1/24/2012
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The rounder the hole, the less ring tension is ne...
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Cylinder Bores – Machining to Sleeving
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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.
Surface Finish
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).
Crosshatch
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.
For more information about boring, honing and sleeving, visit our online archives.
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