Plug This!

(c) 2003 J. Sage Schreiner

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As I came off the track from my fun, but soaking wet, race at Mission BC, I had a serious problem to solve.

 

In the paddock, I just couldn't figure out what was wrong – every time I turned the engine on, the coolant temp looked normal, but the oil temperature would start to peg. Finally, I pulled the radiator cap off – and discovered there was no coolant. At all. No coolant means that the coolant temperature gauge is pretty much worthless, since it’s just measuring warm air. My girlfriend / crew chief poured water in the radiator while I looked for the leak. The hoses all seemed fine, but I was surprised to find that the coolant was gushing out of the block. It’s not supposed to do that. Really.

 

A plug had popped out of the engine block, apparently at the end of the race, and all the coolant went with it. The oil temperature hit 280 degrees, and while that is extremely hot, I kept my fingers crossed that the engine was still sound. Worst case, losing coolant can utterly destroy an engine, or perhaps "only" warp the head. If I was really lucky, I might have escaped with no engine damage at all. The synthetic motor oil in the engine doesn't break down until well over 300 degrees, so even if the motor gets very hot, it may still be sufficiently lubricated to avoid the most serious damage. In order to avoid any further damage, Olona, myself and a volunteer pushed the car back onto my borrowed tow dolly.

 

After returning home, I called my engine builder. I didn't know the purpose of the plug, but he explained to me that a “freeze plug” works by releasing when engine coolant freezes and expands. Theoretically, this can help avoid damage to a car’s block. A freeze plug should never come out on its own. We discussed possibilities for what might have caused the problem and he laid out some next steps.

 

To make sure the engine was still sound, I did a full compression check on all cylinders. Essentially, a compression check uses a tool that looks a lot like any pressure gauge. It screws into the cylinder spark plug hole. Then, the engine is turned over electrically several times, and a pressure reading is taken off the gauge. If a cylinder’s PSI reading is very low, it indicates that it's not sealing properly. This is often because the seal between the head and the block is not complete, either because of a problem with the head gasket, a warped head, or because the valves aren't properly sealing the combustion chamber. In other words, a compression check is a good way to judge general engine health, although it's less useful for diagnosing specific problems.

 

All four compression readings were between 145 and 150 PSI. The Bentley manual suggests that  readings should be between 150-160 PSI, but the slightly lower readings were what I would expect for a loosely built race engine. More importantly all compression readings were reasonably even. If there was a problem, I would expect to see one or two cylinders with a substantially lower reading, below 100 PSI.

 

Since the engine seemed healthy the next step was to hammer in a new brass freeze plug. The engine has eight plugs, two for each cylinder. The plug on the right side of the engine, closest to the firewall, was the one that had popped out. It wasn't quite the hardest one to get to, but it was close. I had to jack up the car and remove the right front wheel. Even then, the exhaust manifold got in the way. The result was a difficult angle to access the plug as I tried to bang, err, “tap” it in with a hammer. The Bentley manual notes that It’s important to not touch the lip of the plug as it’s tapped into place, as that can distort it. Initially, I tried using a large socket that was about the same size as the plug, but because of the wacky access angle, the socket began to cut into the soft brass plug. Next, I used a wooden dowel as an extension. While I was able to get the plug about halfway in, I simply wasn't able to get it further. The angle for accessing the plug was too problematic. I stopped work for the night and slept on it.

 

The following day at work I chatted with a co-worker with a background in electrical engineering and materials science. He told me that the coefficient of thermal expansion of brass is much higher than that of iron. In other words, brass tends to expand more when you apply heat. Or, alternatively, shrink more with cold. This, I discovered, is why brass is used for so many different kinds of seals in the engine. As the engine heats, the quality of the seal improves. Most importantly for me, by applying cold, I could theoretically shrink the plug by enough to seat it a little easier.

 

On the way home from work, I purchased some dry ice. It worked beyond my wildest expectations, unfortunately. Using some tongs, I applied a small piece of dry ice to the half-inserted plug for about sixty seconds, until it was nice and cold. I gave it one solid tap with the dowel - hammer combination, and… Whoops! The dry ice was so effective that the plug went too far by several millimeters. Next time, I'll remember to use less dry ice. After asking a few folks, I came to the conclusion that as long as the plug sealed coolant into the engine, it wouldn't matter that it was jammed in a little too far.

 

As I wasn't quite sure what had caused the initial problem, I replaced the radiator cap. I figured if the radiator cap wasn't bleeding off pressure properly, that could have resulted in coolant system over-pressurizing and pushing the freeze plug out. I also replaced the thermostat because they're inexpensive and easy to replace and a possible a culprit for the system over pressurizing. As a just-in-case measure, I put a small tack of epoxy over each of the freeze plugs to help hold them in place.

 

I filled the engine with coolant, and ran it pretty hard – making sure the coolant got quite hot, and carefully monitored for leaks. There didn't appear to be any problems. I also made a point of carefully checking the oil for any sign of coolant, in case there was a problem with the head or head gasket that the compression check hadn't shown.

 

Happily, it looked like my engine was healthy – which was great, because the second to last race of the season at Portland International Raceway was coming up in a hurry. I was also hoping that the race suspension that I had ordered in July from Ireland Engineering would finally show up, as I had just enough time to get it on the car. Having stiff, low, adjustable race suspension was a major go-fast modification for my car.

 

“Just enough time,” in my experience, is another way of saying, “snowball’s chance in hell.”