How to break in your built Engine

AMSOil Break In Oil for MX5 Miata

Everyone has a different opinion on the correct way to break in a new engine. Some suggest a long and slow period of use with frequent oil changes while others contest that a break in period may not even be necessary! While we don’t claim to know which way is the correct way, we will share our engine break in procedure, here at BOFI.

What Oil should I run?

We recommend using either a conventional mineral oil or a dedicated break in oil to bed the rings and to put some initial miles on the engine, depending on the value of your build we’d always use a dedicated oil.

This oil’s job will last for less than an hour and its job is to seat the rings, using a dedicated break in oil or mineral oil allows the required amount of friction in the bores, your engine must be built with Mineral oil, ATF or break in oil lubricating the bores during assembly, synthetic is too slippery for this job.

After that initial run, drop that oil which is now full of metal shavings from the initial rings bedding in and refill using the same oil for the main break in miles (4-500) and then change it for the appropriate fully synthetic stuff. The final oil weights you require will depend on your oil clearances – typically a looser engine would benefit from a heavier weight oil and a tighter one would need a thinner oil.

Alright, lets get into it

Nearly every component in the engine is suspended on a film of oil to prevent metal to metal contact and lubricate everything that needs to move. The oil film suspended nature of the engine means the only parts that really need bedding in are the rings as they are making contact with the cylinder bores under their own spring tension. As the engine turns over, they (for the lack of a better word) scrape up and down the bores forming a seal to keep the compressed mixture in the combustion chamber. A freshly honed bore has a visually pleasing crosshatch pattern. This pattern provides a slightly roughened up surface for the rings to file away and conform to – I know fresh bores feel smooth, but like an egg they’re actually a rough surface when seen under magnification.

This roughened surface means a newly rebuilt engine rarely makes its final compression numbers until it has been run for a few miles to give the rings chance to form a proper seal. This is why we consider the first start up and drive to be the most important. Under no circumstances should it be allowed to idle for an excessive length of time. In fact, we consider it best to start up the engine for the first time when you are ready to bleed the cooling system and check for leaks. When that is done and the engine is warm – usually only a couple of minutes – take it out immediately for the first drive and get the rings seated.

Now that you’re fully prepared for the break in process, find a quiet stretch of road and perform the following:

  • 2 medium throttle pulls to about 4000 rpm and lift off letting it coast back down to around 20mph / 1500 rpm
  • 2 medium throttle pulls to about 5000rpm and lift off letting it coast down to around 20mph / 1500 rpm
  • 2 hard throttle pulls to about 6000rpm and lift off allowing it to coast back down to about 20mp / 1500 rpm
  • 1 hard throttle pull to 6500rpm and coast back down again
  • drive around for a bit to cool it off and keep an eye on the vitals. Complete about 20-30 miles and take it back home to change the oil and filter.

If  the engine you’re bedding in is boosted, avoid entering boost as much as possible. This may require a MUCH lighter touch on the throttle.

The purpose of all that coasting is to pull high vacuum in the cylinders. This will force the rings outwards and into the cylinder walls and should result in an even wear and great compression numbers.

The second oil change should use the same conventional oil used to bed the rings and the car should be driven sensibly, avoiding high RPM and excessive time running at a constant speed (eg. a motorway). After about 500 miles (usually the recommended clutch break in period), change the oil and filter again using whatever oil you want to run – typically we recommend a full synthetic of appropriate weight for your new engine.

Now go forth and rag the hell out of it, you’ve earned it.

 

2 comments

  1. Hi, I have engine rebuild by shop, I installed turbo kit on it. Problem is, I will be on base map on first start, so should i follow this bedding in procedure with light throttle, and monitor afr, or should I just start the car, let it run for few minutes, see if nothing leaks, and ask the tuner to do this procedure on dyno instead? “The second oil change should use the same conventional oil used to bed the rings” Does this mean, that if I used 10w40 running in oil, I should use 10w40 synthetic? Thanks in advance, Daniel

    1. Daniel Marshall

      Hi Daniel,

      I would recommend you have the car recovered to the dyno or have the tuner to come out to you. The issue and risk is if the car doesn’t start, you wash the bores of their limited oil film. If you’re confident in being able to get the car to start and then subsequently can set the car up to target a leaner mix than normal to perform the break in, go for it.

      The aim is to get the car started quickly, with AFRs in the 13-15 range, idle for 5 minutes to bleed/ get up to temp and check for leaks. Then go out and do the procedure while maintaining that leaner mixture. It isn’t the end of the world if its richer, it isn’t going to outright break the engine but It can and will impact how long it takes to bed in and or the final leakdown numbers.

      You should generally use a 10w-40 Fully Synthetic, we like Millers CFS.

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