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Old 09-17-2010, 12:15 PM
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Join Date: Dec 2006
Location: Chicago, IL
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Drain Coolant from Engine Block on 107, Where's the Drain Plug (screw)

I have a 78 450sl, I want to drain the coolant from the engine block as part of my radiator flush (doing a citric acid flush).

Does anyone have a picture of where the engine block drain plug is for the coolant?

Draining the radiator is easy, but coolant/flush fluid remains in the engine block so I want to get it out.


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Old 09-18-2010, 11:42 AM
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Location: Ocean Isle Beach, NC
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There are two, one on each side. Might be tough to break loose, but it's worth giving it a try. I can't remember if they are regular hex bolts of allens. The service manual shows a photo of one of them - looks like a hex.
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Old 09-19-2010, 05:53 PM
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good luck, could not budge them on my 72
I opted to take off hoses and junction at block, used hose full foce through that until it ran clear, good time to renew the hoses , the small connector hose and tstat
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Old 09-20-2010, 02:00 AM
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Couldn't get mine out, and mine are allen.

When I put the 3.5 heads onto my 4.5 I used a garden sprayer as a "Pressure washer" to hose down the inside of the block & I used a 1/2" ID hose hooked to a Shop Vac to suck the water out. Worked very well and my engine ran MUCH cooler after this. Now, taking your heads off to flush the block is probably a bit extreme, so do what Ron did & hook up a garden hose until it runs clear!

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