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Tightening head bolts

By Mike Allen
November 9, 2008
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Q. Years ago a mechanical engineer told me the best way to adjust your car's ale for maximum fuel economy is to turn the temperature dial up and leave the fan on high. Logic says the fan takes less energy to run than the compressor and, by turning the temperature up, the compressor doesn't have to work as hard.

But I'm now thinking that, when you turn the temperature up, hot air from the engine gets mixed in with the cold air from the ale. So is the compressor either on or off, or is it variable?

A. Second question first: Some cars simply clutch the compressor on and off constantly to maintain the correct refrigerant pressure and, consequently, the appropriate cooling. Others use variable-displacement compressors, and still others use a throttling valve in the refrigerant line.

Some vehicles, especially ones with automatic temperature controls, use air heated in the heater core to blend with cold air from the evaporator to control the temperature of the outlet air. If that sounds inefficient, it is.

Cooling your car with the fan uses less fuel than cooling it with compressor-driven cold air. So leaving the fan motor high should use less energy. In the big picture, I really don't think it will make a huge difference.

Q. I decided to replace the head gasket on my Ford Escort. When I went to the auto-parts store, the counterman insisted I needed new head bolts. I think he's trying to punch up the bill by $30.

A. I checked the price of your head bolt set, and the $30 you quoted seems reasonable. Most parts countermen don't work on commission, so this guy really is giving you the proper advice.

Your Escort, like many modern vehicles, uses a different style of head bolt than we're used to. In the good old days, a head bolt, like most critical fasteners, was gradually tightened in two or three stages to the proper torque. Using a torque wrench permitted the clamping force between the head and the block to compress the head gasket enough to seal it without warping it.

Then came aluminum-alloy heads, bolted to cast-iron blocks. Since these are different metals, the block and the head expand and contract at different rates, making the head gasket's job far more difficult. Problem is, conventional torquing isn't precise enough. Tightening a bolt stretches it, while pulling the head and block together. These bolts elastically deform and return to their original dimensions when loosened. Friction between the bolt head, its washer, and the cylinder head isn't consistent. Two bolts tightened to the same torque may have widely different clamping force.

Your one-use-only head bolts are meant to be tightened to a different criterion - specifically, beyond their plastic-deformation point, the point at which they stretch and don't return to their original length when loosened.

These head bolts need to be tightened in a very different fashion. Normally you would tighten conventional head bolts in a crisscross pattern in three steps, to one-third, two-thirds, and then to the final torque. Stretch bolts are tightened in a torque-plus-angle sequence.

Various engines use similar but different sequences and specification. Here's the procedure for your particular Escort.

Initially tighten all the bolts, in the sequence specified in the shop manual, to 30 to 44 foot-pounds. Next, loosen them in the same sequence by one-half a turn. Again tighten them, in sequence, to 30 to 44 foot-pounds. Next, turn all the bolts, again in sequence, by 90 degrees. Then turn them all yet again, in sequence, a final 90 degrees.

One last note: I'd have someone who knows Escort cylinder heads take a look at yours. If it's warped, it'll simply blow the head gasket again in short order.

Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com.

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