Now that cold weather is here, I get asked quite often, “How long should I warm up my engine before I drive/fly off?”

As always, there is no simple answer.

Part of the reason we warm up our vehicles is to get the heater going. But there are technical reasons to warm up our cars and our planes before we take off.

The first is to get the oil warm enough to flow through all of the passages to properly lubricate the entire engine. However, the new multi-grade oils have really eliminated this as a concern.

The second has to do with the coefficient of thermal expansion of different metals.  Aluminum expands at a greater rate than iron. This means that aluminum pistons shrink in the iron cylinders, so at very cold temperatures, the piston to cylinder clearance increases.

When you start an engine at very cold temperatures, it can result in excessive piston “slap” until the pistons are warmed up and expand to properly fit in the cylinders. This “slap” can overcome the lubricants’ coating capabilities and result in metal to metal contact and, eventually, a scuffed piston. Therefore, it is important to not over rev or overload your engine until it warms up.

The guideline I use is to start my car, check the gauges, radio, mirrors, etc., put on my seat belt and then drive off.  But — and this is an important but — I drive off slowly, being careful not to over rev the engine.

If you are pulling a heavy load behind your pickup, you need to warm the engine up longer. Also, if you have a diesel, you will want to warm the engine up longer to ensure that the fuel filter housing is warm to prevent fuel gelling.

But this is an aviation magazine, so how long should you warm up your aircraft engine?

With an aircraft engine we have another problem. It goes back to the coefficient of thermal expansion thing.

In an aircraft engine, the aluminum crank case shrinks around the iron crankshaft to reduce the bearing clearance. A few years ago, the people at Tanis Aircraft measured a crankshaft and aluminum case. They then rechecked the measurements after the engine sat outside all night at -25°F and they found that the clearance had disappeared.

So when you pre-heat an aircraft engine, it is not only to get the oil thin, but also to expand the crankcase so that there is adequate clearance in the main bearings so that oil can be pumped into the cavity to provide hydro-dynamic lubrication. Otherwise you will experience metal to metal contact, which will greatly reduce the life of the soft bearing material.

The big concern here is in the front main bearing, which carries the primary load of the propeller. This bearing is in the nose of the engine, which is the most exposed to the outside air and is usually least affected by the pre-heat process.

This is especially critical when using a crankcase heater. If you have a crankcase heater without individual cylinder heaters and a cowl cover, the chances are that on a very cold morning the nose section of the engine will not be heated sufficiently and the clearance in the front bearing may be inadequate. This can result in metal to metal contact for the first several revolutions of the crankshaft — which, in turn, can result in increased wear and decreased life of the front main bearing.

So always pre-heat adequately, especially at temperatures below 0°F, and make sure that the nose of the crankcase is properly heated.

Once your engine is started, how long you need to idle it really depends on how long of a taxi you will be making.  If you have a long taxi prior to your run up and takeoff, then you can start taxiing shortly after you get the engine stabilized and all of your engine checks completed.  If you have a short taxi, then I would idle your engine for a period.  The rule that I generally use is that once the oil temperature starts moving up significantly, your engine should be ready to takeoff.

Cold starts are bad for your engine’s health, but you can minimize the negative affects with some common sense and care.