Konrad

Mama Rossi is by just about anyones measure a true high performance engine with interference fitting of the P&L (pinch at the top). The instruction sheet that comes with them had one run the engine rich at first. There was no mention to get the engine up to operating temperature as soon as possible. While some believe that because of the different coefficients of expansion between the materials that the engine needs to be run in a narrow operating range. Now if the thermal load was uniform this argument might hold more weight, but the predominate driver here is hydrodynamic. So as long as there is oil and cylinder pressure there is little need to worry about changes in fit.

For an example I use to fly at what we called the Polar Bear meet, 1st week end in January. One year we held the meet in temps of 15°F. To show how superior the Picco carb was I idled a true ABC Picco for an hour. That engine showed no performance loss that day or for three more seasons of flying.

Now a more meaningful example was the fit of the true ABC P&Ls for the engines I built. (I ran some tests trying to determine the best tapper and fit for my expected power output.) I measured the shape of the piston and liner to within 0.0012 mm (0.00005") and can say that measured across the same axis as the transfer ports hot and cold break in resulted in close to the same wear (break in). Now the only issues I have had with a cold rich running (breaking in) was on one of my 6.5 cc Pylon engine and a 13cc Picco 80 Ducted fan engines. I had set these up so tight that I needed heat the cylinder very fast with a flame to be able to turn over the engine. These engines failed the rods not the P&L fit (well not what I could measure after the rods let go).

Now unless my experiments were set up wrong I have come to the conclusion that running an ABC engine rich is not detrimental to the P&L fit. Yes, I break in my engine with a rich cycle to set the rod bearing then I get the engine up to operating temperature to set the piston as a result of the asymmetric heating from the exhaust flow.

I need to place this disclaimer. These experiments and examples were done with engines made with real chrome approximately 0.12mm thick. Chrome is much harder and dimensionally stable than the soft electorless nickel used in many of today's sport engines (aka ABN).




Konrad

MJD

My Feedback: (1)

Interesting stuff Konrad.

I had a first run rod failure on my West .50, the big end was beat up really badly. I would assume this is from hammering the piston through a tight TDC pinch several hundred times, it was very squeaky and I did not preheat it. It had run 15 - 20 seconds rich and I was in the process of needling it through the 4c-2c transition towards a rich 2c but not there yet. "Zaaaaap!". Jett runs their engines at the plant to get past the potential of a first run rod failure, and to ensure they perform up to spec.

I preheat all my tight engines now using a thermos of water from the kettle, and it makes a noticable difference. The West has maybe 30-40 runs on it and is still tight up top when cold, same for the Jett .50. But not as tight as when new.

Sport_Pilot

While the P&L fit may suffer from running very rich on a highly tapered engine, I have been told that it was broken rods that made manufactures recommend tha the engines not be four stroked during break in years, no decades ago.  The engine liner grabbing the piston as it cools off causes the rod to break. 

Konrad

MJD and Sport Pilot,

Yep, the rod failure was with engines that couldn't even be rotated at room temperature. When I gave the liner a head start with the flame pre heat she would turn over. But as the temps stabilized (both the piston and cylinder) during the run the P&L fit reverted close to that of the room temp fit and over loaded the rod. This would have happened no matter how I would have broken in the engine as I had fitted the P&L way too tight.

Back to the P&L fit as a result of running rich. I have seen little (actual measurements) to support accelerated wear in rear chrome plated ABC engines.

downunder

Coincidentally, I've just finished running in an engine that had the tightest pinch I've ever felt and I had real concerns about the rod. With a very firm grip on the prop and turning as fast as I could I could just barely get it through TDC. Hand starting was out of the question because the piston simply locked up. When I finally managed to get it running it burbled quite happily at 6000 revs in as rich a setting as it could handle but for the first 40 minutes it would stop instantly with the piston jammed into the pinch. After 60 minutes I was finally able to hand start it. Pre heating was out of the question because it had a cast iron piston and steel liner . Much to my relief though, the rod survived unscathed with no detectable wear.

mike109

G'day Downunder. I'm down there too. A question if I may. I have seen that you have a number of Enya engines. I picked up two Eny 40SS plain bearing engines on a certain auction site recently. One is fine and has been run in well and has a nice piston and liner fit. The second was described as "low use etc" but its piston and liner fit was useless so I bought a new P&L from Mr Enya. It is iron/steel and so will need some proper running in.

And now the question. How would you run-in an engine like this? (It is an RC engine if that makes a difference). I imagine lots of short rich runs?

Thanks.

Mike in Dubbo NSW

lablover

Good God man

Tons of info here for a newbie....Think my eyes are bleeding now. Now I'm almost afraid to break in my new 2 stroke OS 55AX I can tell I have tons to learn,....Pinch???? Not sure what that even is yet

Time for more EYE BLEEDING

Patxipt

Mike,

These engines are run-in with a good amount of castor oil (20%+) at an extra rich setting. Run the engine and when it gets warm, pinch the fuel line and let it cool.

