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Cylinder Bores – Machining to Sleeving

The function of a cylinder bore is fairly simple yet quite demanding. The bore is nothing more than a cylindrical void which contains the engine’s air/fuel mixture as it undergoes compression and combustion.

By Larry Carley

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The walls of the cylinder have to be strong enough and rigid enough to withstand high combustion pressures and temperatures (600 to 1,000 psi and 3,500 degrees F for gasoline engines, and up to 2,500 psi and 4,500 degrees F for diesels!). The cylinder bore also has to provide a wear-resistant bearing surface to support the piston and rings, and retain enough oil to keep the rings lubricated.

When boring and honing cylinders, installing sleeves or replacing cylinder liners, three criteria are absolutely critical for good results:

1. Bore geometry and straightness. This refers to the roundness or cylindricity of the bore (which is necessary for good ring seating and sealing), the straightness of the sides of the bore (to minimize ring flex as the piston moves up and down), and its alignment to the bore center and crankshaft. The bore must also be accurate dimensionally so the piston will fit with proper side clearances.

2. Surface finish. The average roughness, peak height and valley depth of the surface must be compatible with the type of rings that are used.

3. Crosshatch angle. Should be within a specified range for the application to provide proper lubrication and oil control.

Cylinder Bore Roundness
The rounder the hole, the less ring tension is needed to seal the piston rings. Lower tension rings are better because they reduce friction. To get a rounder hole with minimal distortion, steel torque plates are bolted to the block to simulate the bore distortion that occurs when the cylinder heads are installed. 

The torque load on the head bolts tends to pucker the bore next to the bolt threads, causing the bore to bulge inward up to several thousandths of an inch. The lighter the block, the more vulnerable it is to this type of bore distortion.

If the block is bored and honed without the simulated load created by a torque plate, the bore may be perfectly round after it has been machined. But it won’t stay that way. As soon as the head is installed and bolted down, the head bolts will pull the bore out of round, possibly causing a poor seal between the bore wall and rings (especially if thin, low tension rings are used).  

That’s why most performance engine builders and even many production engine builders use torque plates when they are machining blocks. Good bore geometry improves ring sealing to maximize power while minimizing blowby, compression losses, emissions and oil consumption.

How much bore distortion is too much? It depends on the application. For  many late model gasoline engines with relatively tight piston to cylinder wall clearances, .0005˝ out-of-round may be too much. But on an older Chevy small block budget motor, higher tension rings and greater piston clearances, you might get by with as much as .005˝ of bore distortion.

Cylinder taper and straightness are just as important. On some late model gasoline engines, the factory specs call for .0002˝ (.006 mm) or less of taper. By comparison, the maximum amount of taper allowed in many older passenger car engines was as much as .010˝ (0.25 mm).

Too much cylinder bore taper is bad because it causes the rings to flex in and out as the piston slides up and down.  Excessive taper can lead to ring breakage as well as interference problems if the ring end gap is not sufficient to handle the change in bore diameter. With gapless rings, good bore geometry and taper are even more important to minimize blowby and compression loses and to maximize the benefits provided by this style of ring.

How do you check bore geometry? A simple bore gauge that’s used to measure the inside diameter of the bore is okay for checking gross dimensions, but it can’t give you a 3D map of what the bore actually looks like.  A bore that appears to be round may actually have some taper, distortion or misalignment in various areas as you go from the top of the bore to the bottom.

Three dimensional bore mapping requires sophisticated (and very expensive) lab equipment that piston ring manufacturers and engine designers typically have but very few shops can afford to own. All you can do is make sure you are using proper honing and finishing procedures so hopefully the bore will be round and straight.  

Keep a close eye on honing pressure so you don’t push the bore out as it is being honed. Diamond honing stones typically deliver the best bore geometry because diamond is more consistent than vitrified honing abrasives – especially with less experienced honing machine operators. Consequently, there’s less risk of machining taper into a cylinder bore when honing with diamonds because stone wear is almost nonexistent.

Diamond honing stones are easier, faster and last much longer than vitrified abrasives. Yes, they do cost more initially but a set of diamond honing stones can typically hone up to 10,000 holes before they are worn out.

A honing machine that offers variable speed stroking and can dwell in the bore while maintaining the same loading reading will produce better bore geometry than a machine that lacks these features.
Using a coolant that is compatible with your honing stones will also improve bore geometry. The coolant flushes away debris while helping the stones maintain a consistent temperature. You can use synthetic water based coolants or honing oil with diamond, but honing oil only with vitrified abrasives.

Controlling Bore Geometry When Sleeving
Good bore geometry is just as important with sleeves and cylinder liners.  Whether you are boring out a cylinder to accept a repair sleeve, or replacing a worn cylinder liner with a new one, the location and alignment of the hole must be right on for a proper fit.

The hole must not only dimensionally correct, but in proper alignment with respect to the block deck surface, adjacent cylinders and crankshaft.  CNC machining equipment can give excellent results for this type of work, provided all of the dimensions are entered correctly and the block is fixtured accurately (level and true).

The amount of interference fit when installing dry sleeves is critical for proper sleeve support, sleeve retention and thermal transfer.  Aluminum blocks have more thermal expansion than cast iron blocks, so they generally (but not always) require more interference fit to keep the sleeves from moving.

Recommendations vary, but flangeless steel sleeves installed in aluminum blocks typically require anywhere from .0015˝ to .004˝ of interference fit. If the block is iron, you can use less interference fit (.0015 to .002˝). With flanged sleeves, however, little or no interference fit may be required.

Using too much interference fit (over .004˝) runs the risk of distorting the block and sleeve. With wet liners, there is no interference fit to worry about.