace the fan cowl and body parts . I put on my BBK without removing the engine from the frame since I could with the whole body off of it . Some you have to drop the the whole engine unit to be able to get to it but is more work to do .
Do you have the head for it ? The 49cc head would be too small . You will need a larger main jet for sure and maybe an idle jet depending on what you have already .
I bought a 72cc exhaust pipe and could not get it to line up with the 49cc engine but it fit like a glove on the 72cc engine . So your stock exhaust may not bolt up like it does with the 49cc and you should get a pipe for the larger piston anyways as the stock is small and restricted with a catalytic convertor where aftermarket just a pipe and muffler and a tuned pipe would require a larger jet once again .
Note street 2Ts use 2 rings where race engine use 1 ring . The dual ring will last longer and hold in compression better , but a single ring can hit higher RPMs since less friction . Since you said it was ported then you must get a new exhaust and here a great write up on several exhaust options
49ccscoot.proboards.com/index.cgi?board=exhausted&action=display&thread=13912Ts are very picky on the jetting and will take some trial and error using a few sizes of jets o fine tune it . A2T you can have a soft sieze if too lean so any bogs upjet ASAP instead of riding and messing it up .
The oil pump should be bleed unless you premix and here is a guide to help you
49ccscoot.proboards.com/index.cgi?action=display&board=yamajog&thread=3151&page=1Hoca is widely used and may be what cylinder you have already
