One of the goals of boring and honing cylinders is to size the bores to a specific dimension. If an engine is being remanufactured to meet specific criteria, that usually means boring and honing to achieve a standard oversize (.010?, .020? or .030?). The oversize will depend on the thickness of the casting, how much boring it can safely handle without the cylinder walls getting too thin, and the amount of wear in the cylinders.
If you’re doing a custom rebuild, then the final bore size will probably be determined by how much boring or honing is needed to eliminate all of the taper wear in the cylinders. If there’s less than a few thousandths of taper wear from the top to bottom in the cylinders, you may not have to do much work at all. Simply breaking the glaze and restoring the crosshatch may be all that’s need to seat a new set of rings.
If you’re finish machining a brand new casting, such as an aftermarket performance block, you’ll be honing the cylinders to achieve a certain amount of piston clearance with a standard sized piston. The final dimensions of the bore will depend on how loose or how tight you are building the engine. Figuring the clearance means measuring the OD of the pistons (perpendicular to the pin at the point specified by the piston manufacturer), then adding some incremental amount of clearance (.0005?, .001?, .002?) to the OD to determine the desired ID of the bores. If the pistons have an anti-scuff side coating on the skirt, you can usually ignore or subtract the thickness of the coating when calculating clearances (at least, that’s what the piston manufacturer’s usually say).
If you’re sleeving a block, the final size of the bore IDs will be slightly less than the average ODs of the sleeves. The difference in these two dimensions will determine the amount of interference fit (which will vary depending on the application and whether the block is aluminum or cast iron). Getting to this point requires accurately measuring the ODs of the sleeves and the IDs of the bores.
Good Bore Geometry
The second major goal of boring and honing is to achieve the best possible bore geometry. This refers to the roundness, straightness and form or shape of the bore (cylindricity). You want the bores to be as round as possible, with no taper top to bottom, and true to the crankshaft centerline and deck surfaces on the block.
Poor bore geometry can cause a number of problems. If the bores are not concentric (especially with thin low tension piston rings), the rings won’t seal as tightly and the engine will experience blowby and oil consumption problems. The compression losses will also hurt performance and emissions.
Bore distortion can be caused by any number of things:
• The age and condition of the block. Seasoned blocks and blocks that have been cryogenically treated or vibrated are more stable and less apt to shift than new, raw castings.• The loading of the head bolts, head gaskets and heads on the block. When the head bolts are tightened down, the metal adjacent to the bolt threads tends to pull and pucker. When the bolts are spaced closely to the cylinders, it can create high spots in the cylinders.
• How the coolant circulates around the cylinders inside the engine when the engine is running. Normal coolant temperatures as well as hot spots can create a lot of bore distortion – up to .0015? or more at 200 degrees F compared to room temperature.
• How the block is supported in the vehicle. The location of the motor mounts and other bolt-ons can even cause small distortions in the cylinder bores.
• The type of honing equipment used to finish the cylinders. Bore distortion can sometimes occur as a result of the design of the honing head, how the abrasives are mounted in the tool, and the consistency of the force applied while the bore is being honed.
Bore distortion can be described by levels of “order.” A first order bore is one that is perfectly round with no distortion in any direction. A second order bore is one with an oval distortion in its cylindricity, typically caused by machining errors or heat transfer. Rings can usually tolerate some second order distortion by conforming to the bore. But the lower the ring tension, the less able the rings are to conform to bore distortion. A third order distortion results in a triangular shaped hole, and is usually caused by a combination of second and fourth order distortions. A fourth order distortion is a bore with a cloverleaf or squared shape. This type of distortion is caused by the location and loading of the head bolts. Unfortunately, a bore gauge will often fail to detect lobes in a bore unless you happen to measure at the exact point where a lobe has raised the surface.
Bore distortion can vary from almost nothing up to a couple thousandths of an inch! With today’s tight piston-to-wall clearances, even .0005? of bore distortion may be too much for some engines. Your goal is to make the bore as round as possible by using the proper boring and honing procedures, and up-to-date and properly maintained equipment.
The roundness of the bores may not be so critical in a budget priced small block Chevy or an entry level Saturday night race motor. A typical roundness spec for this type of application might be .005? or less – which is adequate for that type of engine, but no where near the requirements for a late model engine or performance engine with low tension rings, plateau finished bores and tight piston-to-cylinder clearances. The maximum allowable cylinder taper in a late model Ford 4.6L V8, by comparison, is .0008? (over six times less!).
Some performance engine builders say they want their cylinders to be as round as possible, down to .0001? or less! That’s probably overkill for most engines. But a good number to aim for on performance engines and emissions-sensitive late model engines would be concentricity within 0.01 mm (.0004?) or less.
Measuring concentricity requires an accurate bore dial gauge or ID micrometer. The roundness of the bores is determined by measuring each bore parallel to the direction of the crankshaft, then measuring the same bore again perpendicular (90 degrees) to the crankshaft. If both dimensions are identical, you have a nice round hole. If the numbers differ, the greater the difference the more out-of-roundness exists in the hole.
Taper is another critical dimension to consider. Taper in the bores will cause the rings to flex in and out as the rings slide up and down in the cylinders. Over time, this can lead to metal fatigue and ring breakage.
Taper is determined by measuring the bore ID near the top (but below the ring ridge) where wear is greatest, then measuring the bore ID near the bottom where wear is minimal. Subtract the small number from the larger to calculate the amount of taper in the cylinder.
