I want to say 14mm but dont remember for sure. Used an impact gun i bought from harbor freight for $20, hooked it to compressor. One of the best purchases ive made. Bolts come out quick and go in easily with it and dont have to hold the blade. Use it for everything now

 

The manual for my HRX says 14mm

Don't forget to torque them back to spec. 36-43 lb-ft

 

The manual for my HRX says 14mm

Don't forget to torque them back to spec. 36-43 lb-ft

I'm sure it's the same for all the similar models. Thank you.