I just started my truck '' 79 Bronco '' for the first time this year and noticed what sounded like an exhaust leak, as I was looking at the exhaust manifold I noticed a hole in the head behind the alternator. I put my finger over it and found my leak! I looked at two different 351m engines, one had the hole, but was plugged with what looks like a small freeze plug and the other one didn't have any hole at all. Does anyone know what this hole is for or if its even supposed to be there?

 

It should be plugged. It is the same hole that is on the rear of the heads for the air injection pipes to hook up to. Since the heads can go on either way, they plud the holes in the front side. Your freeze-out type plug must have rotted away. It happens occasionally

 

the a.i.r. system (air pump) just put a new frost plug in it the tubes for the air pump usually go into the back of the head both heads are the same so they have holes on both ends even if your truck doesn't have an air pump later heads all have the holes

 

The Wheezer had air tubes in the back of the heads. On a 351M/400 there is a port in the intake manifold for the air pump.

I had that plug rot out on the rear of the driver side head. I didn't want to pull the engine so I cut a small hole thru the firewall for access. It waa a PITA. I will be plugging those holes with threaded pipe plugs when I rebuild my engine.

 

I had the same thing on my engine. I just rebuilt my 351m and had to get a new head and this head had a much bigger hole then the old one right where the power steering pump bolted on to the head so my original bolt was to small. So i started her up and it sounded terrible I looked and looked for a vacuum leak but found nothing finally I put my finger in that hole and my engine sounded sweet, took it back out and sounded like crap! It was a vacuum but smelt like exhaust. So I went to a Hardware store and got a bolt and put some plummers tape on it and bolted my powersteering pump to the head. Engine runs good and sounds fine, dont ask my what the hole was, autozone didnt know and my old head didnt have a vacuum from that hole so...........Oh well I guess!