In late model engines, the maximum amount of taper allowed is not much at all. On a Mustang 4.6L V8, Ford says there should be no more than .006 mm (.0002?) of taper top to bottom. By comparison, the maximum amount of taper allowed in many older passenger car engines was as much as 0.25 mm (.010?). That’s a huge difference. So if you are rebuilding a late model engine using yesterday’s specifications, you’re probably going to have problems.
The best advice is to always look up the latest engine service specifications on the vehicle manufacturer’s website (which will always be more up-to-date than any printed manual), and follow those specifications as closely as you can.
Achieving Proper Geometry
To achieve the best possible bore geometry, late model engines with thinwall castings and performance blocks should always be honed with a heavy steel torque plate (deck plate) and head gasket bolted to the block. The plate simulates the loads placed on the block when the head is installed. Honing the block in this condition will result in better bore geometry when the engine is assembled and run. Some performance engine builders even circulate hot coolant through the block (a process called “hot honing”) while the block is being honed to simulate actual operating conditions.
Good bore geometry also requires good equipment. You can’t hope to achieve good bore geometry if your equipment is worn out or not aligned properly.
If you are honing with conventional vitrified abrasives, the operator (or equipment) must compensate for stone wear to keep the cylinders straight. Forget to compensate or compensate by too little or too much and you’ll end up with taper in the bore. Diamond honing stones experience minimal wear and will generally produce straighter bores and greater repeatability hole to hole.
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.
Surface Finish Tips
Surface finish recommendations for late model engines vary depending on the application, but are often in the 15 to 20 microinch range Ra (roughness average). For performance engines, the recommendation may be even smoother, say 10 to 15 Ra. Regardless of the Ra number, most rings seal best with a plateau finish in the cylinders.
There are as many recipes for plateau finishing as there are for making pizza. The objective is to create sufficient crosshatch depth in the cylinder wall to retain oil with a relatively flat, smooth flat surface area between the grooves to support the rings. A plateau finish will essentially mimic a broken-in cylinder. This will drastically reduce the time it takes for new rings to seat, and also minimize ring wear during the break-in process for longer overall ring life.
Rings manufacturers say the best surface finish is often achieved by rough boring a cylinder to within .005? of final dimensions (or rough honing to within .003? of final size), then honing with #220 grit stones down to the last .001?, and finishing with a #280 to #400 grit stone (depending on the application). Brushing the cylinder with a plateau honing tool as the final step removes loose and folded surface debris, and does not alter the dimensions of the bore.
Accurately measuring the surface finish in a freshly honed cylinder requires a special instrument called a profilometer. The profilometer drags a tiny diamond-tipped stylus across the surface to calculate a number of important parameters.
Ra = Roughness average
Rpk = Peak height
Rvk = Valley depth
Rk = Core roughness depth
Rmax = Highest peak-to-valley
Rz = Mean highest peak-to-valley
Mr1 = Peak material ratio
Mr2 = Valley material ratio
Many piston ring manufacturers specify a surface finish of 15 to 20 Ra for moly faced rings, which can be achieved by finish honing with #280 grit stones. Cast iron and chrome rings can tolerate a somewhat rougher surface finish (20 to 35 Ra) so coarser #220 grit stones can be used to produce this type of finish.
If bores are honed with #325 to #400 diamond stones, the finish will typically be in the 22 to 24 Ra range – which is too rough for moly faced rings, so a second finishing step is usually necessary. Plateau finishing the cylinders with #600 grit stones, or a plateau honing tool or brush will improve the finish and lower the numbers to the desired range of 20 or less.
A plateau cylinder bore finish is a good one because it combines all the “good” numbers: low peak height (Rpk), plenty of bearing area (Abbott-Firestone curve), and adequate valley crosshatch (Rvk) for good oil retention and ring lubrication.
According to one honing equipment manufacturer’s guidelines, a “good” bore finish should have Rmax and Rz numbers that are about 10 times the Ra number. If the Rmax or Rz numbers are less than one seventh the Ra number, the surface is glazed and won’t retain oil. If Rmax or Rz is more than 12 times the Ra number, the surface has too many deep scratches.
Here are some “good” bore finish numbers to aim for:
• Ra 10 to 24
• Rpk 6 to 24
• Rvk 20 to 80
• Rk 28 to 48
Some performance engine builders run somewhat higher Rvk (valley depth) numbers in the crosshatch to improve oil retention in high revving engines.
According to one ring manufacturer, the recommended surface numbers for a performance engine with a hard block or cylinders (like a typical ProStock or NASCAR motor) are:
• Rpk = 4 to 6
• Rk = 18 to 22
• Rvk = 28 to 32
To achieve these numbers, the bores are rough honed to size with 270/325 metal bond diamond stones at 170 rpm and 35% load. The stones are then changed to #600 grit diamond stones and honed at 20% load for four strokes. The final step is to brush the cylinders with a plateau finishing tool for 6 strokes at 20% load.
The recommended numbers for other types of racing such as sprint cars and drag cars would be a little rougher:
• Rpk 8 to 10
• Rk = 25 to 30
• Rvk = 35 to 40
The same rough honing procedure as before would be used, but the second step would be 6 strokes with #550 grit diamond stones, followed by 6 stokes with a plateau brush at 20% load.
What about crosshatch? Recomm-endations also vary according to the application. The typical automotive application calls for a 42 to 45 degree (included angle) crosshatch, while 20 to 30 degrees is often recommended for performance engines. If the engine has a Nickasil coating, even less cross hatch is needed, typically 10 to 15 degrees.
One final tip with regards to honing: Don’t forget to scrub out the cylinders with hot soapy water and a brush. Rinsing with solvent won’t remove the metal and abrasive residue.