Below in an interesting read for those who have the intention of changing pistons/piston rings be it for high compression or big bore. The process that people then to neglect when dropping in piston is the honing/preparation of the cylinder itself. Read on for the process of engine cylinder bore honing.
When an engine is rebuilt, the cylinders usually need attention. Wear tends to create taper in the upper part of the cylinder that can reduce ring sealing and increase blowby and oil consumption if not removed. The cylinder may also be out of round, scored or have other damage that requires correcting before a new set of rings will seal properly.
The main objective when refinishing the cylinders is to make the walls as straight as possible (no taper), the bores as round as possible (minimal distortion, which is especially important with low tension rings), to have the right amount of crosshatch for good oil retention and ring support, and to produce a surface finish that meets the requirements of the rings. This is done by boring and/or honing the cylinders in one or several steps with various types of abrasives (vitrified or diamond).
After honing, the cylinders need to be cleaned remove residual abrasive and metallic debris that is left in the bores. Washing and scrubbing with warm soapy water will remove most of the unwanted material. But washing alone does not loosen or remove surface "swarf" such as torn or folded metal that can wear rings and delay ring seating. The only way to get rid of this material and smooth the bores is to "polish" the bores after honing with some type of flexible abrasive brush.
Brushing after honing not only helps clean the bores, but can also plateau the surface depending on the characteristics of the abrasive used. Brushing sweeps away the torn and folded metal as well as the sharp, jagged peaks leaving a much smoother surface. The result is a better bore finish with little extra effort. Another way to plateau the surface is to use very fine #600 grit stones or cork to polish the bores after honing.
One of the advantages of a plateau bore finish is that it preconditions or breaks-in the cylinders. This reduces the time it takes to seat a new set of rings as well as initial ring wear, blowby and oil consumption. The engine delivers good compression right away, there is no blue smoke in the exhaust, emissions and oil consumption are reduced, and the rings last longer because they have not had to wear to conform to the bores. A plateau bore surface also provides increased bearing area to support the rings while retaining enough depth in the crosshatch for good oil retention and lubrication. That is why the original equipment engine manufacturers (OEMs) favor this type of bore finish and use it in many new engines (see sidebar on how to plateau hone a Ford 4.6L V8).
CYLINDER HONING CHALLENGES
One of the concerns expressed by OEMs who have engine reman programs is that many aftermarket engine rebuilders may not have the know-how or right kind of honing equipment to reproduce an OEM type of cylinder bore finish. With emissions testing a fact of life for many motorists in many parts of the country, the worry is that a rebuilt engine with cylinders honed the "usual way" may not pass an emissions test. The challenge here is to develop procedures that allow aftermarket engine rebuilders to duplicate an OEM bore finish.
Ring manufacturers are concerned that some engine rebuilders may not be using the proper honing procedures or stones for their rings. Too rough a bore finish will produce a lot of scrubbing when the engine is initially fired up. With prelapped rings, this is not good because it creates unnecessary wear. The challenge here is to use honing procedures that produce the best possible bore finish for a given set of rings.
RECOMMENDED CYLINDER BORE FINISH
Most ring manufacturers specify a #220 grit honing abrasive for finishing the bores when using cast iron or chrome rings because the recommended bore finish for these rings is 28 to 35 RA (roughness average in microinches). A #280 grit stone is generally recommended for moly rings because moly rings like a somewhat smoother finish of 16 to 23 RA. But these recommendations are for conventional vitrified abrasives, not diamond. Diamond cuts differently from a vitrified stone, so higher numbers are generally required for an equivalent finish. A #325 to #550 grit diamond stone may be required for the final honing step to achieve an RA finish in the desired range. One manufacturer says a 500 to 550 grit diamond honing stone will produce a surface finish in the 13 to 15 RA range.
To add to the confusion over which honing stones may be required to produce a certain kind of finish, some vitrified honing stones with identical grit ratings will produce different finishes that may not always agree with the reference charts.
For example, one #220 grit vitrified stone may produce a surface finish of 28 to 35 RA while another may leave a much rougher finish of 35 to 60 RA, which is too rough for most prelapped rings. The difference in actual surface finish is due to the bonding agents and fillers that are used to hold the abrasive particles together. The type and quality of lubricant used during the honing process can also make a difference, too.
The third challenge is profitability. Cylinder bore refinishing is a time-consuming and expensive step in the engine rebuilding process. So anything that can be done to reduce honing costs and streamline the procedure while also improving the bore finish is a step in the right direction.
CONVENTIONAL HONING ABRASIVES VS. DIAMOND HONING STONES
Many shops bore or rough hone cylinders to within .003 inch of final oversize (.010 to .030 in. depending on the application), then finish hone the last .003 in. of the bore with #220 or #280 grit vitrified abrasives. Most shops do not have a profilometer to measure surface finish parameters such as RA, RK (core roughness), RPK (average peak height) and RVK (average valley depth), bearing area and so on, so they rely on stone grits along with the right honing pressure, head speed, stroke rate and lubricant to achieve the desired bore finish. Consequently, there's no way to know if the bore finish actually meets the requirements of the ring manufacturer or the OEM, unless someone complains about excessive ring wear, blowby or oil consumption. But even if you have not experienced any ring problems, it does not mean the cylinders are as good as they could be or should be.
