Probably one of the most argued about topics in engine theory you'll ever come across. Everyone has their own opinion ranging from the 'give it death from the start' to treat it with kid gloves for thousands of miles before using it hard. In fact running in an engine is not a single process. The various internal components take different times to reach full efficiency and require different approaches to make sure they bed in properly.

The first item to consider is the oil used. In years gone by you could buy running in oil which was basically very cheap oil with low friction reducing characteristics. When engines were hand carved out of solid blocks of cast iron by workmen in flat caps using hand files and paid a shilling a month this might have been a good idea. Nowadays any half decent engine machining should be of a high enough quality that comparatively few high spots actually need to be worn off the various mating components. Using a fully synthetic oil from the start is generally considered to be a bad idea because it needs some metal to metal contact to bed things in but component loadings are so high on modern high output engines that a very cheap oil is a false economy. My advice is to use a medium to high quality non synthetic oil and the best oil filter you can buy. The higher the engine output the better the running in oil you should use. Turbo engines generate very high temperatures in the turbocharger and the last thing you want is something that will bake into carbon the first time you start the engine up.

Next thing to consider is running in the camshaft. This will mainly be applicable to pushrod engines or those using overhead cams and rockers. OHC or DOHC engines with flat faced buckets under the cam lobes are much less prone to cam wear problems. 15 minutes at 2500 rpm with the car on the driveway is the way to run cams in. The main thing is never let them idle for the first few minutes. All this is considered in more detail in the article on running in cams.

Now you're ready to take the car out on the road and bed the piston rings in. To do this requires several brief applications of full throttle in a high gear to generate high cylinder pressures and force the rings against the bore walls. Put the car into 4th or 5th at 1500 to 2000 rpm and apply full throttle for about 10 seconds. Coast along for 30 seconds to dissipate any heat generated and repeat. Do this ten times. It should take about 10 minutes and maybe 5 miles if you don't have too many other cars up your chuff trying to get past.

By now you've already worn off more than 90% of the high spots on the various bits that contact each other. However friction levels inside the engine are still fairly high compared to what they'll eventually settle down to. The next stage is to gradually build up throttle usage and rpm limits. This can take place much faster than many people realise. Drive for 50 to 100 miles with gradually increasing throttle usage and rpm. By the end of this time you should be using full throttle and high rpm for brief periods provided fueling and ignition settings are already optimised. In fact on the road it will be hard to hurt the engine because you'll have to back off for a corner, speed camera or plod car long before you melt the engine. On the track, or maybe on a motorway you might be able to use the engine hard enough to hurt it in the first few miles.

Listen to the engine. Does it feel tight and not want to rev or does it feel free? Hopefully it should sound quite happy by now.

99% of the high spots are now worn off after 100 miles and it's time to change the oil and filter again. Use a high quality non synthetic oil. You can leave this in for either another 1000 miles or until your normal oil change interval. At that point swap to your long term oil which might be a fully synthetic if the engine and your pocket warrants it. The engine should be happy to take pretty much whatever you can throw at it after 100 miles if it's been built right. What you might want to consider is that by the time you've pottered about with your new engine for 3 hours (that's probably at least 100 miles) at say an average of only 3000 rpm it's already done over half a million revolutions! Yes I did say half a million. 3 hours x 60 = 180 minutes x 3000 revs per minute = 540,000. How many times do you think one component with a miniscule high spot has to operate against another component before the high spot gets worn down?

If the engine has been modified it might require alterations to the fueling or ignition timing. Don't use it too hard yet if there's a possibility these might be way out. Get it to a rolling road and have it set up properly.

Over the next 1000 or more miles the friction levels inside the engine will continue to fall as the bores glaze up. The harder you use the engine the faster this will happen. Drive like your granny and it might take 5000 miles before the engine is fully loose. It won't blow up because it's not fully loose if you use it hard but the power will continue to increase as the friction levels drop. Also the combustion chambers carbon up which raises the compression ratio slightly and improves thermal efficiency as the carbon acts as an insulator. You might see another 2% or 3% power once the engine is really loose compared to the first run on the dyno after the initial break in period.

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