Small block, easy to install and works very well. I would only recommend for metal stem valves as you have to screw the sensors very tight to avoid air leakage and also leave room for the lock nut. Tires do not appear to be out of balance. The batteries are pre-installed on the sensors, so I'm not sure if they can be replaced. The battery life of the device is excellent, I only charged it once and it's still fully charged after 24 hours of driving (5 days).