cylinder break in

blimpy

blimpy

New member
I guess its almost time to think about what method to use for breaking in a couple of overhauled cylinders.

Seems like the conventional wisdom used to be not to drivev
at a constant speed, idea being ..Im guessing, that when you back off on the throttle more oil moves up to or past the rings temporarily.

how that is supposed to work while flying...I dunno.

Then there is the full tilt boogie school of thought.

And how much run in to do on the ground, and how ?

With no oil filter Im thinkin 1 oil change in the first few hours, and the next as soon as its ok to switch back to compounded oil.

Comments and wisdom ?
 
There is a lot of lore about this subject.

I have been around long enough to know what it was like to pull a jug before compounded oils became common. It was usually accompanied by the sound of pieces of ring hitting the floor. So, I am a great believer in not letting straight mineral oil anywhere near a piston engine.

I do know there is a theory that straight mineral oil is the way to go for break-in, but I wouldn't do it and have never seen any science to back it up. Apparently new car manufactures don't believe it either. If there is good info out there, I'm here the learn. I would tend to favor a little lower viscosity oil and light loading as in the original break-in schedule.

I wonder what John Schwaner of Sacramento Skyranch has written on this subject ? Usually a very reliable source of information.

r
 
The reason engines used to fail early using straight oil is because it wasn't as good as the newer oils with additives... but that's actually a good thing for break-in purposes.
Ron is right, the lore in the tribe about break-in runs wide and deep. Generally it's thought to be just polishing the cylinder walls so the compression is better. That's partly true.
See, when you first install new rings, the pressure they apply to the cylinder isn't even.. consider the diagram below:
pistonringdiagram_zps2ecc786b.jpg

The force near the split is higher than the force opposite the split - as represented by the relative size of the arrows. The idea is to even out the forces to obtain the maximum seal.
The way that is accomplished is to cause the ring to wear. It'll wear more where the forces are greatest - eventually balancing out until the wear (and it's ability to seal) is even.
so, the question is: How to make it wear? That's where straight oil helps. Along with an abrasive cylinder wall (due to it's honed surface), when the new cylinder/ring combo is newly running, the fine scratches in the honed cylinder wall are actually grinding away on the ring. Eventually the rings get fine-tuned to the shape of the cylinder and, Just like good-ol sandpaper, the scratches eventually wear out and leave a nice smooth surface on the cylinder wall. If you use modern lubricity-enhanced compounded oils you interfere with that process, even standing the chance of polishing the cylinder surfaces before the ring has a chance to fully adapt.. Referred to as "glazing", once that happens the ring will never fully break-in.
Sometimes you can get away with using additive oils for break-in but you run a greater risk of glazing.
There's different ways of determining when break-in is accomplished. When an all new set of cylinders, you can often see a drop in oil temp. That doesn't work well when you're only replacing a few.. or one. You can use compression checks looking for it to peak but only do it with the engine stone cold. Or you could just run it for 25 hours or so and let it go at that. It wont hurt an otherwise good engine to switch to straight oil for a few hours.
Once it's broken in, please use modern oils.. no sense in using straight oil anymore.

So why don't cars need to do this? Actually, they do. Auto engine manufacturers "pre-breakin" engines during manufacture. along with advanced manufacturing techniques, they can deliver an engine that's ready to go.
 
Thanks Bill, for a good job presenting the "straight oil school" theory. If there are Tribology studies that back up these and other break-in incantations I could become a true believer in any one of them myself.

In the meantime, from my own experience and observation, mechanics stuff new rings in old ring grooves, scuff the cylinder and hope for the best. This is shoddy practice even for auto engines. Servicing the bottom surface of the top ring land is crucial. I am not talking just theory here but firsthand experience. Here are the facts as I see it:
  • Aircraft pistons are not replaced on a repair or top overhaul because they are too expensive.
    Re-groving the piston and adding a spacer is not approved.
    The new ring is tenuously supported on its very inner diameter by the relatively unworn inner part of the grove.
    The bad geometry cones the ring, either distorting it or breaking it. Or sometimes, luckily, the ring embeds itself enough to survive after a few dozen hours and lives happily ever after.
I think with a lot of the above happening, other explanations pop up like daisies and give legs to the break-in lore. Certainly, various factors such as oils and techniques will help. But having it right to begin with is number one.

Ron
PS
Maybe an STC to re-grove exists. Or maybe it could be done. Hastings has anything needed.

Sorry for the semi-hijack Larry, good luck on your cylinder OH. I'm sure it was done right, so break-in won't be critical and they will be 80 over 80 forever. :D
 
well everything on 2 cyls will be new except the pistons.. which are still in tolerance.
I think it was just assumed that new std size pistons just weren't available... and it wasn't discussed,
since the old ones are serviceable.

Didn't discuss the pistons at any length.

Personally, having wrecked a perfectly good BMW m/c by using non-compounded av oil in it.
I HATE the stuff. I understand it is a necessary evil for getting the rings to seat, but I hate the fact that the rest of my
mid time engine has to be lubed by the crap during this time.

---

You wont believe this.. but a recent EAA magazine showed 3 cylinders.. and the ones where all the hone marks were gone completely was described as "worn out" while those still showing some hash marks were " still good". What a crock !

