If you plan to have your engine rebuilt, or a top-end
rebuild performed, you will probably need to take some of your parts to a
machine shop. Some tasks require special, precise tools and knowledge that only
a machine shop possesses. Most of the time, owners drop off their parts and
then pick up magically rebuilt parts with no clue as to what really happened to
them. This section aims to take some of the mystery out of what happens to your
parts when you drop them off.
Machine shops are especially useful for their parts cleaning services. For less than $100, the shop will clean and bead blast sheet metal, flywheels, heads, body parts--just about anything you want. If you’ve ever sat in your garage with a piece of sandpaper and a block of wood, you will instantly recognize how much time and energy can be saved by having your parts blasted. If, for some reason, you can’t use a blasting procedure (on engine cases or oil coolers, for example), most shops have advanced cleaning tanks that are similar to industrial-sized dishwashers for greasy, oil-soaked parts.
Cylinder head reconditioning is a popular procedure performed at the machine shop. This job generally can’t be performed at home because the process requires too many specialized tools
First, the valves and springs are separated from the head. The head is then placed into a specialized spring compressor tool that compressesthe valve spring, allowing for removal of the entire spring assembly. . The valves can then be removed from the assembly. Finally, the head is either cleaned or blasted until it looks like it just came out of a brand-new BMW box.
The heads are then inspected to see if they need new valve guides. In most cases, the guides will be worn beyond the recommended BMW specifications and need to be replaced. To quickly see if a guide is worn, the valve is inserted into the guide to see if it can wobble it back and forth. If it doesn’t wobble, then a more precise small-bore gauge will be needed to accurately measure the guide.
If the guide is worn, it needs to be removed. Threads are tapped into the guide and a cap screw is screwed in. This screw gives the valve guide puller a grip to remove the valve from the head.
New guides are pressed into the head. Advances in valve guide technology have resulted in new materials with higher wear strengths. Newer guides may look different than the older ones and should last considerably longer. After the guides are pressed into the heads, they are reamed to ensure that the inner bore is within the proper specifications.
The heads contain valve seats, which are steel inserts pressed within the aluminum casting of the head. In most cases, it is not necessary to replace the seat in the head. The seat is machined in precise alignment with the new valve guide. A machine that aligns itself with the new guide cuts the seat at a specific angle so that the valve will seat and seal properly.
The valves themselves are machined as well to match the angles of the valve guides and valve seats. For a valve to be reused, it must still have a significant amount of material on both the outer edge and the valve stem itself. If not, the valve can no longer be used. In general, a valve can be used for one or two rebuilds before it will need to be replaced. Exhaust valves should only be used once, unless they are the more-expensive sodium-filled valves. The sodium-filled valves dissipate heat better than standard stainless-steel valves and thus are less vulnerable to the wear and tear of thermal shock that might affect a steel valve. In turn, the valves are set into a valve grinding tool and precisely ground to the angle that matches the angle on the valve seats.
As you can imagine, the machining of the valve, guide, and seat are precision processes that need to be aligned together. If a machine shop is sloppy, or its equipment is out of alignment, you might be in for trouble later on. In some cases, the cheapest machine shop might not do a quality job. Unfortunately, it’s very difficult to check the tolerances on the valves after you get them back from the shop.
While you’re there, also take your engine case in to be cleaned and checked. Be sure, however, that the case is not sandblasted, as sand may get caught in the tiny oil passages that feed various parts of the engine. The shop will check the engine case to ensure all the bearing surfaces are round and aligned with each other. If they are not, a procedure called “align boring” is performed. In many cases, machine shops will outsource align boring, because the necessary machines can be large and expensive. Align boring increases the outer diameter of the bearings to a specific size, while aligning all the bearing surfaces within the case. After the case is bored out, you must use oversized bearing sets instead of the standard sets.
Also take the crankshaft to the machine shop before using it in a rebuild. Magnafluxing is a common procedure associated with crankshaft inspection. This process exposes all the flaws in the crankshaft, affording detection of any microscopic cracks on the surface. Have your crankshaft Magnafluxed if you plan to reuse it.
