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HOME/Techtalk #2 - Bevel Engine Oil

13 January 2017 by Harm Heuvelman

The subject of engine oil is one of many controversies, developments, improvements and opinions, or so it seems. So many types and brands of engine oils exist and everyone seems to have the best advice for your application. But what is the oil to use in our classic Ducati bevel engines? And why? We thought to take away some of the ambiguity surrounding engine oil.

Engine oil has a number of functions, the most important of course to limit wear between moving parts. This is done by forming a film on the surface of the metal components moving against each other and in doing so, limiting the friction between the surfaces.

By forming a film on the moving components inside the engine, the oil also ‘moves’ heat away from the moving parts as it flows through the engine. Oil therefore also has an important cooling effect on the engine.
Oil also moves any sludge or small metal particles away from the moving components as it flows, leaving these particles preferably inside the oil filter where it passes through or inside the engine’s sump. Oil has a cleaning effect on the engine as well.
Last but not least, oil also limits corrosion inside the engine as it is naturally preserving metal.

So how does engine oil do all these jobs at the same time? By far the most important property of oil is its viscosity. The viscosity of a liquid can be thought of as its ‘thickness’ or as a measure of its resistance to flow. The desired viscosity is depending on the way the engine is engineered (bearing types, oil flow circuit etc.) and on the operating temperatures.

More on this later, but let us first take a look at the different oil types that are available and a little history to how this all came to be. There are 2 types of oils if we divide them by origin: organic and mineral.

Organic oils are derived from plants or animals and are mostly seen in the kitchen but are also used in cosmetics, paints but also as lubricant. It is the oldest type of oil and is used for thousands of years, long before we were able to dig up oil from the deep layers of the earth. Castor oil, derived from the seeds of the Ricinus Communis plant was found to have good properties for lubrication and was used in steam engine and was as such the oil of choice for the early internal combustion engines as well. In fact, the Castrol oil company (now part of BP) derives its name directly from this product and still makes R40, well known among classic racing enthusiasts. Although castor oil has good lubrication properties, the disadvantage is that it tends to form gum over time and therefore is only suitable for engines that need rebuilding quite often.

When crude oil was increasingly used as a basis for carburants, it was soon discovered that it could also very well serve as a lubricant. Mineral oils became the lubricant of choice even, made by refining crude oil and separating the lubricating fractions from the more volatile fractions. With the application of mineral oil in internal combustion engines, manufacturers prescribed a certain oil viscosity based on the temperatures the engine was operated in. The Society of Automotive Engineers (SAE) developed a numeral code for grading the viscosity ranging from 0 to 60 (from high to low viscosity) with the numbers 0 to 25 also designated with a ‘W’ for ‘Winter’ indicating these were more suitable for low temperature use.

All was fine up to this point but it was the US army that had vehicles operating around the world and tried to minimize the large quantities of all the different oil viscosities they had to deal with. The petrochemical industry was asked to come up with a single oil type that could be used in different conditions and a multi-grade oil was what they came up with.

By developing ‘viscosity index improvers’ based on polymers and using these as an additive to mineral (single-grade) oil, a lubricant was created that could function over a range of operating temperatures. This range was graded with dual numbers; indicating both the cold viscosity and the viscosity at the warmed-up operating temperatures.

With engine development going further, more and more plain bearings were used in designs and with this came the need for oils that were more and more viscose. Special additives were developed to improve the properties of oils that became even ‘thinner’ at operation temperatures. Gradually a development towards higher oil pressure and lower viscosity could be seen. This lead to oils that were 100% engineered from petrochemical substances, rather than being a direct product of crude oil refinement. These type of oils can be categorized as fully-synthetic.

To clarify this, the table (picture 2 above) can be used to categorize the different oil types.

So, with all these different types of oils on the market, what is the best oil for any given engine? Over time, the use of plain bearings became more and more standard, requiring higher oil pressure and higher oil flow, requiring lower viscosity. It is a very good measure to take this most important property of oil, viscosity, as a basis for our choice. More modern engines will require other properties as well, following a choice for a fully synthetic oil with additives to reduce friction, improve life-span and therefore minimize running costs.

This however does not apply to our beloved Ducati bevel engines as these were developed in a time when these additives were not even available. The most important factor to keep in mind however is that the Ducati bevel engine has roller big-end bearings. Where later plain big-end bearings rely on a higher oil pressure to separate the bearing surfaces, roller bearings need a much ‘thicker’ oil to stick to the rollers and the bearing surfaces for much longer in time. If an oil is used that is too ‘thin’ this will result in the oil being flung off the big-end before it can do its lubricating job.

The gearbox design of the bevel engine does not have any oil passages or channels and purely relies on a ‘mist’ of oil created by the moving of the crankshaft through the engine sump. Here again, the oil that is flung to the gears must have a high viscosity and must have the time to stick to the gear in order to lubricate it sufficiently.

We sometimes hear fears about the cylinder heads not getting enough lubrication when using an oil that is too thick. However, when we take a closer look at the Ducati bevel engine design, we see that a deliberate restriction is build-in to oil passage to the crankshaft in order to increase the oil flow to the heads. You only need to fit a ‘gear gazer’ to the bevel cover to see how much oil flow goes to the cylinder heads anyway.

So: all bevel engines should be run on a mineral monograde engine oil. As we don’t expect any bevel Ducati owners to take it out in winter time, we recommend to use an SAE50 viscosity for general use. Any good quality monograde SAE50 will do, but we like to use the one offered by Motul

Single cylinder models, as well as the 750 round case engines don’t have an oil filter other than the sump gauze filter. A disadvantage of using monograde SAE50 is that the oil will pollute quite fast as it doesn’t have any cleaning additives. This results in an oil change interval of every 2000km approximately or 1500 miles.

The 860 engine is fitted with an oil filter and it is true that monograde oils don’t filter as well as multi-grade oils. However, these engines are still fitted with roller bearing big-ends and the same gearbox design so the same reason applies here and a monograde oil should be used. Many experts will advise a multi-grade semi-synthetic for this engine type, and base this advice on the fact it has an oil filter. We feel that lubrication is more important than the life-span of the oil so 860 engines should have an oil change interval similar to the earlier round case engines of 2000km.

The only Ducati bevel this doesn’t apply to is the Mille. As this does have plain bearings, a multi-grade semi-synthetic oil should be used with a 15W50 or 20W50 viscosity:Motul 5100 15W50

When changing from one oil type to another, you can always go ‘back’ along the ladder of product enhancement. This means that you can go from a full synthetic to a semi-synthetic to a monograde without any problem to the oil’s performance. The other way around will affect performance so don’t use a semi-synthetic in an engine that used to run on a monograde, or only after a full engine rebuild or at least an internal cleaning.

We are sure to stir some discussion with this article and we are very much looking forward to your experiences and opinions. We hope at least to have cleared some of the questions with regard to the ‘big oil question’.

Tags: Mechanical Restoration, Nut-and-Bolt Restoration, Patina Restoration

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