Anyway... we all want a nice HOT compression test .. except when we are trying to determine if the rings have seated well ?

??? Simba dumb chimp.. no get banana.. or in this case.. the reason behind this one ?

In the end I am gonna do what Dan Torrey tells me to do.. cause he has to put his name on it.

:)
 
Well, Terry Helickson at Gene's Aero Service in Canby Oregon has my 2 cylinders in the mail back to Dan Torrey at Santa Paula Cal,
having gotten the needed cyl liners, rings,guides and exhaust valves, from Susan Prall in Texas.

Sure are a LOT of steps, and a lot of Franklin specific details, which I'm very glad Terry is saavy about.

I rejected LyCon.. a big rebuilder, because they said "we'll do it, but we don't like to."

I know what kind of work come out of that attitude !

I'm glad I used Gene's. The attention to details and keeping me in the loop- Priceless :D

Should test fly it a week from today..and should have it back home in time for Columbia a week later.

:wink:
 
Larry,
I read your essay in my email.
The full throttle application is important, But Dan T. will also say to vary the power some of the time.
Don't do any extended steep climbs. Keep that air flowing over the cylinders.
If you are flexible in routing you can use "AirNav.Com to find the best prices on fuel. There can be more than a dollar/gal difference at some locations and FBOs. You can still find 100ll for less than $5/gal in the mid-west.
Dan
 
Thanks Dan.

We'll see if I can actually pull off the loop up into Ore, Wa. Idaho and return.
Oughta get something fun with it before it goes to a new home.

Be down there thurs, and dan will give me instructions.

the garmin pilot app I use with my Samsung Tab2, has reported gas price, and it will display them
right on the chart !

4.99 is about the cheapest I've seen in the west for avgas.
Many small muni airports keep the prices low, and that's where I like to go.
My airport is one of them.

Seems like backing off on the throttle should draw some oil up past the rings,
flushing out the metal being generated by the break-in process... and helping with cooling,
so seems smart to me.... that was always the drill with cars and motorcycles.
 
Well, Monday we ran the engine for 4 minutes at 1000 rpm, then shut it down for an hour.
Next ran it about 3 minutes, ran it up to 1700 did the mag check, than max static for about 10 seconds,
and 1 min at 1000 to cool down.

Another hour cool down.

Tried another run up prior to flying.. and found a shielded ignition wire shorting P lead intermittently
on L mag. Fixed that. Wait another hour.

Full Run-up.. no problem.

Cool an hour

Trot out to the line, and take off in less than 4 min.
Circle the field at 2400 rpm for 30 min @ 3000 ft.
Then 3 Ten minute cycles.. full throttle for 3 min, back to 2400 for 7
Land and check everything.

Temps, oil pressure stayed good.
No leaks.

Next day flew home 3 hours 15 minutes at 2450 5500/6500 feet.. with periodic reduction to 2300 for 30 seconds or so,
and periodic full throttle ( 2500) for 30-45 seconds. Cruise climbed to altitude at about 110 mph.

Back on the ground.. no new leaks, burned 10 Gal/hr cruise and climb with a taxi/run-up/wait time of less than 5 min
( incl) both ends. Approx 33 gallons at 65% + power for a not so wonderful 10 nm mi per gallon about 12 sm/gal.
Making 125-127 knots gps ground speed with light tail winds.

First tank ran 2:01 with no measurable remaining ( which could be as much as 3 gal).
Second tank 1:14 with approx. 7 gal remaining.

Left with ~7.5 qts of oil.
Too tired to check it on arrival.
Other than the smell of hot new paint.. nothing unusual.

I guess we will fly to Columbia in much the same way. :D
 
Now have about 6.5 hours on the 2 new cylinders.

After the round trip to Columbia, found the fuel consumption running 10 gph total including 2 climbs to altitude
of 6500 / 5500 feet in Hot and humid wx.

Got home and Added the first quart of oil which includes the initial break-in time, and 3 cross country trips.
Only oil leaking is from the long standing leaking of the starter: from a gasket and maybe a bearing oil seal.

No more oil down the belly !

I'll change the oil after a little session of landing practice, still using the Shell 100 mineral oil.
I guess I will continue to run that on the long trip I plan to take around the west. 2000 miles,
and 15-20 hours.

Lots of HOT weather and High Altitude Flying ( well 10-12 K feet anyway). If I go ahead with the plan,
Conditions where I would much rather have the compounded oil for heat resistance and film strength.

My inner voice keeps saying, 10 more hours of this is enough ! But I guess I will yield to the warning voices of
real mechanics.

We'll see how the oil consumption plays out, and how it feels pulling it through cold,
how fast the oil gets dirty, and how it smells.

I think the highest concentration of break-in wear particles has probably already occurred,
so I'm opting for fresh oil to get it out of my engine. What is the point of recirculating that crap past my
bearings, over and over ?

If the rings have in fact seated pretty decently, there should be less fuel dilution of the oil henceforth.
Another good reason to start fresh.

Whether it is wise to attempt to cruise climb to 10-12 thousand to cross the Sierras with only ~ 8 or 9 hours on these new jugs
I don't know, let alone fly across Nevada in July enroute to W. Colorado, I don't know either.

I do know I'm NOT taking any passengers !

:?:
 
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