Magnafluxing is a relatively simple process. The crankshaft is initially magnetized using a large circular magnet. The magnetic field is applied to the crankshaft at a 45-degree angle, so the process will detect cracks that run both parallel and perpendicular to the length of the crankshaft. The crankshaft is sprayed with a special liquid that has a magnetic powder suspended within the solution. This powder becomes trapped in any cracks present in the crankshaft. The crankshaft is then examined under an ultraviolet (black) light in total darkness. Under the black light, the cracks clearly show as bright lines in the surface. Crankshafts typically show failures at the points where the journal bearing meets a center flange.
After the crankshaft is tested, it is demagnetized and then washed in solvent to remove the Magnaflux material. Make sure the crankshaft is demagnetized; otherwise, the tiny bits of metal that inevitably find their way into your engine oil will stick to the crankshaft bearing journals.
It’s also wise to get the crank polished. The bearing surfaces of the crank require a smooth surface in order to properly create a thin oil film to ride upon. If the surface is at all rough, it disrupts the flow of oil around the bearing. Polishing the crank keeps the oil flowing smoothly around the bearing surfaces and increases engine bearing life.
Additionally, the connecting rods need to be reconditioned at the machine shop. New wrist pin bushings should be placed at the rod’s end. The procedure known as “resizing” ensures the size of the rod bearing that fits around the crankshaft is correct. Over the life of the engine, rods sometimes stretch, causing the rod bearing surface to become slightly out of round. In order to correct this problem, the rod cap is removed and a small amount of material is removed from the mating surface. The rod cap is then reattached to the rod, and the bearing surface is machined to original factory specifications. Removing a small amount of material from the smaller rod half is common and doesn’t affect the strength or reliability of the rod.
Finally, it may be necessary to have your camshaft reground. Although most BMW engines don’t exhibit large amounts of wear on the camshafts, some engines may require a regrind and polish close to original specifications. If the camshaft is at all pitted, it may be necessary to weld the pits, regrind the shaft, and retreat the metal to reharden the surface on the lobes of the cam.
Entrusting any or all of these tasks to a reputable machine shop will help ensure a long life for your engine.
Machine shops are especially useful for their parts cleaning services. For less than $100, the shop will clean and bead blast sheet metal, flywheels, heads, body parts--just about anything you want. If you’ve ever sat in your garage with a piece of sandpaper and a block of wood, you will instantly recognize how much time and energy can be saved by having your parts blasted. If, for some reason, you can’t use a blasting procedure (on engine cases or oil coolers, for example), most shops have advanced cleaning tanks that are similar to industrial-sized dishwashers for greasy, oil-soaked parts.
Cylinder head reconditioning is a popular procedure performed at the machine shop. This job generally can’t be performed at home because the process requires too many specialized tools
First, the valves and springs are separated from the head. The head is then placed into a specialized spring compressor tool that compressesthe valve spring, allowing for removal of the entire spring assembly. . The valves can then be removed from the assembly. Finally, the head is either cleaned or blasted until it looks like it just came out of a brand-new BMW box.
The heads are then inspected to see if they need new valve guides. In most cases, the guides will be worn beyond the recommended BMW specifications and need to be replaced. To quickly see if a guide is worn, the valve is inserted into the guide to see if it can wobble it back and forth. If it doesn’t wobble, then a more precise small-bore gauge will be needed to accurately measure the guide.
If the guide is worn, it needs to be removed. Threads are tapped into the guide and a cap screw is screwed in. This screw gives the valve guide puller a grip to remove the valve from the head.
New guides are pressed into the head. Advances in valve guide technology have resulted in new materials with higher wear strengths. Newer guides may look different than the older ones and should last considerably longer. After the guides are pressed into the heads, they are reamed to ensure that the inner bore is within the proper specifications.
The heads contain valve seats, which are steel inserts pressed within the aluminum casting of the head. In most cases, it is not necessary to replace the seat in the head. The seat is machined in precise alignment with the new valve guide. A machine that aligns itself with the new guide cuts the seat at a specific angle so that the valve will seat and seal properly.
