Glow Plugs, Part 2

by Ben N on July 16, 2014

When last I left you, I had an engine with the glow plugs removed, and the remaining holes gunked full of carbon. Everything I’ve read so far strongly suggests reaming out the carbon when replacing glow plugs.

I had looked online and found that a specialty reamer tool for glow-plug holes were over $50! I just want to clean out these holes, not clean out my pockets at the same time!

The next day, I was over at the auto parts store (for something completely unrelated) and saw that they had a set of five screw extractors for $14. I don’t already have any screw extractors and thought to myself “What are the odds that one of these fits the 240D glow-plug holes?”. So, I spent the money and excitedly hurried home to try it out.

DSC_2219Sure enough, I took some measurements of an old glow plug, a new glow plug, and the two larger screw extractors of the five in the set. It looked the the largest (#5) was just a hair bigger diameter than the old style glow-plug – just perfect for reaming out the hole.

The first hole reamed-out just fine. I was able to put the #5 screw extractor straight in the glow-plug hole. I easily turned it by hand, and saw black flecks of carbon dust get pulled out. The next one was a little more difficult, and I didn’t have a good handle on the reamer. I did notice that it was the same size square connection as a medium socket set. So, I got out a male hex socket (which happened to be 10 mm) and then put a regular 10mm socket onto that, and lastly connected a socket wrench. I now had a reamer connected to a nice big socket wrench handle and I could easily wrench left while applying a little pressure to push the reamer into the hole. Who needs to spend $50 to do that?

I reamed out the other two holes and then finished the job by putting a long, skinny air nozzle into each hole, and applying compressed air. Blasts of black carbon dust shot out like tiny mortar rounds, leaving only a clean aperture behind.

I was now extremely confident that the glow plug holes were in prime condition for the new plugs.

DSC_2223I applied a little bit of anti-seize to the threads of one of the new glow-plugs and started threading it in by hand. Oddly, the plug stopped turning before it was all the way to the engine block. Confused, I once again compared the old and the new and noticed that the threads on the new plugs did NOT extend as far as they did on the old. It looked like the new plugs were NOT designed to screw all the way in to the engine block. They would instead extend out from it just a bit.

After running all four of the spark-plugs in, I set to wiring them up. This was pretty darn easy. All I did was put a short, heavy wire from each spark plug to the next. I found my purple-and-white wire that went to the original glow-plug relay connection, and hooked that up as well, even though it wasn’t going to a switch or battery.

DSC_2227After finishing installing the new glow plugs, I reinstalled the fuel hard-lines. I love the way these look. Kinda steam-punk, only genuine awesomeness instead of faux-Victorian imaginary!

Before starting this all, I also tested one of the new glow plugs to see how much current it would draw. I simply connected it to a known good 12V battery and completed the circuit through my multimeter, set to the DC Amps setting. The meter read about eight and a half. I figure that if each plug draws up to ten amps, I’m going to need a 40-amp fuse and relay to run the glow plugs.

So there you have it, fancy new pencil-style parallel-wired glow-plugs in a four cylinder engine from a 1976 Mercedes 240D. This “Super-Truck” project has been a slow one, but it really feels good to be chipping away at it and learning as I go.

Til next time, stay charged up!


{ 5 comments… read them below or add one }

1 Bob July 16, 2014 at 10:00 pm

Hey Ben, are you using the Mercedes engine as a part for a truck, or are you making the Mercedes a hybrid, because you call this project “super truck”?

2 Ben N July 17, 2014 at 9:20 am

I just have the engine, not the car. This DOES make it easier to work on the engine without having a car in the way! My master plan is to combine the engine from the 240D with a big DC motor and install them in a light pickup truck. That way, I can run on diesel/biofuel/electricity. I’ll be able to tow and haul, and still make trips into town without even turning on an engine! See my concept at:

3 John John July 17, 2014 at 4:31 pm

Hey ben,

SInce you are pretty handy with things, I was wondering if you could make your next project a solar steam generator, where you put a solar panel that concentrates heat onto a pipe where water is flowing, and that creates high pressured steam, and that can be used to power a medium sized turbine which can create lots of power. This would be a really big step in affordable efficient solar power.

4 Ray November 15, 2017 at 10:41 am

Excellent write up and photos,Ben.Will try this out. Just concerned about blowing air into the plug hole and carbon fragments going further in.
Any advice ?
Regards Ray ( South Africa)

5 admin November 15, 2017 at 11:25 am

Just blow the air really good BEFORE pulling the old plugs out then. No dust or anything can get inside the cylinder while the plugs are still in it.

Leave a Comment

You can use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

Previous post:

Next post: