Engine Break-in Procedure (Installation check-out)
Note: Click 'Technical Stuff' on the left to go back.

To break-in an engine means to introduce all of the new moving parts to each other in such a manner that the contours of the metal matrix of the new mating surfaces form a compatible relationship with the adjacent surface. This relationship provides the least amount of wear with the best interaction of the two surfaces.

Engine (or cylinder) break-in concerns itself, primarily, with the seating of the piston rings.  When piston rings are replaced at a engine overhaul, top overhaul, or single cylinder overhaul, it becomes necessary to break-in the new rings and cylinders.  Engine or cylinder break-in is nothing more than the physical mating of the engine's piston rings to it's corresponding cylinder wall. That is, we want to physically wear the new piston rings into the cylinder wall until a compatible seal between the two is achieved.

In steel, nitrided, or Cermi-Nil cylinders, the rings are the harder of the two surfaces. The objective is to create combustion chamber pressures high enough to force the rings against the cylinder walls and wear into a good (great) sealing of the rings to the cylinders. We need to break-in the ring to cylinder mating surface so that the established mating surface provides the best seating without excessivley wearing the surfaces or glazing the cylinders.

Installation Checkout
Engine Test Runs
Flight Test
Post Flight


1. Install engine in aircraft making all connections
2. Check operation of all engine controls for full travel and proper operation
3. Check torque of engine lift eye hook bolts, use torque wrench. Torque to 8 ft/lbs
4. Service aircraft battery
5. Install the cowling. Running without the cowling could overheat and damage new cylinders.
6. Flush fuel system. (Prior to connecting fuel feed line to engine)

a. Use a clean 5 gallon bucket, hang engine fuel feed line in bucket.
b. Turn on electric boost pump.
c. Drain approximately one gallon from each tank.
d. Check fuel in bucket for contamination.

7. Connect fuel line to engine, do not turn fuel on

Before Test Running the engine:

1. Remove lower spark plug leads and spark plugs.
2. Service engine with proper amount of Aero Shell straight weight non-detergent mineral oil.

NOTE: It is common practice to use non-detergent and mineral oil during the "BREAK-IN" period because 'in-the-olden-days' that's how it was done.  It was felt that the rings would seat quicker without the film strength additives. More recently, there has been a trend to high speed and high temperature engines, cam lobe and tappet loads also have increased to a point where it is important to use heavy duty oils which contain an EP (high pressure) additive right from the start. Rings will seat properly when moderate loads are applied as noted in the flight test section.


Comment: I personally don't use mineral oil for break-in. Lycoming recommends an Ashless Dispersant oil from the start in turbo-charged engines because the compression pressure (manifold pressure) can be kept higher. Why not break-in all engines at higher than 65%-75% power?* In the 35 years that I've been building engines, automotive, marine, motorcycles and aircraft, I've never run a mineral oil. I think it's a myth. I use AeroShell 15w-50 from the very first start. Using the procedures I outline here, I've never had an engine take longer than 20 hrs to break-in. My experience has been stabilized temperatures in less than 10 hours.

*By-the-way, TCM recommends breaking in the engine at as high a manifold pressure as possible.

3. Use the proper weight oil based on air temperatures and time of year. It is preferable to fill engine with WARM oil.
. . This will lower its viscosity and insures that the oil filter bypass valve will not open up on initial engine start.

NOTE: Let's face it, if you use a multi-grade oil from the start, the temperature of the oil will not be an issue.
Make your own decisions. I base mine on experience and not "Because, that's the way we've always done it."

4. Pre-oil engine by turning the engine over using aircraft starter.  Keep to less than 30 seconds.  Let cool.

a. During the first 30 seconds of engine spin up, you should get an indication of
. . oil pressure on the oil pressure gauge.
b. Spin the engine up three more times after getting oil pressure indication.
c. Allow short cool down periods for the starter motor
d. Check Oil level.   Add oil as required to bring it to full capacity minus one quart.

. . Note. It may require up to 3 quarts to fill the oil filter, oil cooler, and oil lines

5. Check for oil leaks
6. Install spark plugs and ignition leads
7. Install fuel line.  Turn on boost pump and check for leaks.
8. Check priming system for leaks

Initial Engine Start and Check-out:

1. The engine cowling should not be removed for any test run. Adverse cylinder heating may occur.
2. Begin engine test run as soon as possible while the oil is warm
3. Face aircraft into the wind
4. Position an observer to monitor engine for oil and fuel leaks with fire extinguisher at hand
5. Just crack throttle open! We do not want engine to start at high RPM at all
6. Prime engine as little as possible, usually two or three strokes of prime.
. .Note: On fuel injected engines, prime enough to establish fuel flow.
7. Start engine, adjust throttle so that engine idles at 800 RPM
8. Check for proper oil pressure indication within 30 seconds.
. . If there is no oil pressure within 30 seconds, shut engine down immediately and determine the cause
9. Verify throttle and mixture operation.
10. Shutdown the engine.  Check for leaks.

 I want to comment on items 5 and 6 above. 

It is common practice to crack the throttle and prime the engine prior to starting the engine.  A few of problems:  First, how big is a crack?  Second, how much is too much prime and how much is not enough?  Third, at most, you have only one valve open for that prime.  That means the rest is running down into the intake manifold.

I think it's a common misconception that the engine needs to start on the first blade.  If you think about it for a few seconds, item number 6 under "Before Test Running" says to pre-oil the engine by cranking it.  Why not do that every time?

I have a radical approach to starting the engine.  I let the engine decide when it's ready to start.  Well, sort of. 

1. Leave the throttle closed.  Engines like to draw their own air fuel ratio. 
2. DO NOT PRIME AT ALL.     I repeat, Do Not Prime The Engine.
3. When you are ready to start, keep the throttle closed and start cranking.
4. If it doesn't fire in a few (6) blades,
    rapidly open the throttle wide open and close the throttle while cranking.
5. The engine should start when you close the throttle.

There are a few engines that require two pumps of the throttle, especially when cold, before the engine starts.  There is no requirement to hurry the start.  Be patient.  It'll start.