The valves themselves are machined as well to match the angles of the valve guides and valve seats. For a valve to be reused, it must still have a significant amount of material on both the outer edge and the valve stem itself. If not, the valve can no longer be used. In general, a valve can be used for one or two rebuilds before it will need to be replaced. Exhaust valves should only be used once, unless they are the more-expensive sodium-filled valves. The sodium-filled valves dissipate heat better than standard stainless-steel valves and thus are less vulnerable to the wear and tear of thermal shock that might affect a steel valve. In turn, the valves are set into a valve grinding tool and precisely ground to the angle that matches the angle on the valve seats.
As you can imagine, the machining of the valve, guide, and seat are precision processes that need to be aligned together. If a machine shop is sloppy, or its equipment is out of alignment, you might be in for trouble later on. In some cases, the cheapest machine shop might not do a quality job. Unfortunately, it’s very difficult to check the tolerances on the valves after you get them back from the shop.
While you’re there, also take your engine case in to be cleaned and checked. Be sure, however, that the case is not sandblasted, as sand may get caught in the tiny oil passages that feed various parts of the engine. The shop will check the engine case to ensure all the bearing surfaces are round and aligned with each other. If they are not, a procedure called “align boring” is performed. In many cases, machine shops will outsource align boring, because the necessary machines can be large and expensive. Align boring increases the outer diameter of the bearings to a specific size, while aligning all the bearing surfaces within the case. After the case is bored out, you must use oversized bearing sets instead of the standard sets.
Also take the crankshaft to the machine shop before using it in a rebuild. Magnafluxing is a common procedure associated with crankshaft inspection. This process exposes all the flaws in the crankshaft, affording detection of any microscopic cracks on the surface. Have your crankshaft Magnafluxed if you plan to reuse it.
Magnafluxing is a relatively simple process. The crankshaft is initially magnetized using a large circular magnet. The magnetic field is applied to the crankshaft at a 45-degree angle, so the process will detect cracks that run both parallel and perpendicular to the length of the crankshaft. The crankshaft is sprayed with a special liquid that has a magnetic powder suspended within the solution. This powder becomes trapped in any cracks present in the crankshaft. The crankshaft is then examined under an ultraviolet (black) light in total darkness. Under the black light, the cracks clearly show as bright lines in the surface. Crankshafts typically show failures at the points where the journal bearing meets a center flange.
After the crankshaft is tested, it is demagnetized and then washed in solvent to remove the Magnaflux material. Make sure the crankshaft is demagnetized; otherwise, the tiny bits of metal that inevitably find their way into your engine oil will stick to the crankshaft bearing journals.
It’s also wise to get the crank polished. The bearing surfaces of the crank require a smooth surface in order to properly create a thin oil film to ride upon. If the surface is at all rough, it disrupts the flow of oil around the bearing. Polishing the crank keeps the oil flowing smoothly around the bearing surfaces and increases engine bearing life.
Additionally, the connecting rods need to be reconditioned at the machine shop. New wrist pin bushings should be placed at the rod’s end. The procedure known as “resizing” ensures the size of the rod bearing that fits around the crankshaft is correct. Over the life of the engine, rods sometimes stretch, causing the rod bearing surface to become slightly out of round. In order to correct this problem, the rod cap is removed and a small amount of material is removed from the mating surface. The rod cap is then reattached to the rod, and the bearing surface is machined to original factory specifications. Removing a small amount of material from the smaller rod half is common and doesn’t affect the strength or reliability of the rod.
Finally, it may be necessary to have your camshaft reground. Although most BMW engines don’t exhibit large amounts of wear on the camshafts, some engines may require a regrind and polish close to original specifications. If the camshaft is at all pitted, it may be necessary to weld the pits, regrind the shaft, and retreat the metal to reharden the surface on the lobes of the cam.
Entrusting any or all of these tasks to a reputable machine shop will help ensure a long life for your engine.
No comments:
Post a Comment