So, I just did my first ever head gasket change on my 2000 Civic Dx 1.6L. I made a classic mistake. I have one bolt left over, and I can't figure out where it belongs on the engine. Its a 10mm head, 30mm length. Any idea where to start looking on the engine?

 

And you only changed the head gasket ??? It looks like one of the longer bolts that hold the water pump to the engine block. It fits where the dowel pins are.

 

Cam cap bolts all screwed in?

 

Water pump bolt or thermostat bolt

 

That bolt is too long for the thermostat housing... however, it is also the proper length for the upper water neck that bolts to the front corner of the cylinder head.