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I am trying to get my 72 LT-1 ready for storage and in the process of draining the coolant from the block I noticed that the two drain plugs are a bit buggered up. These are 9/16" but my question is what is the thread size and is it a tapered plug? Will a plug from something as simple as Home Depot or the like fit? Since these are hidden behind the "boomerang" shields, I doubt anyone is going to look at them. Mine do not have any head markings but are definetly 9/16".
They are pipe threads - 1/4 NPT IIRC. Any hardware store pipe plug of the same thread size will be functionally correct. Use plumbers "pipe dope" on the threads. This will help prevent them from seizing in the future, and don't tighten them to more that about 10 lb-ft.
Use a six point socket to take the old ones out if the hex is bunged up.
I went to the auto parts store (not AutoZone) and purchased two in brass for the 71 after they were severely boogered up upon removal. Now they won't rust in place next time and will be much easier to remove.
Interestingly enough, the original ones on the 72 have come out easily both times I've done the coolant drain thing.
Patrick
Vice-Chairman (West), Michigan Chapter NCRS
71 "deer modified" coupe
72 5-Star Bowtie / Duntov coupe. https://www.flickr.com/photos/124695...57649252735124
2008 coupe
Available stickers: Engine suffix code, exhaust tips & mufflers, shocks, AIR diverter valve broadcast code.
I did not put my "boomerang shields" back on my 70 after removing the them to remove the drain plugs since you have to deal with motor mount and removing the starter. Wanted to have less trouble to change coolant and installed radiator drain cocks on each side. My 70 did not even come from the factory with a drain cock in the radiator. It has a plug just like the ones in the block, learned early that I had to back up the drain with another wrench to remove it, so it is not installed real tight anymore.
As long as it doesn't leak, it is tight enough. Anything more than what it takes to keep it from leaking will only cause removal problems "down the road".
A couple of year ago I did some research looking for "standard torque values" for pipe threads - like those listed for the various Grades/sizes of bolts and nuts.
There were none that I could find - the only advise being to "tighten enough not to leak" like Joe said.
I think most mechanics way overtighten pipe threads. If you use pipe thread dope (I always do) they only need enough torque to not loosen from vibration, which is not much, and pipe thread dope will go a long way in preventing seizing.
One other note. Many refer to pipe threads as "quarter-inch", etc. Pipe threads are dimensionless. Way back when someone invented a tapered pipe thread that was called "one-pipe". Successively smaller sizes were called "half-pipe", "quarter-pipe", etc. and this evolved into the National Pipe Thread standards, that are usually listed as 1-NPT, 1/2-NPT, 1/4-NPT, etc.
If you are old enough to have worked on steam locomotives or spent some time in a steam power plant I suppose there are also 1 1/2- NPT, 2-NPT, on up for the really serious steam pipes.
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