That’s the question…
By Francisco Aristizabal
In the overhaul process of current monoblock diesel engines, a frequent question at the time of rebuild of the cylinder/piston assembly is do we sleeve, or do we bore? The answer is always bore as far as possible in order to use oversize pistons. However, there are myths and misunderstandings around this process, as well as limited supply in some markets (as in Latin America for example), of pistons available outside of the standard size.
The use of oversize pistons allows you to extend the cylinder block’s life keeping its structure and avoiding the use of a salvage sleeve. This is the final option after 0.010” (0.25mm), 0.020” (0.50mm) or even 0.040” (1.00mm) pistons have been considered, without sacrificing any differences regarding performance and or fuel/oil consumption.
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Nevertheless, all the OEM procedures and specifications must be accomplished to assure a properly reconditioned assembly, , all these include considerations about piston and block materials, piston expansion fit and form, and others, including torque plate use and a correct and proper honing procedure.
TORQUE PLATES
Torque plates are becoming an important part of cylinder finishing. Caterpillar and Cummins just to name a few have increased their use and is a mandatory step mentioned in the service manuals to prevent cylinder distortion and piston scuffing. Although it is a part number included in the portfolio of most OEMs, this is a tool that could also be made locally in the shop. It is just necessary to have enough thickness (1-1/2″ to 2″) to allow proper location and torque of the head bolts, and using an old head gasket.
HONING
The days of running a manual ball brush through the cylinder and reassembling the engine are over. Today, modern equipment can be found to be automatic and programmable, have versatile sizes of grit/type hone heads, and full cooling is essential. Start by milling the block to a correct preliminary dimension, usually 0.005” less than the desired bore for the oversize piston. Then hone to final size in 2 or 3 stages to achieve the final dimension using either diamond, silicone carbide, and carbide plateau using the surface texture and hone angle recommended by OEM.
AND IF SLEEVING IS NEEDED?
Well, there are more topics to consider, some salvage sleeves are offered only by the aftermarket. The material and their compatibility with the block regarding thermal expansion and conductivity is basically the most important, including the press fit installation tolerances. Most of the failures in this process are related to sleeves that are so thin that they “copy” any block distortion and can’t transfer heat properly. There can be serious consequences even with adequate width but installed with a wrong press fit, the results are equally not good.
INSTALLATION
The interference press fit usually must be around 0.0005” – 0.0015”, freezing of the sleeves is recommended beforehand, the use of a sleeve driver to press or drive smoothly the repair sleeve into the cylinder bore until repair sleeve contacts the step in the bottom of the cylinder bore is preferred. Avoid hitting the sleeve; some manuals approve the use of retaining compounds such as Loctite 620, always check the OEM manual for detailed specs.
Obviously all the operations (measurement, honing, etc) must be performed with the torque plate installed as mentioned previously. All the pistons must be the same size (standard or oversize) mixing sizes is not recommended, this could cause balancing and performance issues.
PISTONS
Pistons are among the most heavily stressed components in diesel engines with temperatures around 2000°C (3600°F) and pressures up 200 bar (2900 PSI). Several designs including cast aluminum materials and now forged steel are especially suited for use in piston manufacturing. The fitting clearance is the difference between the cylinder diameter and the largest piston diameter. The fitting clearance should be as small as possible, so that consistently smooth running is achieved under all operating conditions. Due to the difference in thermal expansion, this goal is most difficult to achieve for the combination of an aluminum piston and gray cast iron blocks. Because of high injection pressures, steel pistons are used more and more frequently, therefore it is important to always check the skirt oil clearance according OEM specs.
The reusability guidelines are another topic to consider, aluminum and steel pistons have different criteria beyond piston diameter and piston ring groove wear; the condition of the coating (mainly in steel pistons), ring lands, and pin bores are important too and have several check points to evaluate.
As always, we mention all this info as it is intended to serve as a general guide. Specific procedures may vary depending on the equipment manufacturer / specifically application. Always check their literature and any other OEM service recommendations, the AERA tech line is also available to solve questions in these matters.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2024EPQ3/#p=96