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16th March 2005


Tech: Cylinder Bore Roundness

A banana, yesterday.Your old bike's engine will have at least one cylinder which should, in theory, be cylindrical. Hutch explains how cylinders can become squared off, barrelled, or even (gulp) banana-shaped...

We all know that size is important. You need the right size and thus a good fit of piston and rings to cylinder bore for engine efficiency, low blow-by (an excess of which can shorten the life of your crank bearings) good compression, and acceptable oil consumption.

The surface finish or roughness is also important for oil consumption and longevity of cylinder life.

Measuring the roundness of a not very round circle.It is accepted that worn cylinder bores are not round and that being round is also important for achieving a good seal between the rings and the cylinder wall, again minimising blow by, oil consumption and maximising compression. So roundness is important too.

In engineering terms 'roundness' is a two dimensional parameter, while 'cylindricity' is its three dimensional counterpart.

By engineering definition, roundness is the diameteral difference between two perfect, theoretical, concentric circles between which the measured circle can just sit; it is only valid for any one distance down the bore.

130 microns is about 5 thou.Similarly, cylindricity is the radial difference between two perfect, theoretical, concentric, cylinders between which the real cylinder exists.

My experience shows that a cylindricity within 130 microns (0.130 mm) will be acceptable and slightly more is probably OK on older engines where the expectation of oil consumption and such is different from that of modern engines.

A demonstration of geometric intloerance.Picture diagrams showing roundness and cylindricity definitions can be found by looking up geometric tolerancing on the Internet, it's a big subject!

Unfortunately cylinders do not wear evenly and just get bigger in diameter, they distort and diameters can also get smaller as cylinders go oval.

Three dimensionally, cylinders tend to wear into a banana shape. The position of maximum cylinder wear is the point of maximum piston velocity on the thrust side of the cylinder. This is where a cylinder will wear smoothest and will have, therefore, accelerated wear over other areas on the cylinder wall.

The other two points of high wear on a cylinder will be the inner and outer ring reversal points (six points really - on a three ring piston) as these are the positions of maximum piston acceleration and deceleration. This is why the fingernail test for a ridge at the top of a cylinder for wear is used.

A cylinder, yesterday.'Barrel-ing' can also happen on engines, especially those with ally rods, as seen on a lot of later big Brit twins.

The small ends wear out of line with the big end bore, a rebore will only rectify this for a short while as the lack of parallelism between the big and small end bearings on the conrods will give accelerated cylinder wear.

A difference of any more than 1 to 2 thou across the ends of a gudgeon pin fitted in a rod stood vertically means the rod needs straightening and re-bushing.

Unfortunately it can be difficult to spot 'banana-ing' and 'barrel-ing' of a cylinder bore by just measuring diameters.

A very large and expensive machine, yesterday. There are very large and expensive machines specifically for this type of measurement but these aren't accessible to most.

It is good practice, however, to measure a cylinder at a number of positions down a bore and in a number of positions at each height, not just two positions at right angles to each other.

Remember a bore need not be either round or oval it can be triangulated, squared, 'threpenny-bitted' or other shapes for which I don't know the name!

As with most engineering, cylinder bores are not perfect when new or re-bored, even when done well. It is just a question of how much deviation from the theoretical perfection is acceptable for good function.

There are a plethora of reasons why a cylinder may not be round and doesn't have good cylindricity when machined. Even if a cylinder is as near to perfect form as possible after machining, the chances are that it will distort considerably when assembled onto the engine.

Pulling down a barrel onto a short motor or a cylinder head onto a barrel will distort it considerably; this can be improved by machining the bore with a 'dummy' head bolted in place.

Engines with head bolts that thread into the barrels will invariably distort the cylinder at a position level with the threads.

Tool Stuff on eBay.co.uk

A barrel shaped barrel.The typical four head bolts round a cylinder will give a squaring of the bore, probably right at the mid point of the barrel, which is also the point of max piston speed and wear.

Again this could be minimised by having tightened bolts in them when machining the cylinder.

All these distortions can be very small, but can have significant effect on the performance of an engine, its oil consumption, life, etc.

What it does mean is that over tightening of head bolts will have more of a detrimental effect than you may think.

A banana shaped barrel. Also check for yellowness.Some distortions can be easily coped with by piston rings distorting to take up the shape of the cylinder. Oval, tri-lobed or triangular out of roundness (known as second and third order harmonics of roundness), can be coped with quite easily by piston rings - up to a certain magnitude of distortion.

Fourth order harmonics of roundness (squaring - remember the headbolts) and above can't be coped with by rings nearly as easily, so these will cause increased oil consumption, blow by, etc.

So what can be done about all this? Some of the problems are design based and little can be done during a rebuild.

Measuring for cylindricity.Some things can be improved to get better life and performance from a cylinder. The basic points are:-

  • Don't over tighten your head bolts.

  • When measuring a used cylinder try to get an idea of form as well as size.

  • Take a measurement below the lower ring reversal point (unworn bit) to get some idea of how round your cylinder was when new.

  • Measure a cylinder a second time with a head bolted on if possible - and be prepared for a shock.

  • If your cylinder is barrelled, look at the rods.

  • If you bore / hone yourself, try to replicate the forces applied in assembly.

  • Use a reputable engine machine shop for your re-bores.

    No hand-drills, please.

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