I would scrap that 15T and stick with the 14T, get a good piston head and plunger with steel teeth. Don’t get that compressed metal crap. If your inner barrel is long, 28cm plus start looking for a 100% cylinder and maybe you will need to port it to 90%. 18-1 is a good ratio but if you fit that it would be a toss up as to bearings or bushes. Just my take on it. Change all the O-rings, even if they are new for good green ones and a little silicon oil helps when fitting and during operation. Because CYMA has a mongrel propriety spring retainer with four locking pins, you will not find a suitable replacement easily, add to that the fact that the buffer tube screws into it as well. J9 Spring metal retainers with the bearing on them are best fitting and allow the spring to spin as it compresses instead of eating up your plastic/ nylon/polymer original retainer. FYI, the original motor should have been a 480 anyhow and if true to form the barrel will be an inside diameter of 7mm. They are a loosely based Gen 2, but a mongrel as well.
I have just finished mine, it runs a 350mmx7mm barrel and I use the full 100% still using the all nylon gearbox.
Your spring sounds like it is too heavy for the housing, The original should be a 1.2mm equal at 15cm +/- 1cm.
My first move was to an M100 SHS 1.3mm-15.5cm unequal spring and whilst it worked, it was unforgiving on the whole blaster. I have now fitted an M90 1.25mm-14cm unequal spring. It still hits the same distance but more control and less hammer.
Good luck, watch some videos on stripping and assembly before you dive in, enjoy it. Oh, have some lubrication for it too, you can buy it from these places as well and the greases are labelled.
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