Engine Cylinder Design and Function - Part 7

Ring Ridges
The wear that isn’t there
  
The piston doesn’t travel the entire length of the cylinder, be it sleeved or in-block. The two main causes of cylinder wear are piston slap, which we’ve covered and ring wear. The piston rings eventually wear away at the cylinder and wear themselves. However when rings fail people usually just replace the rings and sometimes the piston, but rarely the entire cylinder, especially the multiple cylinder engines. This means that the cylinder will wear through 2 or even 3 sets of rings before that receive any attention.

Cylinder with heavy ridging, carbon has not been cleaned from this surface highlightening it.

This wear can be corrected, as in some cases has to be. The ring ridge starts where the pistons stops on the upstroke. Most engines are used in transportation and rarely rev high enough for this to become a problem. However, unbeknownst to most people connecting rods stretch under heavy load at high rpm. If the ridge is deep enough and an engine is revved hard under load the rings will pass over this ridge and this sudden impact can break piston rings causing major damage to the cylinder and piston.

 FAE of 3 different styles of connecting rods. This analysis is focused on rod stretch.

To remove the ridges from a cylinder the inner walls need to be machined. This is done with a cylinder boring machine obviously. These machines are self-centering and bore the cylinder to a larger diameter. Usually a oversized piston is fitted to fit take up the slack, most manufacturers sell oversized pistons and rings in 0.50mm increments.

Boring is also the method to remove the oval out of worn cylinders. With sleeved cylinders the sleeve is usually replaced.



 
Honing a bore
Smoothing out the cut

After the cylinder has been bored out to a larger diameter the surface is too rough for piston rings to be fitted. The inner cylinder wall needs to be smooth to ensure good sealing and smooth running. So you’d think the cylinder would be polished to a shine, this would actually be bad for the engine causing excessive heating and probably a seize.

Honing marks in a cast iron sleeve.

The surface also has to be able to retain an oil film to reduce wear. This is where honing comes in. Honing is basically precision grinding, but removes very little material. The bore is machined close but not to the final dimension. This excess of material is left for the honing process.

Honing uses stones of various grip that are fitted to a spring loaded tool that rotates inside the cylinder bore. These stones lightly grind at the inner surface. The honing tool is moved up and down the bore similar to the movement of a piston. As this is happening the honing stones are rotated constantly. The rotating and up and down stroking ensures the stones grind a perfect bore with parallel walls. 

Honing head with stones attached.  

The stroke rate of the honing head is carefully controlled to grind the cylinder in a particular pattern, this is know as cross hatching. Cross hatching is usually at 45˚ along the cylinder wall as pictured above. This angle helps retain oil on the cylinder wall. Too step and angle and the oil will run off the cylinder wall. Too level and the rings will flutter along the bore. Some engines run 60˚ hones with good results, although the angle is harder to control.

 Flex-hone

Other tools available are known as flex-hones, however they should be renamed as they don’t really hone to cylinder wall in the same fashion as a proper cylinder hone. These flex hones are ceramic balls attached to wire brush attachment. They and then fitted to a hand drill and spun in the bore in a similar fashion to a honing head.

Flex hones are only good for removing carbon build up and some cooked on oil residue which is called glazing. Flex hones shouldn’t be used as a substitute for professional honing.