Grade 8 bolt to allow the engine and frame to rotate relative to each other reducing vibration and loading. These are laser cut 3/16? They match factory measurements so the engine remains in the same location as stock.
Do not overtighten the main center bolt. It uses a locking nylon nut and will not back off. Overtightening can compress the mount which may result in an increase in vibration. To the lower 48 US states.