One of the limitations of vitrified abrasives is that they wear rapidly. Depending on the grade of stones and the hardness of the block, a set of vitrified honing stones might do three V8 blocks (24 cylinder bores) before they are worn out and have to be replaced. And with each cylinder that is honed, the operator or equipment must compensate for stone wear to keep the bores straight. If you fail to compensate, you can end up with taper in the bores.
By comparison, metal bond diamond honing stones wear very little. A set of diamond honing stones might do 300 V8 engine blocks (2400 cylinder bores) before they have to be replaced. The slower wear rate means the stones tend to cut straighter (less taper) than vitrified stones.
The slower wear rate of diamond versus vitrified abrasives helps to more than offset the much higher initial cost of diamond stones. Using the above figures, a set of $14 vitrified honing stones cost about 58 cents per hole if they do 24 cylinder bores. A $300 set of diamond honing stones cost about twelve and a half cents per hole if they set does 2400 cylinder bores.
For these reasons, many production engine rebuilders (PERs) have switched to diamond honing. Diamond lowers their honing costs per cylinder, saves labor (fewer stone changes), and gives better overall bore geometry (straighter and less distortion).
RECON RESULTS
"We have had excellent results with diamond honing," said Tom Wilson of Recon Automotive Remanufacturers, Philadelphia. Wilson says Recon uses diamond stones in Sunnen CK-21 honing equipment.
"The type of stones we use is dictated by the type of rings that are going into a motor. We have tried various stones before we came up with the best combination. For an RA of 20 to 25 with moly rings, we use #325 grit diamond stones. For a finer finish in the RA 15 to 20 range, we sometimes use #500 grit diamond stones.
"Diamond cuts differently from vitrified abrasives. It rips the metal out and leaves a lot of microscopic fuzz on the surface. So after honing, we brush the bores with a hand drill eight to ten strokes. Brushing does a good job of cleaning the debris off the surface and eliminates any break-in period. We have also found that it improves the RA, too, getting it down around 18 or so."
Wilson says Recon uses a water-based synthetic lubricant with the diamond stones, which he says runs "clear as water." He says the lubricant is filtered to take out the dirt, and monitored constantly to prevent any bacteria growth.
FRANKLIN POWER
"As an OEM supplier to Navistar, the only way we can meet their cylinder bore specifications is to follow a three-step diamond honing procedure, says Jim Ormsby of Franklin Power, Franklin IN.
First, we rough hone to within .005 of final size with coarse grit #200 diamond stones, then final hone to size with find grit #600 diamonds. The last step is to brush hone the bores eight strokes with a plateau honing tool.
"We believe we actually get a better finish and maintain closer tolerances than the original OEM bores because we pay close attention to every bore we do. OEMs are not set up that way. They turn on a production line and let it go."
Ormsby says it is easy to be consistent with diamond honing, provided you have the proper equipment.
Honing Hard Engine Block and Cylinder Liner Materials
In recent years, Nikasil coatings have provided a challenge for engine builders. Nikasil is a hard coating of nickel and silicon carbide about .0025˝ to .003˝ thick that is applied to cylinder bores to improve wear resistance. Invented by the German firm Mahle, Nikasil was originally developed for the Mercedes Wankel rotary engine. It has been used by BMW and Porsche in some of their engines, and is also used in many chain saw engines, some motorcycle and marine engines, and even many NASCAR Winston Cup engines.
Goodson's Jensen says PERs have had success honing Nikasil treated cylinders with diamond. But for smaller shops that have only portable honing equipment, you can�t exert enough pressure with diamond to hone Nikasil. The best advice here is to use #220 silicone carbine and just do a couple of strokes to deglaze the cylinder. If a cylinder has to be bored to oversize, cut it out with a boring bar and then hone in the usual manner to achieve the desired dimensions and finish.
Ed Kiebler of Winona Van Norman in Wichita, KS, says new harder coatings on cylinder walls are forcing shops to change to diamond honing and to upgrade their equipment.
"I see a lot of shops who are interested in diamond but who don't fully realize what is involved in the diamond honing process. Diamond takes a lot of pressure to cut. Some people use diamond on portable hones, but realistically you can't get enough pressure to make the diamonds perform well. Having said that, I truly believe the new harder cylinder coating materials are going to force people to go to diamonds," says Kiebler.
"The two-cycle stuff is all Nikasil. Now the outboard engines are going to Nikasil, too. All the NASCAR Winston Cup shops are using Nikasil cylinders. If it is good for NASCAR, it won't be long before you start seeing it in OEM engines," Kiebler explains. "The time is coming when you are going to have to use diamonds if you are going to hone Nikasil cylinders."
Kiebler says all most shops do is slightly roughen Nikasil cylinders. "You don't really remove much material. The Winston Cup shops are running some of these motors five races before they redo the cylinders. The Nikasil coating really extends ring life and cuts down on ring wear."
taken from: http://www.aa1car.com
