Your description sounds like you are doing a differential compression test. That type of test will almost always show some type of leakage around valve seats and piston rings. You already know where the trouble is so there is nothing you can do except take it apart. 6sally6 is right. You could have a stuck valve, a burnt valve, a blown head gasket, a cracked head, broken rings or piston, and even a cracked block. To find out it has to come apart and hope for the best. If it's not to bad you can fix just the one side and be back on the road in short order. Take it apart then let us know what you find and go from there. Pics tell a good story, post some as you go along.

TW, to pump air into to a cylinder and check for leaks around valves and rings, you will need to remove / back off the rocker arms or crank the motor over until that cylinder is on a compression stroke to know both valves are closed.  Otherwise air will surely be escaping out through the intake or exhaust.  

Pulling a head is not that difficult. Plenty of help here, books, and you tube videos. The specialty tool for reassemble will be a torque wrench, which can be rented from most auto parts. With the head off, scrub around on the top of the pistons for a part number or stamping. If it's been rebuilt previously, the pistons will show the manufactures part number and .020, .040, or .060 as the overbore. 

If you don't know the history of the motor, take some pictures and post here. Casting dates and part numbers will tell when parts were made, heads and blocks mainly. You can at least determine what decade it manufactured. It's a good time to pull the oil pan to check a rod bearing and main cap bearing. Post those pictures here as well. Use that rented torque wrench to make sure things get properly tightened. Most of those rented tools do have a time line on returns. The O'Reilly's by the old house didn't care, a week or a month was OK. The O'Reilly's by the new house gave me grief when I returned something about 4 hours after the 48 hour deadline. 

You mentioned in the other post about going to a bigger radiator. I don't know that overkill on a radiator is a bad thing and say do it. prior to the problem, what was the condition of the water in the radiator? The color of the antifreeze mixed in, straight water, rusty looking? Leads to the condition. 

I haven't seen where you described the events prior to the problem. For all the cars I had that blew a head gasket, they got too hot. 

If you pull the hood, it will make it easier to work on the car. I use a pencil to mark the hood hinges to the hood and the bolts washers to the hood hinges to help me realign it when it goes back on. Mine is currently stored in one of the bedrooms for safe keeping. 

 

Pull the heads see what you got . Sounds like you are on the right track.

TremendousWand wrote:

I'd already done a compression test to figure out that I have no compression in cylinder 5.  I was running pressure into the cylinder to try and see if the air was escaping from a valve or into the cooling system.  I have a camera scope.  I positioned the cylinder at TDC from visual inspection into the spark plug hole but maybe not precised enough.

In talking to a buddy, summit sells re manufactured assembled cast iron heads for pretty cheap....$300.  I'm considering going that route.  Swap the heads, replace the head gasket, and get a new bigger radiator for less than $1000 and I can do it all myself.

BUT if the problem ends up being with something on the bottom head or catastrophic like a busted cylinder or cracked block then I would be wasted money as I'd need to source a crate engine.  

It seems like whichever way I go, pulling the heads is a necessary step.

Not necessarily.  You could just buy a shortblock and use the heads you'd planned to use on that engine if you're okay with stock.  The tendency with a crate engine would be to buy something with a little more get up and go than what you've got, but everyone isn't after that. 

Personally, if the heads are salvageable I'd have them rebuilt locally.  The heads you're going to get from Summit are done in a huge parts rebuilding house.  Attention to detail is not their strong suit, and if there's a problem you may have  a warranty, but you also have to pull it back apart gain. 

The shop vac is your friend.  Another tip, drain the water out of the cooling jacket in the engine.  There are bock drains on the sides of the block.  I remove the factory plugs and install brass petcocks in them.  This makes draining the block easy in the future and they don't rust like the factory steel plugs do.  These come in real handy if you ever have to flush the cooling system too.