Originally Posted by SlowLST2

Don't use a drill. Get a rotostarter or similar. The nitro-specific rotostarters aren't as strong as a drill and as such not as likely to cause damage in the event of a hydraulic lock.

If your OS uses an external one-way clutch bearing like the Traxxas engines do, you might be better off with a pull starter or looking into the rotostart system from a Losi 350/3.4 engine. I just don't know if the Losi 3.4 uses an internal or external one-way clutch bearing.

An OS .21 will probably do fine with a Pullstart. If you're able to pick up on what it's perfect priming habit is, you could have it down to a one-pull start. That's how I have gotten my 3.3 TRX engines to since I swapped to Pullstart from EZ-Start.

YMMV.