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The hot honing unit is plumbed to the block on the honing machine. The intention is to simulate engine operating temperature (at least in terms of coolant temperature) during honing.
THERMAL CONSIDERATIONS DURING HONING
Another aspect of obtaining a bore that will remain as true as possible during engine operation involves heat. Consider that although a bore may be correctively trued while the block is cold, it stands to reason that bore shape may be affected as temperatures rise and fluctuate. In that regard, does it make sense to hone cylinders at room temperature, or at an elevated temperature that more closely resembles levels experienced in a running engine? Some argue that the change is minimal; or that the act of honing generates surface heat on the walls anyway. However, others are of the opinion that the block should be heated with circulating water in replicate the effects of hot coolant flowing through the block (and adjacent to the bores).
Although a torque plate affects the upper 1.5″ or so of the bore, the dimensional change of an average bore between hot and cold can be twice that of a torque plate, or more. Because of the complex interaction between engine design, casting processes, uneven wall thickness, variations in metal composition and internal metal stresses, dimensional changes of the cylinders within a block can vary significantly. Not only are there thermal dimensional changes, but these changes are far from uniform throughout the bore. The major effect is the gross dimensional change in bore diameter, which can change 0.0005″-0.001″, per inch of bore diameter, at temperatures between 65 and 210 degrees F. For a common 4″ bore, this equals 0.002″-0.004″ of growth.
The rest of these variations within the bore-thermal distortions-are largely non-uniform and can create critical concerns. These distortions include the local diametric variations in the walls, the barrel or hourglass effect, bores departing from circular and becoming non-round and the bowing or arcing of the bore from a true cylinder. A normal bore gauge can detect all but the last of these distortions.
Since the cylinder walls are not of uniform thickness and the cylinder is captured at each end, thermal expansion of the metal can and does cause distortion of the cylinder walls. A normal cylinder wall will assume a slight barrel shape as the temperature increases, with high and low spots that vary from cylinder to cylinder, causing a non-circular, non-uniform bore as the engine heats.
Temperature is not the only effect on a bore. Under normal conditions, the engine cooling system operates at 10-14 psi. Normal engine pressure forces the cylinder into an hourglass shape. While 10-14 psi may not seem like much, it compresses the cylinder walls in the center around 0.0005″. The normal pressure effect from the cooling system tends to partially offset the barrel effect of this thermal distortion. Because of these offsetting effects, hot honing must be done with the block pressurized to achieve the best possible true bore.
Although a few attempts at creating a “hot honing” environment have been seen in previous years, an excellent example of a viable system is KW Products’ “Hot Hone 2000.” This is a completely self-contained system that features a stainless-steel cartridge pump, brass tank and armored safety hoses with quick (“Kwik”) connect couplings. The system includes a fully adjustable electronic thermostat, pressure and temperature gauge, automotive-style filler, soft plug and block adapters for the small-block Chevy. According to KW, the unit is adaptable to all honing machines and uses standard torque plates.
Block adapters for other engines are also available or can be fabricated by an individual shop.
Torque plates reproduce the stress in the upper 0.5″-1.5″ of the bore that would otherwise be produced by bolting the cylinder head in place while still allowing access to machining the bore. The actual measured amount of distortion in the areas around the bolt at the top of the block is seldom more than 0.0005″-0.001″. This is however a significant amount when you consider that the average high-performance engine builder won’t accept a bore that is less than 0.0002″ straight and true.
It is usually necessary, especially with plain-Jane OE blocks, to torque-plate the block when you bore it if you plan to use torque plates when honing. The hone may jerk from the distortion if it wasn’t originally bored with torque plates, and sometimes the bore is distorted enough that by the time the machinist straightens the bore, it may be larger than desired.
When a block is heated to its intended operating temperature and the subsequent related pressure, the amount of distortion will vary depending on many factors. An obvious few are temperature, pressure, block material, varying density of material, quality of casting, Siamese bores, sleeved blocks and offset boring.
The average change in size of a cylinder bore heated to 190 degrees F from an ambient temperature of 65 degrees F is between 0.003″-0.004″. This anticipated change is not necessarily linear, as different areas of the cylinder bore may change by varying amounts.
According to research done by KW, in addition to the amount of average bore size increase, the average difference from cylindrical in the heated and pressurized bore seems to be around 0.0015″. This change can be several times the change seen with torque plates. These are average figures and the real revelation will be those areas of the bore that move outside of these norms without obvious reason.
Theoretically, with the engine block heated to operating temperature during the honing process, the bores should remain fairly stable and stay on-size throughout the process.
Various factors can affect cylinder bore geometry changes
Factory original cast blocks:
• The density of the material varies
• Core thickness varies throughout
• Machining tolerances are not within acceptable performance guidelines
• Webs or reinforcement of critical areas are not optimal
• All core sand, ties, stands and machining debris is not removed
• Galleys are not sized or routed optimally
• Water jackets and passages do not allow for even heat transfer
• Flow problems in molds create weakened areas
Aftermarket blocks
(all materials):
• Some are just copies of OEM and suffer the same ills
• Some are far superior in most of the above areas
Tags: BORE DISTORTION, BORE GEOMETRY, HONING, SUNNEN