Although refering to diesels, the procedure is the same for these cast iron piston/ steel sleeve engines.

http://www3.bc.sympatico.ca/dieselco...ut_diesels.htm

Konrad

Pinch= Is the interference fit of the piston in the cylinder bore near the very top of the bore. This is normal for a ringless aluminum piston such as ABC, AAC, ABN and ABL systems.

Taper= Is the change in the cylinder bore getting smaller towards the top of the cylinder. This again this is normal for a ringless aluminum piston such as ABC, AAC, ABN and ABL systems.

For your OS don't worry about the details the manual covers what you NEED to know other than some description of terms. Just don't turn over the engine without a glow plug installed. And do use a fuel with close to 20% oil content and at least half that oil should be castor oil for your first few tanks (20 minutes of run time).

All the best,

Konrad

blw

My Feedback: (3)

Yeah, I figured that would have happened a good ways back.

What we call pinch is where your OS engine is different than your car. Your car has cylinders with straight walls. It has the same diameter at the top and bottom. The compression comes from the rings as they squeeze against the cylinders. Your engine doesn't have rings, so it has to make compression some other way. This is done by making the cylinder slightly tighter at the top. The piston hits this tighter area as it comes up and this creates compression.

The compression is necessary to squeeze the fuel/air mixture for being burned.

Konrad

Compression comes from the piston moving to create less volume in the cylinder/combustion chamber. The rings and oil film create the seal between the piston and cylinder. The taper is there to control the dimensional characteristic of the cylinder so that the cylinder is straight during operating conditions (temperature) A ringless aluminum piston does NOT rely on the taper to seal. It is the film strength that results in the seal. This demands great accuracy in control of the piston to cylinder clearance. And it is this demand for accuracy that make the breaking of ringless systems so critical (no rings to take up the slop).
In an ABC/ABN system the tapers and interference (pinch) control (address) different concerns with regard to drag and sealing ability at operation temperatures. Now most of this is controlled at the point of manufacture so one has little control over these. But one can control the break in (stress relief {thermal or machine}, work hardening) and as such should be done on the bench to avoid a lean run during this critical phase.
Now many high performance engines were available with different size pistons to take into account the various fuels and power setting one might expect the engine to perform at. No one fit is suitable for all applications.



Konrad
FYI: I set up all my full sized air cooled car engines (2.0L to 3,3L) with cylinder taper and these also had rings. Again this was to control the difference in size at the top of the cylinder as a result of heat.

lablover

Thanks Guys

Now I get it

blw

My Feedback: (3)

That's what counts since this is your thread.

Konrad

Not a dead horse. The OP has his question answered about the 2 to 4 cycle break. It was addressed several ways.

He now knows a little of the "why of break in". He also knows a little about the two schools of break in. He also knows a little about the fit and clearances that define a ringless aluminum piston system. And if I'm lucky he will be questioning the materials in his next purchase.

After all the quintessential thing that defines an engine is how it seals the piston to the bore, and for some, how the valves seal four cycle engines. The rest of the engine really is secondary. This is why it is so important to know and understand the construction and material type of the engine. With this knowledge one can extract the most from ones engines, ether life, power or if skilled both.

To me it looks like he is much better off for asking the question!



Konrad

Ladyflyer

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In larger production engines the fit along the hopefully parallel bore is handled by proper piston design. A good piston design will have cam and taper that have been very carefully worked out. Today's engines can run very decent mile numbers .
The cylinder seal in non ringed model engines is , as you say a different animal all together.
Today's model engine makers have made the most of many different advances as well as the realities of manufacturing as dictated by so many influences.

MJD

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There is no harm in open conversation spawned by a simple question. This thread is no different than many others - the original question gets answered, perhaps in varying manners, and the question leads to round table discussion and perhaps some debate. Knowledge and insight is often gained through debate of opposing viewpoints.

I found it very interesting that some folks have measured p/l wear fit in varying mixture conditions - food for thought and somewhat in the face of conventional "wisdom", though I don't have a compelling reason to change my methods and opinions for now.

Konrad

I know that some Audi engines had piston clearances of around 0.018mm for 90 mm bores. I know of no model piston that is cam shaped with expansion plates cast into the piston across the wrist pin bores, this is how a cam piston works. Now most of the model engine pistons I made had all sorts of relief cuts, kind of like sewing thread spools, with 0.005 mm diameter changes. Now the two cycle piston needs to allow for changes in crankcase pressure (seal across the exhaust port)

Ladyflyer

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quote:

ORIGINAL: Ladyflyer
"In larger production engines "
-"The cylinder seal in non ringed model engines is , as you say a different animal all together."


The comment was a response to blw's comment regarding automotive engines. Automotive engines and similar larger engines.

blw

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This is getting silly. The original poster indicated that he was somewhere between beginner and intermediate stages, so I tailored my responses to him. I didn't load him down with semantics, trivia, or theoretical talk that was outside of what he requested.

As pointed out, a free flowing extension of the original topic is fine. So is giving a straight answer to a straight question.

Ladyflyer

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Silly ? Nothing new to web forums . You are right,the question had pretty well been answered.
I just re-read some of the comments.

TTFN