I need to drain/flush the cooling system on our '78 L48, and I'm not sure
whether I have the stock, or a later aftermarket replacement radiator.

From its general appearance, and the state of the rest of the car, I'd guess
it's the original. But the drain plug is small, black, and seems to be hard
plastic, with what looks like a rubber gasket. It's strictly a screw in/screw
out item--not brass or a petcock style, which makes me think that the
radiator is in fact an aftermarket.

Given the words of warning on here about possibly damaging the solder
joint by trying to remove the stock drainplug, I'm very wary of attempting
to drain the radiator the usual way. But I want to back-flush the system,
and I don't think I can do that by loosening the lower radiator hose.

Any advice/info on this drain plug? Is it stock or not? If it's not, and the
radiator is an aftermarket, would the same warnings about possibly
damaging the solder joint probably apply?

If you have a plastic, butterfly looking drain, there is no worry about damaging the radiator. You'll break the plastic long before you damage anything else.

Most butterfly type drains do not come all of the way out of the radiator to drain it. Just loosen it until it flops around, and you're done. Then just thread it back into the hole to seal it back up.

As far as being the original, I'm not sure, but I think a '78 would have this type of drain.

Joel, you described it a lot better than I did (no surprise there!)--it is a
"butterfly" plug, although a very small one (the 'wings' are a lot tinier than
the aftermarket butterfly drain plugs I'm familiar with).

I'm relieved to know it should be OK to loosen. It seems like it's pressing
against a flange of the radiator support, but that may just be a result of
my looking up at it while flat on my back. So I'll go ahead and give it a
shot. Just to be on safety's side, I'm heading to the Chevy dealer
tomorrow to see if I can order an OEM replacement to have on hand..."just
in case"!

Thanks, as always, for your quick and on-the-money answer...hope
you're feeling better now that you've had a little time to reflect on the job
situation. Best wishes to you on that front--I know it's not easy.

My 78s have (had) a brass plug, no petcock. I've replaced them with brass petcocks that I can attach a hose to. The plastic plugs seem like something more recently in use (wife's 98 Malibu has plastic, a green plug with a tab to grip with a pliers) but not sure.  Mike

Thanx for that info, Mike!    I wasn't sure on the later cars.

My '74 also has a brass plug, no petcock, from the factory. I wound up ripping the whole fitting out the one time I tried to drain the rad.    I just took it to a rad shop and had them resolder the fitting.

Thanks for that additional info, Mike. A while back I actually picked up a
brass drain plug at a local auto parts store. It was listed as being a
replacement for GM cars up through 1975, and I figured it might just
work for later versions as well. But when I got the car lifted and got a
closer look, I saw the (apparently) smaller diameter plastic plug that's in
there now. So maybe I have an aftermarket radiator after all...

In any event, I'm going to try to loosen the plug--hopefully without
breaking it off!--and drain the cruddy contents out; replace the upper
and lower radiator hoses; back flush the system, and then button it all up
again.

Fingers crossed...

If you have a plastic plug with different size threads sounds like you indeed have an aftermarket radiator. If you put the plastic plug back in make sure you put a little silicone around the seal and don't over tighten. Thread tape might not be a bad idea either. Terry

Funny coincidence, Terry--for no good reason whatsoever this morning I
was thinking about wrapping the plug in Teflon plumber's tape when I
reinsert it...so your advice came along at just the right time! Thanks for
sending it in.

FYI for any readers of this thread, current and future, who might be
interested in this saga, one of the better auto parts stores in the area had
three different sizes of drain plugs listed for a '78 Corvette. Since that
didn't help, I drove to the to the parts department at my local Chevy
dealer down the block from there, where they identified the correct plug
as 1/4"--but of course they no longer carry it in stock.

Armed with at least the accurate size information, I headed to the local
NAPA shop way across town--where they had plastic drain plugs that
looked 'close' to what's in the car now (I haven't taken the plug out yet
because I want to make sure I have a replacement in hand first before I
do). Problem was, there was no size identification on any of the
packages--so no good way to tell if it was 1/4" or not.

This little drain plug was proving to be more of a challenge to find than I
could have imagined.

Off to two Corvette parts places yesterday for a whole bunch of things.
Turned out neither of them had any drain plugs--but Corvette Clinic
believed that the plastic plug I described sounded like an original factory
item. The guy there suggested what others had as well--going to a
radiator repair shop.

Which is where I'm headed today. Two local shops. And maybe, just
maybe...I'll come home with an actual drain plug that fits.

Maybe.

Any advice on adding a plug? The radiator in the 72 has no such provision for draining. Pulling the lower hose is about as exciting as watching grass grow.