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.
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.
Some suppliers of sleeves for aluminum engine applications say bore distortion can be minimized by installing the sleeves with only minimal interference (.0005 to .001?) and by using an anaerobic sealer to prevent the sleeves from moving.
One way to ensure good heat transfer between the sleeve and block is to lightly hone the block with #280 grit stones after the hole has been bored to size. This will produce a smoother, flatter surface for supporting the sleeve.
On some air-cooled small displacement engines (motorcycle and small engines, for example), more interference fit may be required because the cylinders run at higher temperatures. We have heard of engine builders using as much as .006 to .008? of interference fit to ensure the sleeves stay in place.
Something else to keep in mind is that if you are sleeving only one damaged cylinder in a block to repair it, the sleeve may distort the adjacent cylinders somewhat – especially if you use a lot of interference fit. The result may be ring sealing problems, compression losses and blowby in the adjacent cylinders.
When installing dry sleeves in a block, you can reduce the risk of creating bore distortion is to cool the sleeves and/or preheat the block so the sleeves will slip into place more easily. Never pound on a sleeve to drive it in as this can damage and distort the sleeve. If it needs force, use a mandrel and hydraulic ram to press it in.
The type of surface finish that’s required will vary depending on the application. For a typical stock gasoline engine with moly faced piston rings, honing with #280 vitrified abrasives or #320 grit diamond stones and brushing afterwards will provide a good surface finish. If you want to plateau the surface, use a two-step honing process.
After the initial hone with #280 grit vitrified abrasive or #320 diamond, lightly hone the bore with #400 grit vitrified abrasives or #500 diamond stones, then finish with a brush. Brushing cleans off the broken peaks and debris from the surface. Plateauing the bore with a two-step honing process leaves a flatter surface with more bearing area that improves ring seal and reduces the time it takes for the rings to seat.
After the cylinders have been honed, don’t forget to scrub them out with warm soapy water and a brush to remove all honing and metallic debris. This is an often overlooked step that can ruin a new set of rings in a hurry.
By The Numbers
If you are using a profilometer to check surface finishes, here are some ballpark recommendations to aim for. For stock, street performance and circle track applications with moly-faced rings:
• Ra (roughness average): 10 to 20 microinches
• Rpk (peak height): 5 to 20 microinches
• Rvk (valley depth): 30 to 65 microinches
• Rk (plateau area): 30 to 50 microinches
Surface finish recommendations can vary greatly depending on the type of rings, the application, the hardness of the block and whether the bores have any type of wear-resistant coating. For example, on blocks with nickle/carbide hardened cylinders or a thermal spray coating, the coating has craters that retain oil better than an uncoated surface.
Because of this, the surface finish can be much smoother (as little as 2 to 4 Ra) and less crosshatch can be used. With compacted graphite blocks, surface finishes can also be smoother (12 Ra range) with less valley depth to retain oil (Rpk around 15).
For most stock gasoline engines, ring manufacturers typically recommend a crosshatch angle of 42 to 45 degrees (included angle). On performance engines with low tension rings, many engine builders go with a shallower crosshatch angle of 20 to 30 degrees. With Nikasil and other coated bores, the crosshatch angle is often reduced to 10 to 15 degrees or less.
When refinishing cylinders, bore to within .005? of final dimension, or rough hone to within .003? of final dimension, then finish hone to size with a finer grit abrasive (#280 or #320) followed by a plateau hone (#400 or #500 grit stones) and a brush. If a cylinder is bored or rough honed to within .0005? or less of its final size, the final honing step won’t leave enough depth in the valleys to provide adequate oil retention. The crosshatch will quickly scrub off resulting in high oil consumption and wear.
According to one ring manufacturer, honing should leave the cylinder with a surface that distributes oil, serves as an oil reservoir and provides a place for worn metal and abrasive particles to escape. The surface must also have enough flat area (plateau) to act as a bearing surface on which an oil film can form. Most ring sealing and oil consumption problems are the result of an improper surface finish and/or the wrong crosshatch in the cylinders. Here are some problems to avoid:
• Crosshatch grooves are too wide or deep. This can result in abnormal wear, excessive oil consumption and a prolonged ring break-in period. This can be caused by using a stone grit that is too coarse, poor stone breakdown, coolant viscosity too high or excessive stone pressure.
• Crosshatch grooves are irregularly spaced. This can prevent proper oil distribution and slow ring break-in.
• Crosshatch grooves contain a lot of folded and fragmented metal. This can slow ring seating, cause scratching and high wear, and increased oil consumption. The cause is insufficient dwell strokes at the end of the honing cut, or using stones that are too coarse.
• Plateau burnish. This can slow ring seating, increase blowby and hurt fuel economy and performance. The cause is honing with loaded vitrified stones that are too hard, contaminated coolant or wrong coolant viscosity, excessive stone pressure, or too long a dwell.
• One-directional cut crosshatch. This can cause excessive ring rotation, rapid wear, poor ring seating and increased blowby. The cause is excessive play in the honing equipment or hone head.
• Crosshatch angle too low. The result may be poor oil distribution, high impact forces on the rings, slower ring break-in and increased ring wear. It is caused by low reciprocation rate compared to honing rpm.
• Metal pulled from bore surface. This creates pits that increase blowby and oil consumption. Can be caused by using stones that are too hard, excessive stone pressure, or not enough honing time.