1/1/2000
Click on a thumbnail to see the full-size image
Newer OBD-II Blocks Require Much More Attention To Proper Honing Procedures
By Norm Brandes
Smoother finishes, tighter tolerances mean stricter standards and processes
Want to guarantee a comeback? When you are honing the cylinders on a late model engine, use the hardest cutting stones you have. Run the hone as fast as you can so you finish the job quickly. Working hard and fast generates a lot of heat on both the cylinder wall and the stones, so you’ll need a lot of cutting oil. But oil is expensive, so save money by using the cheapest oil you can find. Check an industrial supply catalog; you can get some great deals on cutting oil. Finally, check your work by measuring cylinder out-of-round to within 0.001˝.
When you’re finished, button up the engine, reinstall it in the customer’s car and send it on down the road. After a few months or a few thousand miles, you can expect that engine to be back on your doorstep. If you’re lucky, you’ll only be out the cost of redoing your work. If you are not lucky, you won’t be able to repair the damage you did to the block. You’ll be the one paying to replace the block.
Honing procedures that worked on an older engine can cause a disaster on a late model engine. To comply with OBD-II emissions standards, while at the same time meeting fuel mileage requirements and satisfying owner demand for performance, engine designers have been pushed into using new material and new technology on modern engines. The result is that OBD-II engines have smoother finishes, tighter clearances and less tolerance for machining errors than an early model engine.
The more involved you become working on newer engines, the more amazed you will be at what you have to learn and the changes you must make in your shop procedures. The differences between working on an older engine and working on a newer one are so great, I am considering a two-tier pricing structure in my shop. There will be one price for work on older (1994 and earlier) engines and another, more expensive price for OBD-II (1995 and newer) work.
Cast-in-place
To save weight, many new engines use a sleeved cylinder. The block is a lighter-weight alloy, while the more wear-resistant sleeve is a cast iron material designed to withstand the stresses of heat, pressure and friction inside the cylinder. Typically, manufacturers design the sleeves without a lot of extra metal. If the sleeve is damaged, or badly worn, there often isn’t enough sleeve wall to rebore the cylinder and install oversized pistons. Sleeve replacement is the only option available.
In some cases, sleeves cannot be replaced. Most sleeves are pressed or pinned into the cylinder. A skilled rebuilder with the right equipment can replace these sleeves. But a "cast-in-place" sleeve is impossible for anyone to replace.
A "cast-in-place" sleeve is inserted into the mold before the engine block is cast. As the molten metal is poured into the mold, it totally surrounds the cast-in-place sleeve. There are ridges on the outside (the side in contact with the block) of the sleeve. The molten metal fills these ridges, forming an interlocking bond between the block and the sleeve.
Removing a cast-in-place sleeve without damaging the block is virtually impossible. Even if a rebuilder somehow managed to carefully machine away the sleeve, then cut out each ridge, without damaging the block, it would be impossible to insert a new sleeve. The ridges on the sleeve are not threads, but equally spaced ridges that are wider than the cylinder diameter. There is no way to screw, press, or otherwise force the sleeve into the cylinder. If even one cylinder is badly damaged or worn on a cast-in-place sleeved engine, replacement of the block is the only option. And you won’t know how bad the block is until after you have disassembled the engine.
When quoting work on a block with sleeved cylinders, I recommend quoting separately for the removal and disassembly of the engine, with a separate quote given after you determine exactly what work must be done.
Inspecting the block before you quote actual repair work also enables you to avoid some unusual problems that I’ve encountered with sleeved cylinders. I don’t know how widespread the problem is, but I have encountered blocks with sleeves that were not positioned properly in the block. As a result, when the sleeve was machined, there was a difference in sleeve wall thickness from one side to the other. The difference was not one or two thousandths of an inch that could only be detected with a micrometer. I am talking about a difference that you could see from 10 feet away once the block was torn down. Because of the variation in wall thickness, no repairs were possible – the block was scrap.
Typically, two types of cast iron sleeves are used in automotive and light truck applications, gray iron and ductile iron. Each type requires a different honing procedure.
