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Adrian Duncan making that fine adjustment for a perfect engine run.


 Bench testing a Webra Mk1



Bench testing an Oliver Tiger.


Breaking in a new diesel engine
 By Adrian Duncan

The rules for Vintage diesel combat require the use of a non-schnuerle .15 cu. In. diesel motor (plain bearing or stock PAW .15 BR) having a cast iron piston running in a steel sleeve. We call this a ferrous piston/cylinder set-up, since both cast iron and steel are ferrous metals. If the best possible performance and longevity are to be obtained from such a motor, a proper break-in is essential. However, most folks are so eager to get that new motor into the air that they ignore this simple yet critical procedure. As a result, they never experience the best that their new motor has to offer.

In the Pacific Northwest, the PAW .15 BR's used almost exclusively by the combat flyers from the Vancouver area are often the subject of comments on their excellent performance compared to other similar motors.

Our Vintage diesel rules require the use of a stock motor, so tuning is not the reason for the results achieved - the motors are all bog stock. Our only secrets, which we're happy to share, are that we break 'em in right and use the right fuel and models. Fuel and models are covered elsewhere. Let's look at the how and why of break-ins, diesel style!

In order to fully understand the break-in process, it is necessary to appreciate some of the properties of the two materials involved, and how they work together:

(A) Cast iron and steel expand virtually identically when heated. This is why a close-fitted ferrous metal piston/liner set-up does not seize or lose compression when the motor warms up. So we want to maintain a close fit throughout the break-in process.

(B) It is a well-established engineering principle that wear in metal-to-metal situations is minimized if the two metals involved are dissimilar. In particular, one should be harder than the other, at least in the initial stages. What happens is that the initial friction between the two new parts wears off microscopic particles of both materials, and microscopic particles of the harder material become imbedded in the surface of the softer material, turning it in effect into an ultra-fine lap (or hone). As a result, it is the harder material which wears at the fastest rate, contrary to most peoples' expectations!

The wear on the harder part keeps the softer part charged with new hard particles, keeping the lapping process going. The parts are eventually honed by mutual action to a glass-like finish. Cast iron in its newly-manufactured state is very much softer than the hardened steel used in most diesel cylinders. Therefore, cast iron and steel are an ideal combination from a wear standpoint and work together perfectly as described above. Meehanite cast iron, which is very close-grained and has a high graphite content, is the particular form most used in model diesels.

(C) If the above process were to continue, the steel cylinder would wear out relatively quickly and the piston fit would soon become unusably slack and "leaky". Fortunately for us, cast iron, particularly the Meehanite form, has two very convenient properties which have a great bearing on the break-in procedure. Firstly, the continued application of friction upon its surface has the effect of making it become harder over time. This effect is called work-hardening. After some running hours, the surface of a Meehanite piston will test as hard on a test rig. This increases its wear resistance, of course, and reduces its tendency to trap more particles of the cylinder material and continue to hone or lap the bore larger. This greatly slows the wear rate of the cylinder material. The other property of cast iron is that it tends to "grow" dimensionally over the first several dozen heat cycles to which it is exposed. This allows a cast iron piston to "grow" to accommodate the slightly larger cylinder which results from the initial lapping process.

Adjusting compression and needle valve.

D) The key point of all this is that the cylinders of model diesels are generally of hardened steel, and the cylinder remains hard throughout the engine's life. The piston, on the other hand, starts out soft and becomes hard over time. During the hardening process, it laps the cylinder to a glassy finish and grows a little to maintain the correct clearances as the cylinder wears, but then stabilizes both in terms of hardness and dimensions. At this point, we have two hard materials at an optimal fit and finish working together - perfect conditions for long life and good performance. This is what our break-in tries to achieve. But it takes time, since we need both the work-hardening time and the heat cycles to achieve stability.

In summary, what we want to accomplish during break-in is to use the piston as a built-in lap to optimize the finish of the steel cylinder (but not to wear it unduly). This of course expands the cylinder bore microscopically, so we want the piston to expand a little to take up the slack caused by the lapping of the cylinder. We also want to end up with a nice hard wear-resistant piston, and we want the work-hardening to be completed at the point where the piston has stabilized dimensionally and the cylinder is correctly finished. Tricky, huh?? Not at all, as it happens, due to the fortunate combination of properties of the two materials involved! 

To achieve this, the following procedure works best:

(1) Use a prop that keeps the revs well below what would be used in the air, at least initially. A 9x6 prop is good for a PAW .15BR, and the good flywheel effect is an aid in getting a tight new motor running. You can switch to an 8x6 nylon after 4 or 5 runs. Use a high-quality fuel with at least 25% oil, most or all of it castor. Add extra castor oil if you need to.

(2) Start the engine and run it slightly rich and a touch under compressed (smoky and missing slightly ) for 4 minutes or so. This allows the piston an opportunity to lap the cylinder surface effectively with plenty of lubrication, minimal wear and not too much heat. We don't want to lap too much off the cylinder or wear the piston down!! It also allows the rod and shaft bearings an opportunity to bed themselves in under easy conditions.

(3) Keeping the engine running, adjust the compression and needle valve for fastest speed (leaned right out). The purpose of this is to get the piston as hot as possible to help it to grow to its stable dimension as discussed above. Run it like this for one minute maximum, then stop the engine by disconnecting the fuel line or closing the needle (NOT by choking or backing the compression off - this cools the piston!!).

(4) Allow the engine to air-cool slowly but completely before restarting. Have patience!! This allows the piston to go through a complete cold-hot-cold cycle (or heat cycle) and begin the process of thermal stabilization discussed earlier.

(5) Repeat this process a minimum of 8 times or until the motor feels nice and smooth throughout the stroke when turned over slowly. At this point, the motor can be flown using an 8x6 prop. For the first 4 or 5 flights, keep the needle slightly rich and allow complete cooling between flights so that the piston continues to experience complete heat cycles. At this point, the motor is to all intents and purposes broken in.

I have seen many other approaches to breaking-in, including a number of "instant" break-in procedures. All of these in my experience have resulted in excessive premature piston wear during the break-in process, and motors broken in "on the fly" in this way neither perform nor last anywhere near as well as those which receive the treatment recommended above. I hope it will be clear from the above explanation that time, and time alone, can achieve a proper break-in with a ferrous piston/cylinder combination due to the unique properties of these two metals in combination. You've paid for it - you might as well get the full performance potential for which you've paid! My 1961 Frog 349 is still going strong on the original bore, runs like smoke and feels like new with well over 100 hours of running on it - the original piston fit is still totally leak-free. It's all down to the break-in!

Break it in right and you'll have an engine that will stick with you through thick and thin!!


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This page was last updated on 02/21/04.