RV9A Engine Mount to Firewall Bolts

[DWright]

I have the new style engine mount and the original owner of the RV9A project I recently picked up attempted to drill the engine mount to firewall for the 3/8" bolts called out in the plans. Unfortunately it appears the holes were drilled vs. reamed to final size and there is some slop in the bolts. Have tried AN176 close tolerance bolts and they solve the issue in some of the holes but not all. Has anyone had an issue here and successfully upsized the mount attachment to 7/16" bolts vs. 3/8"?

 

[lr172]

I have the new style engine mount and the original owner of the RV9A project I recently picked up attempted to drill the engine mount to firewall for the 3/8" bolts called out in the plans. Unfortunately it appears the holes were drilled vs. reamed to final size and there is some slop in the bolts. Have tried AN176 close tolerance bolts and they solve the issue in some of the holes but not all. Has anyone had an issue here and successfully upsized the mount attachment to 7/16" bolts vs. 3/8"?

A bit of slop in a bolt hole is not typically a bad thing in structures like this, especially when they are hand drilled. If the hole is real tight and you drilled the hole a few degrees off perpendicular (typical), you create a lot of stress on the head. Vans instructions call for hand drilling there, not reaming. Their engineers have accounted for the slop. If they wanted it dead straight and tight, they would have instructed you to use drill guides and a reamer.