Whats the best method for replacing the engine coolent? Is it good enought just to drain the radiator and engine by disconnecting the lower radiator hose and replace coolent? Or should you also try to drain the coolent in the bottom part of the block? Don't really want to take the plugs out the the bottom of the block. By the way, now many drain plugs in the 396 block? Are the plugs those with the allen wrench holes?

Or is there another way to get most of the coolent out than takeing the block plugs out. Does this sound like it might work? Drain as much out as possible though the pet cock by removing the temp guage in the top of the intake, fill with water and replace tem gauge, run engine up to let the thermostate open and water circulate, drain & fill with water, run engine until water circulates, drain again, fill with the proper ratio of water and antifreeze. Does this sound offbeat? thanks.

Disconnect lower radiator hose will drain the mostest the fastest except for that which might remain in the block. I will not attempt to remove the block drain plugs on my cars ('65 El Camino, '72 Valiant, '89 Toyota pickup) because they have been in since day one and it would be my luck that one or more would get busted off or stripped out. So how I do it is drain, refill with tap water (we have good water where I live; few minerals), run, and drain again. Do that three times. Then fill with distilled water, run and drain, twice. Then put in one-half the system capacity of fresh coolant and fill the rest to capacity with distilled. Fill the overflow reservoir about half way to the low line with 50/50 solution and warm it up. Shutdown, and after it cools, bring the level in the overflow reservoir up to the full line. Never in 50 years had any need to "burp" a system or had any problems with entrapped air. Keep in mind these are stock systems with downflow radiators. Probably gross overkill, but I only do it every 24 months. Distilled water is cheap at WalMart. Other places might try to hold you up. My $0.02

James, I know you are looking for different options here on this subject, but if you will change out the fluid in the radiator (thru either petcock valve or lower radiator hose) every two years, you wouldn't have to mess with all of this (alot of trouble).

I have found that it is very hard to correct the anti-freeze/ water ratio any time you use more of one fluid than the other.

I have also learned that it is impossible to completely drain the cooling system (engine block and heater core).

If you really want to get the coolent in your system as clean as you can without doing something that might damage seals and/ or components, I would sugest drain the radiator and refill with a 50/50 mix of distilled water and anti-freeze. Then drive your classic a couple of times, then go thru this same process of draining your radiator and refilling with the subject 50/50 mix again.

Onced you have done this, just make sure to change this fluid out every other year and all will be well with you and your very nice Chevelle.:thumbsup:

Thanks Lance and JJ"65. Definetly will stay away from removing the plugs from the block. Hadn't thought abought the distilled water; but that makes a lot of sense since it should have the minerals removed. It's raining today here in MN, we need it. So the project will wait untill next week.

Ok, I'll rain on this parade.

Why not pop the two block drain plugs out, instead of flushing the coolant five times? It is NOT that hard.

Six-point socket, extensions/universal joints as needed. Rip 'em out, throw 'em away. Replace with brass plugs or install draincocks.

You may find that the plugs come out, but no coolant drains. Common problem--poke a screwdriver into the holes to break up all the rusty crap that settles to the bottom of the block.

Engine was rebuilt in late 80's with only about 1000 miles each years. Runs good, good compression, no usage of oil. Might there be a problem with the removal of the drain plugs? How many should there be on a 70 402 block. Thanks.

Might there be a problem with the removal of the drain plugs?
Anything MIGHT happen. I wouldn't expect a problem. Rip 'em out.

How many should there be on a 70 402 block.
Two. One on each side. Factory plugs were iron, and had 9/16 external hex heads. Aftermarket plugs could be iron, steel, brass; and have any wrenching surface you can imagine--external hex, internal hex, external square...you get the idea.

Well I actually started JJ'65 method before I read Lances comments. So I have added my second fill of distilled water, will drive for a couple of weeks as Lance suggested on the fill of distilled water. Then down the road I will rip out the do an engine flush and rip out the plugs as Schurkey suggested. Thanks to all.

Well I actually started JJ'65 method before I read Lances comments. So I have added my second fill of distilled water, will drive for a couple of weeks as Lance suggested on the fill of distilled water. Then down the road I will rip out the do an engine flush and rip out the plugs as Schurkey suggested. Thanks to all.

Well, you don't want to drive it with just water. Corrosion (rust) will set in IMMEDIATELY. Flushing is a poor description. Drain and fill is more like it. Anything loosened upstream of the radiator essentially gets filtered out by the radiator tubes. http://www.jag-lovers.org/xj-s/book/CoolantFilters.html
My $0.02