GOOFIT Big Bore Cylinder Rebuild Kit for GY6 50cc 139QMB Racing Scooter Parts 64mm Valve

Very good product. Tip of the trade here. Kit comes with oilpump, same as original, if your oilpump works just set the oilpump aside and install only if needed. Did install an A9 cam with this kit. Recommend the cam. The head is small valve, did porting and valve shroud ( the valves sunk in a bore and the bore shrouding the breathing dremel out the shroud and dremel out the port. however AVOID ENLARGING THE CHANNELS, very little wall material on some spots.just try to takendown the big wart that is cast to hold the pressed in valve bushing.On a 50cc base engine this head will do the same as a big valve head, so keep the head and dont be dissapointed that its a small valve head.Engine displacement. You will get a bit over 80cc with the 50 mm bore.(Formula: Radius X Radius X Stroke X Pi (3.14)) or as with this kit 25 X 25 X 41.4 X 3.14 = 81.44....or 81.5cc. Its called 100 cc kit but everyone and his brother markets it that way.Carburetor, Do NOT buy a 24 mm carb, its overcarbed and you will have issues. An 18 or 19 is perfect you might get away with a 20. If you have a cheap sealed carb, toss it and get a carb you can adjust, and open up to change Jets. With this kit, you might want to start with a main jet around 80-83 somewhere. Watch Youtube, plenty of videos on how to install and tune. My scooter is now doing a bit over 60 mph and have a very good response and pick up. I do recommend you put in a 2000 rpm torque spring (the red one) with this kit. Run as high octane as you can get hold of because you will bump up the compression pretty significant with this kit. A word of warning, many buys a race CDI and have a hard time starting, and don't know why. Many of the race CDI don't retard the timing on very low RPM (as when youre starting), if you want to put a race CDI on your scooter, find one with advancing and retarding curve, otherwise you blame this kit, or get into a long carburetor fix, that doesnt fix anything. Stick to the stock CDI (stock CDI will retard at start) until you have found an advancing/retarding CDI. This kit comes with more gasket you will need. And the parts are well done. I would take my time to mirror polish the top of the piston, and the combustion chamber, careful not to damage the valve seat. The valve seat has a 3 angle valve cut already when it comes so nothing is needed with the valve seat. You can , and I recommend that, grind the valve and get a vitnessmark where the vakve will seat, and do a back cut on the valve up to and close to that mark. You can also make the circumference of the valve head , the edge..make ut round, and you will get a better flowing valve. If you have a steady hand, you can put a piece of protective leather around the valve stem and put the valve in a hand drill, and work the valves with a file.All in all, this kit rocks, and if you do all those little extras, you will beat all big valve heads.Im very happy with this kit and I strongly recommend it.You can just bolt it on and get a much faster scooter or do all the small tweaks and get a rocket.Word of warning...you will have a bigger engine, and thus you will have more heat. When putting on your cooling shroud, be very anal, it will pay out, go over small cracks and openings with black silicone.Good luck to you all

Installed this almost 3 months ago. For the price, this is unbeatable. The hardest parts were getting the little rings onto the piston and seating the piston into the jug, but other than that it was a cinch. The whole process took roughly 4 hours from start to finish. Make sure you have a good startup grease to coat the jug the piston slides into so you do not experience startup issues.I do not get oil leaks, and the power increase was felt the moment I cranked her up. I was actually a little scared it would fall apart! The weights that come with it were 9 grams, and that was a little heavy, so i switched them out with 6's and she runs ridiculously well uphill, in wind, everything. Couldn't be more satisfied with my purchase.That said, i do have mechanic experience so if you do not, expect to run into some problems. Be careful and take it one small step at a time. There are tutorials for these sorts of things, make use of them!!All in all i was skeptical about the cheapness of this BBK, as others of the same caliber are much more expensive, but i'm glad i went with it. You should, too. Happy scooting!!EDIT: (5/5/16) Hello again! I installed this kit about 6 months ago, she still runs great! I am leaking oil but not its probably a faulty gasket and my own negligence, not the kit itself. I did want to add something i missed in my review; Where you screw in the spark plug has some sort of jam half way through that i never noticed until yesterday. Normally the requirement for spark plugs is "Screw hand tight, then use the tool for 1/4 to 1/2 turn, depending on a few factors". However, hand tight on this head was only about half way! I had issues with my spark plug ejecting itself lately, and it was baffling. I understand now that I never actually screwed the plug entirely flush with the head, which is obviously very important! I'm surprised she was running at all, much less as well as she was.It is DEFINITELY RECOMMENDED that once you receive this, before putting anything on the bike, that you take a brand new spark plug (NGK CR7HSA, take that up to the counter at advance/autozone/nappa/etc) and thread it into the clean, new head itself. I was afraid it would strip like the my old one, but it did not. Initially it spins 3 or 4 times hand-tight, but again that's only half way. Using the spark plug wrench, without much force, i am able too screw the plug flush with the head itself. I recommend doing this before installing, mainly so you can see what you're doing and completely avoid strippage.I had to buy the kit again, mainly for the head but I figured its only 60 bucks and more spare parts couldn't hurt. Don't make my mistake and assume the plug is good on the initial hand-tightening, because it is not. Mine ejected a few times and it ended up stripping the head. Maybe your heads will be different, but the two i received arrived in this manner. Make sure the spark plug screws in completely before you put it on the bike!I am open to any and all questions.Happy Scooting!!Other than that, lovin' this kit.

Its a 50mm piston which is around 82ccs theres very few kits that are any bigger ive seen an 52mm which was about 86ccs. This kit is pretty nice however mine was damaged upon arrival. It had been dropped at the factory i was unable to install as one of the two bolts that hold the chain tensioner down was D shaped instead of round making it impossible to screw the tensioner down. This was due to the item being dropped clearly as there was a perfectly flat spot where it should have come to a point, was smashed down. right where the flat part of the D shaped hole was. Also the head had a piece broken off it. I think this was minor and it would have still run fine. Also the intake screw was bent a few degrees and wouldnt fit the intake withouth some bending. I got a bad kit. However ive installed two of these in the past and they worked great. Im returning this one and i just bought one from a.shop. for those of you who are scared of doing this yourself. It is SO easy. Watch a video 3 or 4 times. If you can, dissassmble your stock head and cylinder, take out your cam,etc. Put it back together. If it works you just did the exact same thing you will be doing installing the bbk. Only difference is you will be putting the piston on the rod yourself. But have no fear this is very easy.