Cycle World magazine runs a "Service" column where readers can post questions to the editors. Technical editor Kevin Cameron answers the question below.

I've been riding since the mid-1960s and, like everybody else, have heard horror stories about oil breaking down from heat and pressure. Can you tell us, simply, what that means? Does 40-weight oil become 10 weight or kerosene? Is the breakdown permanent? When oil is re-refind, does it come out as a lesser quality or viscosity?

 

Oil breakdown could have several components but the simplest is the "falling out of grade" that happens to multi-grade oils. These are oils whose viscosity index has been enhanced by the addition of a percentage of "variable geometry" polymers. When cold, these effectively roll up in a ball, making little contribution to viscosity; but as the oil warms, thermal activity causes them to extend progressively to become long-chain polymers that slow the natural loss of viscosity with temperature.

In olden times, skeptics could point to early breakdown of such polymers, but better ones have been developed and now even the diesel people accept multi-grade oils as valuable. The idea is this: that you choose an oil for a given engine based on its ability to support loads in the hottest places - the top ring groove and the exhaust valve guide. If the oil has adequate viscosity for those hot zones, it certainly has too much viscosity elsewhere, such as in crank bearings, the lower ring grooves, cam-and-tappet, etc. Ideally you'd like an oil whose viscosity was right everywhere, and muti-grades are the best approach to that.

The polymer added to make a multi-grade can be broken by rapid shear, such as what takes place between gears and between cams and tappets. It can also be broken down by excessive temperature, although oil makers claim you get what you pay for here.

 

Many oils for machines and motors are sourced from mineral oils. Do you know some other sorts of oil?

Oil is a liquid lubricant. Some lubricants are solid. Can you name some? What are solid lubricants used for?

Search the internet for an explanation of "viscosity index"?

Why were diesel people at first reluctant to use multi-grade oil?

Cams and tappets are used to operate the valves in a four-stroke combustion engine. Search the Internet for a picture of cams and tappets in a cylinder head. Copy the picture to your sheet.