There comes a time in an engines life when it will require reboring or relining. This operation is called for when the engine has lost power due to the piston ring to cylinder clearance exceeding there maximum operating limits. The first port of call in accessing worn cylinder or barrels is to look at the area of the top of the barrel. You will see a dark annular area with carbon deposits. Just below this area a clean shiny section will appear, this will signify the uppermost part of the barrel that the top piston ring reaches.
When the ring on the piston reaches its penultimate height it has to take the force of an explosion caused by the spark plug creating a spark that will react on the fuel air mixture that has been created by the carburettor and then passed through a valve into cylinder head.
The explosion of fuel be it ethanol or methanol will cause the piston ring to take a proportion of the explosive force. The piston crown or top of the piston will take a lot of the heat and force produced during combustion. During the short burning process of fuel, about 45 percent of the explosion force in an ordinary combustion engine is transferred to the top ring. The action of the explosive force on the ring is generally on the part of the ring that protrudes from the outer periphery of the piston. Pistons are always smaller then the barrel or cylinder that they run up and down in, it’s the rings that do most of the work. The top ring only takes as previously mentioned 45 percent of the explosive force work, but due to its small section in comparison to the piston mass, this is a lot of work. Not only does the top ring have to take down forces due to volatile mixtures exploding above it but it also has to take forces produced within its inside diameter that push the ring outwards on to the bore of the cylinder
This outwards force is at its greatest when the piston is near the top of its stroke. This action can cause a ridge just below the carboned up area at the top of the cylinder. Prolonged running on a cylinder with a ridge will cause the ring to move in and out of its groove in the piston causing wear to the top and bottom of the ring groove this will then cause ring flutter. Ring flutter is an upward and downward movement of the ring in its groove. Signs of ring flutter can sometimes be seen some distance from the top of the cylinder and will look like a series of annular marks. Wear on the outside of the ring will in turn cause the ring gap to open allowing blow by. Blow by will be evident below the top ring as either a light brownish to black colour depending on limits of wear.
The ring is not the only thing that can cause wear to the cylinder wall surface, pistons can also be the culprit. When we remove a piston from its cylinder we will notice small lines running from the crown (top) of the piston down to its skirt (bottom) even if it’s a relatively new piston. The lines are caused through particles of grit or dust entering the oiling system. These lines are not generally associated to piston wear. Deep lines or scores on the piston can be removed by the use of an oil stone run annually around the piston in the area of the damage. Galling may occur, this is a more serious condition that can be caused by dirt, old oil, lack of oil caused by a faulty or worn oil pump or leaks somewhere on the engine. Galling on pistons usually has an adverse affect on the cylinder wall and piston rings, this calls for a rebore job with an oversize piston and rings.
Piston skirts may distort due to there not being enough running clearance between piston and cylinder. Gudgeon or small end pins may push or wear through the circular clips that keep them located causing irreversible damage to cylinder and piston. Rings may break causing damage to the cylinder. Small end bushes may seize due to incorrect sizing or overheating causing loads of damage. Pistons may seize due to bad running in. At Stotfold engineering we have seen engines with holes burnt through the crowns of pistons due to preignition caused by to lean a carb mixture, I.E. Too much air to petrol mixture. Petrol plays a massive cooling role in the combustion chamber. A liner is employed in engines with aluminium or magnesium barrels, liners can be made from close grain cast iron or steel. Liners are put into barrels that have been heated and the liner cooled, the liner is then slid into the barrel casting and allowed to cool, we have made liners as thin as 2.5mm thick.
There is also the wet liner , this is a liner that has quite a thick section, the cylinder block, usually a car, has water ways in it to aid cooling. The block is machined or bored out breaking into the waterways then the wet liner is let into the block so when fitted, the liner has cooling water around a large area of its body.
There are of course coatings that can deposited on the cylinder bores, usually on 2 stroke engines. The barrel is cast in a foundry and then machined or bored to a given size, the barrels are usually cast in magnesium or aluminium. After the boring process is done the barrel bore is then Nikasil coated employing an electroplating system or it can be sprayed internally with ceramic coating, both processes will need diamond honing tools to finish the bores.
The general rules of reboring cylinders.
A cylinder or barrel is presented to us and we will assess the amount of wear that has taken place, usually this will be the next size up for the new piston, +.010″ for British bikes or .1mm for European or Japanese bikes we will then place the barrel or cylinder block on the boring machine and bore them out using a tungsten carbide cutting tip. To get the best finish we leave about .0005thou smaller than the finished size that we need. The .0005thou is needed for the honing process that will leave a swirl pattern within the cylinder bore. Prior to the 1960’s barrels were not honed as the cylinder boring process would give a perfect enough finish. Almost all Japanese motor manufacturers for instance hone there barrels which gives a nice “run in” finish with nice swirl patterns within the bore.