The Ford Barn - View Single Post - What is Engine 'Break-in'??

BRENT in 10-uh-C · 03-04-2021, 06:27 PM

I engaged in a small post on social media where several people were recommending to a hobbyist that a rebuilt engine needs to go thru a 'break-in', ..and (more emphatically) during that time, the engine RPMs needs to be varied (-from idle to up & down the RPM range.). I asked "why" does this all need to happen, ...and no one could seem to give me an (intelligent) answer other than auto manufacturers suggested this.

So I'll ask here where I know the responses to my question will be much more accurate!!

Let me start by giving my theory as to why their advice is a bunch of nonsense on 21st century Model-A engine rebuilds;

The term Engine 'break-in' is really just an old phrase justifying the reasoning for allowing an engine to use internal friction of mismatched components to prematurely wear to a specification instead of using proper machining methods to obtain that desired clearance specification. (See Point #4 below)
The engine RPM does not need to be varied, -and it is likely preferable to run at a single RPM with enough RPM speed to provide splash oiling that all parts receive a constant coating. Altering this engine speed does nothing to increase longevity or better ring sealing on a properly machined & assembled engine.
After initial start-up of a rebuilt engine, the engine only needs to run long enough where various metals and gaskets can thermally expand. If the deck surface of the cylinder case and the cylinder head have a good RA finish, -and if the head gasket is of quality, then upon the components cooled to room temperature, the stud nuts can be retightened to compensate for any gasket compression. The (mis)use of gasket sealers (Copper Kote, et/al) often hinders the thermal expansion & contraction of the gasket which will affect fastener torque ratings being met.
Use of inferior machining techniques and/or worn machines causes cylinder bores to not be truly cylindrical. The use of 220 grit Diamond stones at roughly a 35° cross-hash will produce a finish that a cast iron ring can easily seal to. When poor quality machines and/or stones are used, the concentricity of the bore is compromised creating a need for the rings to scrub against that cross-hashing longer so the ring(s) can shape themselves to conform to the non-concentric shape of the cylinder bore. Additionally, a properly burnished cast bearing does not need additional run time for it become 'fitted' to a crankshaft journal pin. If a rebuilt engine requires a 'run-in' period for the bearings to clearance themselves, then either poor machining practices or machines themselves (crankshaft grinder or line-boring) were used.

The above are just my opinions, ...and I welcome theories that either corroborate or contradict my reasoning for Model-A engines not needing a rebuilt engine break-in period with the machining technology of today.

03-04-2021, 06:27 PM	#1
BRENT in 10-uh-C Senior Member Join Date: May 2010 Location: Eastern Tennessee Posts: 11,508	What is Engine 'Break-in'?? I engaged in a small post on social media where several people were recommending to a hobbyist that a rebuilt engine needs to go thru a 'break-in', ..and (more emphatically) during that time, the engine RPMs needs to be varied (-from idle to up & down the RPM range.). I asked "why" does this all need to happen, ...and no one could seem to give me an (intelligent) answer other than auto manufacturers suggested this. So I'll ask here where I know the responses to my question will be much more accurate!! Let me start by giving my theory as to why their advice is a bunch of nonsense on 21st century Model-A engine rebuilds; The term Engine 'break-in' is really just an old phrase justifying the reasoning for allowing an engine to use internal friction of mismatched components to prematurely wear to a specification instead of using proper machining methods to obtain that desired clearance specification. (See Point #4 below) The engine RPM does not need to be varied, -and it is likely preferable to run at a single RPM with enough RPM speed to provide splash oiling that all parts receive a constant coating. Altering this engine speed does nothing to increase longevity or better ring sealing on a properly machined & assembled engine. After initial start-up of a rebuilt engine, the engine only needs to run long enough where various metals and gaskets can thermally expand. If the deck surface of the cylinder case and the cylinder head have a good RA finish, -and if the head gasket is of quality, then upon the components cooled to room temperature, the stud nuts can be retightened to compensate for any gasket compression. The (mis)use of gasket sealers (Copper Kote, et/al) often hinders the thermal expansion & contraction of the gasket which will affect fastener torque ratings being met. Use of inferior machining techniques and/or worn machines causes cylinder bores to not be truly cylindrical. The use of 220 grit Diamond stones at roughly a 35° cross-hash will produce a finish that a cast iron ring can easily seal to. When poor quality machines and/or stones are used, the concentricity of the bore is compromised creating a need for the rings to scrub against that cross-hashing longer so the ring(s) can shape themselves to conform to the non-concentric shape of the cylinder bore. Additionally, a properly burnished cast bearing does not need additional run time for it become 'fitted' to a crankshaft journal pin. If a rebuilt engine requires a 'run-in' period for the bearings to clearance themselves, then either poor machining practices or machines themselves (crankshaft grinder or line-boring) were used. The above are just my opinions, ...and I welcome theories that either corroborate or contradict my reasoning for Model-A engines not needing a rebuilt engine break-in period with the machining technology of today. __________________ . BRENT in 10-uh-C . www.model-a-ford.com ...(...Finally Updated!! ) .