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6th December 2005

Tech: Cylinder Bore Honing

If your old bike engine needs a rebore, then what happens to it during the honing process? Is this the same as glaze-busting? Can new rings improve oil consumption? Hutch explains...

Honing is the final process used in the machining of cylinder bores, either during manufacture or in re-sizing (re-boring). Honing is used as a process to both remove the final amount of metal to get a cylinder bore to within the required size limits, and to put a surface on the cylinder bore which will give good life span, and aid lubrication and oil consumption characteristics in use.

Honing is not only carried out on cylinder bores for internal combustion engines but on compressors, hydraulics components and probably dozens of other applications.

Engine manufacturers realised long ago that the honing on their cylinder bores was one of the key components to engine longevity and emissions control (and oil consumption). A lot of money is spent on large machines specifically for honing cylinders, developing the optimum finish for their requirements and measuring and assessing the effect differing quality of the honing finish.

Is it just me, or does anyone else see a scary techno-owl when they look at this picture?

New engines are now measured on their emissions not when they are new but when they have completed the equivalent of 100K miles on test. It is comparatively easy to produce an engine that is low on oil consumption when new! So honing is not a final, last-thought process applied after a rebore just to get the right size, as it often appears to be in some engine machine shops I have visited.

Obviously, there are other components of engine design and manufacture that control life and emissions / oil consumption. Some of these are design based and there is little you can do to change them when rebuilding an engine. Other cylinder characteristics, such as the size, roundness and cylindricity, as well as the honing quality, are very important and you can affect when rebuilding an engine. I only intend to talk about honing here.

It is very difficult to assess honing quality easily and affordably. The three components that are specified, characterised and measured relate to the honing angle, depth and surface finish, or surface roughness.

The honing angle and evenness of depth are generally assessed visually. What you are looking for is a constant angle between the cross-hatch lines all over the cylinder, that they intersect such that the more acute angles on each cross are at the sides of the bore, not the top and bottom. The angles should be in the region of 35- to 45-degrees. Angles greater than 45-degrees can increase oil consumption, less than 30-degrees can accelerate wear (although there are other factors such as piston speeds and windage that will also help or hinder this).

Honing depth should be even for both directions of honing lines. One set of honing lines are cut as the hone passes down the bore and then the other as the hone rises in the bore. If you look at the honing then one set of honing lines should not appear to be significantly 'behind' the other but should be on the same plane, ie; the depth of cut was the same in both directions.

Surface finish? Nothing a bit of a rub down with some wet and dry won't sort out...

Surface finish, or surface roughness, is a very big subject. There are literally hundreds of surface finish parameters which can be calculated from the raw data produced when a surface roughness trace is taken for measurement. Fortunately all calculations are made by computer these days, as it a pure mathematician's dream (as opposed to an engineer's nightmare!)

The calculated surface finish parameters can tell you many significant details about the surface of the cylinder bore such as:-

  • *The volume of oil retained;
  • *The percentage of contact area you will have when run in;
  • *The amount of wear before the contact area becomes too great and wear accelerates greatly.

    The list is a very long one and only of detailed interest to real anoraks...

    Surface finish is measured in microns (thousandth of a millimetre) and points of microns so it is very fine -- and unlikely to be able to be measured in your average rebore shop or engine rebuild workshop.

    What all the measured results of the surface finish parameters can tell you are a good estimate of a bores expected life, oil retention and oil consumption.

    When a bore has been run in, all the peaks will have been cut or worn off, leaving the honing grooves. These grooves act as oil reservoirs for lubricating the bore and spreading the oil round the cylinder wall as the rings pass over them. Too much oil retention (if the grooves are too deep or there are too many of them -- the cylinder is 'rough') and the oil consumption will be high.

    Not enough oil retention (so if the honing grooves are too shallow, not enough or at an angle where the oil drains too quickly) and the cylinder bore will wear prematurely. This will be a 'smooth' bore, in which wear occurs because there is too much contact area for the rings against the bore, which in effect gives micro-friction welding and breaking of the rings to the cylinder wall.

    The life of a cylinder bore in terms of wear, excepting mechanical failures, can probably be broken into three distinct phases:-

  • *Running in - where all the high spots from the machining process are worn off, ending in a run in working surface.
  • *Normal useful operational life - where the bore is properly lubricated and the bearing ratio between the bore and the rings doesn't produce undue wear.
  • *Worn bore - where through size changes and wear, oil consumption increases; the hones are either worn away or full of 'glaze' (by product from oil combustion on the bores) with reduced bore lubrication leading to further, accelerated, bore wear.

    One way to reduce oil consumption and raise or retain compression is to increase the ring pack tension; so fit stronger rings that scrape the bore more. If rings have lost their tension over time, which they can do, this is what fitting new rings will achieve. However if you fit rings with greater tension than standard it will improve your oil consumption straight away, but it will also greatly reduce your bore life, I believe this is one of the ways in which Cordiflex rings used to work.

    So correct, good quality honing is important to engine life. It is not the same as 'glaze-busting' which is cleaning out the existing hones during a rebuild, which is probably also a good thing to do.

    Are those finger prints?

    So how effective is the honing you get when you have had a rebore done at the local engine machining shop?

    Well, size wise the bore will probably be pretty good. This is easy to measure. The roundness and cylindricity may be questionable once the barrels are bolted onto the engine, but that is another big subject.

    If the honing looks good, as described, and even all over then it is probably OK. What the standard of honing was when many old engines were new I don't know; nowhere near the requirements of today I would guess, which is why new motors tend to do so many more miles before being worn out. It all depends on how many miles between rebores and new rings you are happy with.

    Boring Tales?

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