I rounded off the long-suffering hex head on my engine block coolant drain plug, so I ordered a replacement. When I got the new one it became evident why the head rounded off... the plug appears to be made of aluminum.

I FINALLY got the old plug off turning it with a vice grips. It had really jammed in there and for a while I had my doubts whether it was going to come out or break off in there.

The plug was so corroded that I had to use very strong pressure almost all the way out. It was complicated by tight quarters, I was only able to get about 30 degrees of movement per clamping with the vice grips, and with hot coolant dripping on my face of course. Took me nearly an hour.

The threads on the old one are all mashed up, with corrosion evident. It left behind some shreds of aluminum in the engine block hole.

I flushed out the hole with a garden hose feeding into engine block from the other side to get the aluminum shreds out. Then I tried to scrape the threads clean using a little Dremel wire wheel by hand, while simultaneously flushing water through to keep any debris from going back into the block (and onto my face instead).

But... when I tried threading the new plug in, it still only went in about half a turn before starting to get hard to turn.

Should I just crank that puppy in anyway, i.e. is it supposed to turn "hard"? The threads on the new plug seem very tall and sharp, so maybe they just get compressed down to fit? Maybe a little anti-seize on the threads would help?

Or should I buy a (big) tap of appropriate size, and try to clean the hole out that way? I'm not sure if that would work since the point of a tap is usually quite tapered, i.e. I may not be able to screw it in far enough to engage the full diameter of the tap into the threads. Maybe I could snap the tapered end off the tap if I had to, but given the size of the tap it would probably be a $20 experiment. And I'm also worried I might screw up the threads given the tight and awkward area I'm working in.

Another thought I had was to get a big metric bolt and thread that it in and out a few times, maybe with some penetrating lube on it to try to break up any crust in there.

Any other thoughts appreciated, the sooner the better... obviously my car isn't going anywhere with a hole in the engine.

Old and new bolts photo below.

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1992 300E Sedan (Sold)
1999 E320 Wagon (Sold)
1995 E320 Sedan
1995 E320 Wagon

if you do find a metric bolt that size use a grinder and cut some slits in the threads like you would find on a tap, and see if that would clean the threads for you.

I'd probably opt for a bottoming tap which is flat on the bottom. I'd be worried that a bolt woold just screw up the threads some more.

Len

Ah... bottoming tap. Learn something new every day! Of course the odds of finding that locally in a jumbo metric size in my small midwest city... I should probably just buy a lotto ticket instead and buy a new car. I'll give it a try though.

Otherwise I'll try the poor man's version. Grinding grooves in a bolt sounds like an excellent idea over an unmodified bolt.

Thanks for the ideas.

__________________
1992 300E Sedan (Sold)
1999 E320 Wagon (Sold)
1995 E320 Sedan
1995 E320 Wagon

A Graingers, http://www.grainger.com/Grainger/wwg/start.shtml, which you might have locally or Maryland Metric, http://mdmetric.com/, would probably have it to you in a couple of days.

Len

Thanks again Len... wonder-of-wonders, I did find an appropriate size tap, although not a bottoming tap. I was prepared to grind off the tapered part to give me better penetration. And yes I know I'd likely ruin the temper but I figured it would last long enough for my mild thread-cleaning job. Cost about $12.

I was NOT able to find a bolt. It's a bizarre size... 18x1.5. Most bolts of that size are x2.0 or coarser threads (I'm not sure why they had a tap of the correct size hanging around). I did get a smaller x1.5 bolt that I was contemplating running around inside the hole in an attempt to scrape out the threads. Cost 80 cents.

I was leaving the store when I had a flash of inspiration, back in and with a helpful clerk and an around-the-store tour, also came away with a copper pipe fitting brush, which is a stiff steel cylindrical brush. Cost about $3.

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__________________
1992 300E Sedan (Sold)
1999 E320 Wagon (Sold)
1995 E320 Sedan
1995 E320 Wagon

Figured I'd try the brush first, after a quick mod to give me clearance. Screwed it in and out of the bolt hole a couple of times and flushed.

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__________________
1992 300E Sedan (Sold)
1999 E320 Wagon (Sold)
1995 E320 Sedan
1995 E320 Wagon

And... the gods of frustration were apparently occupied elsewhere... I coated the threads of the plug with anti-seize, took a deep breath, and it THREADED RIGHT IN with hand pressure. If anything, almost a little too easy, if I wanted to get paranoid about it.

Aaaaah... did you ever see a prettier picture?

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__________________
1992 300E Sedan (Sold)
1999 E320 Wagon (Sold)
1995 E320 Sedan
1995 E320 Wagon

That IS nice.

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Norm in NJ
Next oil change at 230,000miles