Solid cast iron cylinders were standard fare on early snowmobiles. They were heavy and were used on many single cylinder engines until the late 1960s. The cylinders were actually cast of nodular cast iron or malleable iron, which is a little higher tech than basic white or gray cast iron.
The iron cylinders did a dependable job of making about 12 hp from roughly 300cc of displacement, but when engine modifiers started squeezing more and more horsepower from the engines, the cylinders started squeezing more and more pistons. A better means of removing heat from the cylinder had to be found.
Iron Sleeves Boost Durability
The first change made to improve heat transfer from the cylinder was accomplished by using a cast aluminum cylinder. Aluminum conducts heat about 3.5 times faster than iron and offers a simple means to improve heat transfer from the engine while reducing its weight. The problem was that cast iron piston rings wore the aluminum cylinder bores in a hurry, so the bore of the new aluminum cylinders had to be lined with a cast iron sleeve.
Those cast iron sleeves were generally about 0.125 inch thick, but they varied greatly based on how the aluminum cylinder casting was used. Sleeves were kept thin as possible to make heat transfer and expansion as rapid as possible. The sleeves, however, had to be thick enough to maintain strength and to allow for an over-bore or two in the event repairs were necessary.
One problem with using an iron liner in an aluminum cylinder is that the two metals have different rates of thermal expansion. The aluminum “grows” away from the iron liner and heat transmission between the two drops to zero. To eliminate this problem, the iron liner is given an “interference” fit with the aluminum cylinder casting. When the cylinder and liner are at room temperature, the outside diameter of the liner is about 0.004 inch larger than the bore in the aluminum casting it is to be placed into.
To fit a sleeve into a cylinder, the aluminum casting is heated to about 400 degrees F while the liner is chilled to shrink it. In a fast, slam-dunk maneuver, the cold liner is slipped into the hot cylinder and quickly aligned.
There is a tremendous squeezing force of the cylinder on the liner when the two are at room temperature and heat transfer is the greatest between the two when pressures are the greatest and the irregularities of the surfaces are the most intimate.
As the temperature increases on iron-lined, aluminum cylinder engines, the pressure beetween the liner and the aluminum cylinder decreases, as does the ability to transfer heat from the liner to the cylinder. The reductions in heat transfer are small but they exist.
Coatings And Platings
The first coating I remember seeing was Teflon used on the 1970 Ski-Doo Blizzard piston domes. The layer insulated the dome of the piston, reflected heat back to the cylinder head and reduced deposit accumulations.
Chrome plated bores in snowmobile engines appeared in the early and mid 1970s. A layer of hard chrome, about 0.002 inch thick, was plated onto the bore of the cylinder and finished with a ceramic hone. The chrome plated cylinders worked reasonably well, but the brittle material commonly flaked off the intake side of pistons with light seizures and scuffs, especially in wet conditions where just a drop of water could displace the oil film from the chrome bore surface.
A number of techniques have been used by engine manufacturers to put materials such as carbon, molybdenum and nickel directly into the bore of the aluminum cylinder. Some of these “electrofused” techniques had success, but they were very thin and wore quickly if subjected to minor lubrication failures, among other problems.
In the 1970s, Mahle GmbH, a company in Stuttgart, Germany, worked with a nickel-silicon-carbide coating on the sealing surfaces of a Wankel engine and had success. It saw applications for the coating in two-stroke engines. Mahle named the coating “Nikasil,” which is a registered trademark of the company.
Nickel-silicon-carbide coatings can be electroplated or chemically deposited on surfaces. Most companies now use an electroplating process where nickel is plated onto the aluminum surface in a bath containing the silicon-carbide. This creates a nickel matrix with evenly distributed silicon-carbide in it. It is applied 0.001 to 0.003 inch thick.
The Nikasil coating is much more pliable than chrome and can even withstand the abuse of a hard piston seizure. The coating is extremely hard and requires a diamond hone to finish it. This hardness results in long life that can withstand even dusty conditions that might be experienced by a grass drag racer. Piston skirt scuffing that was experienced with chrome-plated bores was virtually eliminated with nickel-silicon-carbide coatings because microscopic depressions in the layer create places where oil can collect.
An oil film actually separates the piston, rings and cylinder lining while the engine is running, but during start up and shut down or an oil film failure, mechanical contact occurs. Nikasil (spelled and labeled in other ways to avoid Mahle’s trademark) seems to solve many problems for engine manufacturers. So why do many new engines still use cast-iron-lined aluminum cylinders? While a Nikasil plated cylinder provides the ultimate in performance, a cast-iron-lined cylinder is often more practical for the average consumer.
In the past, some manufacturers pledged to not force their customers to purchase new cylinders if a severe seizure, broken ring or other piston problem damages the cylinder bore. While a Nikasil plated bore can be restored, it’s an expensive and complicated repair. Sometimes it’s easier to replace the entire cylinder. A cast-iron-lined bore can be bored out and an over-size piston installed, or a new liner can be installed at far less expense.
Coatings today are used to lubricate, to shed heat and to act as thermal barriers and friction-reducers. Today, ceramics and metallic ceramics, rather than Teflon, are used as thermal barriers on piston domes. These coatings are sprayed or printed onto the piston and baked-on in ovens.
Teflon-molybdenum coatings are solid lubricants still used on piston skirts that reduce friction and increase heat transfer. They help reduce the chance of cold seizure as well. Manufacturers use many of the latest coatings in their engines and aftermarket companies can also apply many of the new coatings on new or old engine parts.
What have engine manufacturers done to prevent cold seizures?
Pistons are made of aluminum alloys and the factors that affect thermal expansion between the cylinder and the liner also exist between the liner and the aluminum piston. In a cold engine, as things get hot shortly after startup, the fast-growing piston is inside the slow-growing liner. Silicon is added to the aluminum piston material to reduce the thermal expansion rate, but it also makes the alloy more brittle and subject to piston skirt failure. There is a fine line of how much silicon can be used to control piston expansion.