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Engine basics => Engine set up tips => Topic started by: frank mccune on October 01, 2017, 11:01:53 AM
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Hi All:
In reading old engine tests, I noticed that the running time prior to tests were from one to four hours. How much time should I spend on the bench breaking in an engine and how will I know when it is broken in and ready to fly.
I have always wondered about this and an incident a couple of days ago, reignited my curiosity. I have had Fox engines that seemed to wear out before breaking in and other engines to run well with a little bit of run in time. I evaluate the piston and cylinder fit by how long will it hold compression and how easy it is to hand start.
I do not purposely fly my engines in anything that approaches a lean run. I also noticed that the more expensive the engine is, the better it performs over its live! Lol You get what you pay for! Sometimes!
All of the best,
Frank McCune
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I used to waste a lot of fuel on bench running
Got a work horse round n round airplane and only use the bench run to find and set a needle that reliably (with my prime method) starts...then I run the with a air cooled load pulling a plane
BUT when I did bench run a lot of fuel...my method was many short, up to temp runs...on the rich side, with 10~20 seconds of timed line pinch to leaner high rpm...then full cool down...repeat... Like MM said, when it held a steady wet 2 stroke I figured it was ready
However one thing I did do for bushed engines was feel for hot nose... once it stopped burning my finger I figured the bushing crank shaft had clearance enough
I fly so few flights a month that is actually fun to feel/hear some of my newer engines as they just get better and better each flight
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When it will hold a steady 2 stroke.
When it will return to 4-stroke rapidly after you release a line pinch.
Brett
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with an old iron piston, you are not only trying to lap the fit, but also harden the piston. Try this.
1. first run rich and slow for 1 min. Let it cool completely.
2. run at a fast 2 cycle for 30 seconds, then reduce to 4 cycle for a min. Then run it as lean as you can for 30 seconds to get it hot. Shut it off and let it completely cool. This will grow the piston but also will harden it.
3. Run at a rich 2 cycle setting until the engine will hold a lean setting. This is the final fitting of the run in and it may take awhile. If you do not heat treat the piston, the engine will still break in but it will not last as the piston will lose it's fit prematurely from being soft.