Two of the same compressors are pictured. The one on the left has a red manifold/valve on the top of it and the one on the right is missing this valve. There are two bolts that hold this piece onto the compressor, and the pressure hose attaches to this piece. Does anyone know what size these bolts are? I first tried 5.5mm and almost rounded off the bolts on my old compressor. 5mm is too small and regular standard sizes also do not fit.

 

From my experience honda makes their own bolt sizes/threads , nothing metric or standard may fit , See the dealer for the bolts