I changed coolant for the first time since I got my car this weekend. As expected, I think the coolant was about as old as the rebuild (about 20 years).

Its a 1973 SB and the coolant capacity is (supposedly) 18 quarts. I had to struggle to get 2 gallons back into the car (8 quarts). Any ideas why? Advice?

The car has never had a problem running hot, even on 90+ days.

As usual, thank you for the help

 

you normally can't refill it with the full listed capacity unless you had the engine motor, rad, and heating system out. Simple draining of the rad doesn't remove every bit of the old coolant. Chances are there was still coolant left in the lower areas of the block (which you could remove by taking out the freeze plugs on the block), some left in the heater box, etc.
What remains shouldn't be a big deal though.
Just refill to the proper level as well as you can than leave the radiator cap off and start the car and let it run and warm up as you watch the coolant level. Once the t-stat opens up and allows the coolant to start flowing your level will probably drop so add more in. It's also a good idea to turn the heater on to full high allowing the coolant to flow thru the heater box which will also give you a more accurate coolant level plus help eliminate trapped air pockets.
As the car keeps warming up just add more coolant as needed. When it won't take anymore put the radiator cap back on and take it for a nice drive 9keep an eye on your temp gauge just to be safe.
afterwards, park it back in the garage and let her cool down. Once she is cooled down check the level in the overflow/expansion tank and make sure it's at the correct "COLD" level. If needed, add more in the overflow tank to bring it up to the correct level when cold and you should be good to go.
This is how I always do it on my '78.

my '65 is a bit different of a process since I don't have a cap on the radiator.

 

Did you just drain the radiator? If that's all you did then (as Barry says) you left a lot of old coolant/water/crap in the system. When I drain my radiator I also backflush the system until the water coming out runs clean, so at least I have cleaned out the old stuff. I realize that I am leaving a fair amount of hose water in the system, even tho I top it off with distilled (and antifreeze).

For me it's just too much trouble to pull those plugs at the bottom of the engine which would allow me to drain the whole block.

 

yeah, me too! that's why I don't.

when I had the motor out of the '78 over the winter it was easier though to just tilt the motor on the stand and let the excess coolant drain out of the waterpump - probably got amost a gallon out that way although I probably SHOULD have just removed the drain plugs but i didn't.

PRNDL's suggestion of a backflush is an excellent idea - it will help get all that extra crud out of there. As he mentioned, leaving a bit of the hose water in there won't hurt anything even though you should use distilled water when you mix your coolant 50/50. Afterall, just how "clean" do you think that distilled water stays as soon as it hits the motor and old radiator?

 

If concerned about clean coolant here is a good read.
Keep in mind that 18 wheelers, buses etc have them as well as in 1954 some chevy cars even had a bypass filter.

http://www.jag-lovers.org/xj-s/book/CoolantFilters.html

 

Personnally, I would flush the system. Lots of crud builds up in 30+ yrs. I would use some radiator flush. There are 2 basic kinds. A detergent and a acid. The detergent is pretty safe. The acid type works much better, but could give you a leak if a bad spot is held together by corrosion. This is not all bad, because it is that close to leaking, it may be better to do it now when you are close to home. The bottom of the block can get a huge ammount of sludge. IF you can remove the pipe plug (3/8" I belive) near the bottom, close to the oil pan, you can flush it out. Some times the plugs come out and sometimes they don't. If they are rusted in place, you'll probably just round off the heads. If you do get them out, replace with brass and a good sealer. Either way, flush repeatedly. Fill with water, run to warm, then drain and do again. Doesn't have to get too hot, just enough open the thermostat and run for a few minutes. I'm always a little worried about draining a hot engine, so sometimes I let it set for a while, then drain it.