Sorry I thought you were talking about the bolt pattern that you bolt the standoffs and engine to the fire wall. Not the standoff themselves. I used the stand offs and cut hole in firewall to clear the carb on my pattern . made and kept temp for later use. The wide foot on the standoff goes to the firewall and the 11mm on the engine. The lenght is going to vary per mount as far as the hole drilled in them as long as they are flat which I found nothing wrong with mine they do'nt have to be perfect. If I made my own out of square hardwood and was going to box the carb in then you would want to be more accurate in drilling the mounting holes.