Wow !! initialD thanks a lot for your concern man !! I appreciated it, and you too Traj

I asked that question because NS15S5 manual book doesn't mention anything about NS15S5 and even NS12S3/S5. The manual book is not up to date

But I find only these info,

NS15S5 :
Crankshaft Diameter = 11.50 mm SG
Bore x Stroke = 14,90 mm x 14,25 mm
Carburator = 5.4 mm
Glow Plug = Turbo

NS12S3/S5 :
Crankshaft Diameter = 11.50 mm SG
Bore x Stroke = 13,80 mm x 14,04 mm
Carburator = 5.4 mm
Glow Plug = Turbo

What I meant is this ( best case scenario ) :
I don't mind my NS15 will become NS12 after engine overhaul. I have cool looking NS12 with big heatsink

or worst scenario : the sleeve, piston, and conrod are actually the same except the crankshaft ( different bore and stroke ). However, I'm not sure about that because the cc is determined by the area of piston's surface, ie. larger diameter for sleeve, piston, and crankcase !!