I dont know where to post this Q but ill post it here. My question is how do i know if i need to replace my piston? My engine runs fine but it has no compression... Also, where can I get (online) a piston? BTW i have an RB concept .12

When you say it has no compression, what method are you using to determine this?

Are you using a compression gauge?

Are you turning the flywheel over and feeling for resistance with the glow plug in?

Are you turning the flywheel over with the glow plug out?

Are you turning the flywheel and listening to the exhaust port for blow by (hissing sound of air escaping the combustion chamber?

If you have no compression, it shouldn't want to idle when at operating temperatures.

Im turning it over by hand with the plug in. It feels like it has very little compression with plug in, with the plug out it has absolutly no compression. I hear a very small hiss coming from the exaust port. The engine idles fine...
Mabey its suppose to have that much compression? My .40 has a LOT of compression...

doesn't sounds like the piston is bad spit some after oil in the engine or wd-40 works well and that should give you a little tighter compression but it doesn't sound like you need a new piston.

Thanks for the added info. If you're hearing a faint hiss from the exhaust port and it's much easier to turn over than your .40, you're on the virge of needing a rebuild.

My RB X12 has a similar feel (but with only half as much force required to turn it over) as my OS .40 FP (I know, I know, I should have paid the little extra for the OS max but my local shop only had the FP in stock).

Pretty soon, your 12 will not want to idle once it's up to temp. You should start shopping around for a rebuild kit and pick one up when you find the best price. You should replace the piston, sleeve, connecting rod, and wrist pin. You should also get some extra wrist pin clips if you are assembling the rod to the piston (try to get the pre-assembled complete rebuild kit if at all possible). When it starts to get finicky at an idle, strip down the engine, inspect the bearings, and throw in the rebuild kit.

Good luck.

Ok, Where can i find a rebuild kit? Can i order directly for RB?

Throwing in after-run oil or WD-40 will only improve the seal between the piston and sleeve until you fire it up and run it.

Here's the site for ace hardware hobby. They sell RB engines, and should be able to get you a rebuild kit.

www.acehardwarehobbies.com

If your engine, "...runs fine...", why would you want to replace the piston or rebuild it? Why go through that expense if it is not needed? Don't take you engine apart unnecessarily. A good indication for a rebuild is difficulty with tuning and idling.