Which school of thought is right for breaking in engines - keep RPM below 3000 for the first 3000 miles, or rev up to redline?

Question

My friend has overhauled his engine recently and he sought my advice about the most efficient way of breaking in his new engine (Peugeot 405 1.8L Petrol).

It is usually said that you should keep RPM below 3000 in the first 3000 miles and avoid revving the engine to prevent damage. This is the traditional and the most prevalent belief about engine break-in.

However, there are some people who believe you should rev an engine up to redline during break-in period to let piston rings rub off the roughness of cylinder walls and seal better in the long run. They recommend revving a totally warmed up engine in short burst accelerations (like revving the engine in 2nd or 3rd gear and letting off the throttle immediately). They say if you don't rev your engine during break-in period, you will end up with lower compression (= less power and higher oil consumption) in the future.

These two schools of thought are completely opposing! One says rev it, the other says avoid it!
I don't know how to guide my friend during his essential break-in period!
Who is right and who is wrong?

Solar Mike · Accepted Answer

I used to keep to below 3000rpm for the first 500 miles, then rev the engine to 4500 sometimes but not thrashing it hard in any gear for long periods of time. So not doing a hill climb race in second at 6000rpm at full load for 10 minutes...
Built several engines and ran them in gently but worked them and no problem. But some drivers will always cane an engine ie rev to the max, change late etc...

alephzero · Answer

Just follow the manufacturer's recommendations.
I asked the dealer (at a main dealership) the same question last time I bought a new car, and what the mileage was for the oil change after running in. The response was "just drive it, and the first oil change is with the first service at 18,000 miles". (And the car didn't have any problems following that advice.)
You might consider that hire car companies send out brand new cars to their customers (I once picked up a hire car with only 10 miles on the clock) and their customers are unlikely to drive them "carefully" because they are running in.
However, running in a refurbished engine where you don't really know what condition it was in before refurbishment, and with a mix of new and old parts afterwards, is a different situation from an all-new engine straight off the production line.

Which school of thought is right for breaking in engines - keep RPM below 3000 for the first 3000 miles, or rev up to redline?

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