To the naked eye, both types appear to be extremely smooth. Under a microscope, however, there is a clear difference between gray iron and ductile iron. Gray iron has a more textured appearance, with a surface made of larger, coarser-shaped granules or graphite flakes. Ductile iron has a smoother appearance, made of smaller, more rounded graphite nodules.
When you hone metal with an abrasive stone, you are not "cutting" the metal. Instead, what you are really doing is "fracturing" or tearing off microscopic pieces of metal. The key to good honing is to control the fracturing; and the properties of the metal determine how you best achieve that control.
In my shop, we use a three-step honing procedure, but the steps are different for gray iron than for ductile sleeves.
With gray iron, we finish our boring or rough honing process (70–80 grit) with .003˝ of material to be removed. The second set-up we use is a medium grit stone (220–280 grit) and sizing the cylinder to within .0009 to .0007˝. We then finish the bore with a 400 grit stone depending upon the bore finish.
With ductile sleeves, because of the smoother cast structure texture, we run the rough finish to within .003˝. The second cut with the medium stone is done to within .0004-.0005˝ of the final cut. And then >we finish with the fine stone. The smoother surface of ductile iron allows us to be slightly more aggressive in our honing because the fracturing of the metal can be better controlled.
Tighter standards
The precise nature of newer engines means more precision is needed when measuring cylinder walls. As one honing expert told me, “When the clearances are specified down to 0.0005˝, you can’t get away with measuring your work to within 0.001˝.”
Not only are tolerances tighter, the tolerances now apply to the length of the cylinder wall. You now must measure at a series of points along the wall to ensure the wall is straight and true along its entire length.
Finishes are also much smoother. A few years ago, piston rings pressed against the cylinder wall with 12 to 15 psi pressure. Today, rings exert only 7 to 9 psi against the wall. A ring with 12 to 15 psi will tolerate a less smooth finish better than a 7 psi ring. But the lighter pressure ring causes less friction, which improves performance and fuel economy.
Honing tips
No matter what type of sleeve we are working on, our basic procedures are the same. I’ve developed these tips based on the experience gained in my shop and conversations with experts at various tool and parts manufacturers.
Start with a softer stone. This will give you greater stone breakdown to cylinder wall, which increases your honing time but will keep you from glazing or burnishing the cylinder walls. If the honing is going smoothly, you can move up to a harder stone or increase the cutting speed. Don’t go to a harder stone and increase speed at the same time!
Keep a record of your procedures so you know what hardness and speed works best the next time the same engine comes into your shop.
The best honing procedures use up the stones quickly.
Use the oil recommended by your equipment supplier. Don’t use industrial grade cutting oils or other substitutes. Each manufacturer develops a specific cutting oil formula to match its abrasive stones. Manufacturers’ cutting recommendations are based on using their
stones with the correct oil. According to a technician at one of the major equipment manufacturers, using the wrong cutting oil is one of the most common causes of problems when honing.
Take care of your abrasive stones. Don’t use kerosene or other strong solvents to clean the stones – you may weaken the bonding agent in the stones, causing the stones to wear or fail quickly.
The honing process wears away both metal and abrasive. A good honing job, with the stone grit and cutting speed properly matched to the job, will cause rapid stone wear.
Control heat when honing. I am a little obsessed with heat control. When we are working on a high performance engine, I try to schedule the work so that the second, medium grade stone cut is done at the end of the day. The block then sits overnight so its temperature can stabilize, and the final cut is made the next morning.
When anyone says I make too much of heat control, I ask them if they have ever been to an engine manufacturing plant and watched how the OEs hone cylinders. The honing is done in multiple steps, and all cutting is done inside large, chilled cabinets that keep the new block, cutting oil, and abrasive stones at a constant temperature, usually around 60° F. I can’t match all of the factory procedures in my shop, but I figure if temperature control is that important to them, then I will do whatever I can to control heat build-up when I am honing a cylinder.
Norm Brandes owns and operates Westech Automotive, Inc., a machine shop and vehicle repair service business located in Silver Lake, WI.