Procedure for breaking in a new shortblock?

LBO said:

I just bought a new OEM shortblock for a Lawn-Boy DF commercial engingine. Any tips on breaking it in properly would be greatly appreciated.

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Use dino based oil like Lawn-boy or Legend oil. I always mixed new blocks at 32:1/30:1. Start and run part throttle for 5 minutes. Let completely cool. Start run 10 minutes part throttle. let completely cool. Start run 10-15 minutes full throttle no load. Let completely cool. Then run under load 10-20 minutes. Let completely cool. I was always able to get VERY NICE compression umbers with this system. Then move to 32;1 dino oil for first season. Only use synthetics after a complete season.

Thanks, George. Per your recommendation, am using Legend I almost all the 2 strokes - Stihl premix in the Kombi. I will mix it a little on the rich side for the breaking in process.

I do mine much like GTP as I am a FIRM believer in heat cycling a new short or long block. I do cycle mine a little different varying rpms. Good luck with the break in LBO!

Thanks, BDM. I'm going with Legend's top end oil 30:1 and start
slowly - low r.p.m. to start, let cool
All the way down, repeat, cool, etc.
Short block went on today - need to rebuild carb.

Just out of curiosity did all new 2 cycle mowers already come pre broken in from the factory? I don't ever remember reading anything in any of my owners manuals about any particular break in procedures. If they were pre run and broken in then what about NOS shortblocks? Are they test run before shipping? I can understand about rebuilding an engine and then wanting to break it in properly but wondered about stuff direct from the factory. Bill

Yard Nazi said:

Just out of curiosity did all new 2 cycle mowers already come pre broken in from the factory? I don't ever remember reading anything in any of my owners manuals about any particular break in procedures. If they were pre run and broken in then what about NOS shortblocks? Are they test run before shipping? I can understand about rebuilding an engine and then wanting to break it in properly but wondered about stuff direct from the factory. Bill

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Bill, this is a topic that has been debated as long as engines have been around. Ask 10 people how they "break in" an engine and you'll get 10 different answers. Most are split in 2 groups. They either have a dedicated procedure, or they run them right out of the gate like normal. I personally like to break them in because it was what I was taught, and have had great success doing so.

A few thoughts into your question... First off I can't say for sure if all these were run before leaving the factory, but I suspect many were. Its a simple quality control step that prevents any mistakes from getting into customer hands. A new engine of any type will always require some kind of break in whether you realize it or not. Rings need time and pressure to properly seal to the cylinder wall. The idea of heat cycling is to quickly seat components through a range of differing running conditions. New parts will temper and change properties under thermal load. Many new cars will tell you right in the owners manual that using up several quarts of oil before the first oil change can be expected. This is because the rings have not totally seated. I could go on forever, but instead I'll link to a few good reads.

https://en.wikipedia.org/wiki/Break-in_(mechanical_run-in)

Break In Secrets--How To Break In New Motorcycle and Car Engines For More Power

Engine Break-In: What You Need to Know - autoevolution

Thank, Premix. Better to err on the side of caution. Many engines are pre broken in today, but why take a chance? Won't hurt to do it again. Thanks for the links.