how to know if a nitro engine will fit?
 

is there a specific way to know if a ntiro engine will fit the rc car?and what engines will fit the 1/10th tamiya mighty ballista?anything bigger than stock engines (.15 ).

RE: how to know if a nitro engine will fit?
 

Is this an electric car that you want to convert to nitro?

If it's a nitro car that you're just looking to replace the motor, you can measure the mount. Then you can compare to the specs of the engines that you're considering.

By and large, the mounting dimensions are standard for 1/10 scale size motors. I.e. an OS will fit an Associated mount, a OPS will fit an Associated mount.
The range you're looking at is .12 to .18

When you get to .21 and larger, then you're looking at different mounts altogether.