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Pelican Technical Article:

Machine Shop 101

Time:

As long as it takes

Tab:

$50 to $1,500

Tools:

A good quality machine shop

Applicable Models:

 
Porsche 911 (1965-89)
Porsche 912 (1965-69)
Porsche 914 (1970-76)
Porsche 930 Turbo (1976-89)

Parts Required:

Valve guides, valve seats, new valves " if needed

Hot Tip:

Find a machinist that has time to answer your questions

Performance Gain:

Tightly machined parts for a better running engine

Complementary Modification:

Blueprint your engine
101 Projects for Your Porsche 911

This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.

If you are planning on having your motor rebuilt, or having a top-end rebuild performed, then you will probably need to take some of your parts to your local machine shop. Some tasks require special, precise tools and knowledge that only a machine shop possesses. Most of the time the parts are dropped off, and then returned magically rebuilt 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.

One of the most useful services of a machine shop is their ability to clean parts. For less than $100, the shop will clean and bead-blast sheet metal, flywheels, heads, body parts: just about anything you want. If you have 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), then most shops have advanced cleaning tanks that are similar to industrial sized dishwashers for greasy, oil-soaked parts.

One of the most common procedures to be performed at the machine shop is the reconditioning of cylinder heads. This is a job that obviously cannot be performed at home because there are many specialized tools that are used in the process.

The first thing that needs to be done is to separate the valves and springs from the heads. The heads are placed on a specialized spring compressor, which compresses the valve, and allows the removal of the retaining clip that secures the valve and spring together. The valves can then be removed from the assembly. Then the heads are either cleaned or blasted until they look like they just came out of the Porsche box brand new.

The heads are then inspected to see if they need new valve guides. In most cases, the guides will be worn beyond the recommended Porsche specifications, and need to be replaced. One quick test to see if the valve is worn is to insert the valve into the guide and look to see if it can wobble back and forth. If guide 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. One of the most common methods of removing valve guides is to tap threads into the guide and screw in a cap screw. This screw now gives the valve guide puller a grip for removing the valve from the head.

New guides are pressed into the head. Advances in valve guide technology have resulted in newer types of 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 in order to make sure that the inner bore is within the proper specifications.

The heads contain valve seats, which are steel inserts that are 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 the valve seats. For a valve to be reused, it must still have a significant amount of material on both the outer edge, and also on the valve stem itself. If not, the valve can no longer be used. In general, a valve can be used for about 1-2 rebuilds before it needs to be replaced. Exhaust valves should only be used once, unless they are the more expensive sodium-filled ones. The sodium-filled valves dissipate heat better than standard stainless steel valves, and thus are less exposed to the wear and tear of thermal shock that might inflict a steel valve. 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, the guide, and the seat are all precision processes that need to be aligned together. If a machine shop is sloppy, or their equipment is out of alignment, then you might be in for trouble later on. In some cases, the cheapest machine shop might not do the best quality job. Unfortunately, it's very difficult to check the tolerances on the valves after you get them back from the shop.

Another common procedure at the machine shop is the honing of used cylinders. The honing process places a crisscross pattern on the inside of the cylinders that allow the rings to seat properly when the engine is being broken in. The honing process also helps to insure that the cylinders are round when the new rings are installed. There were two types of cylinders primarily used on the later 911s, Nikasil and Alusil. The Nikasil cylinders, manufactured by Mahle, can be easily honed on a standard machine, whereas the Alusil cylinders cannot. The Alusil cylinders, manufactured by Kolbenschmitt, have a coating on the inside of the cylinders made from a special coating material that doesn't respond well to the honing process. Make sure that your machine shop understands this, and can identify the two different types prior to honing your cylinders.

You should also take your engine case into the machine shop to be cleaned and checked. Make sure that the case is not sand-blasted, as the sand may get caught in the tiny oil passages that feed various parts of the engine. The shop will check the engine case to make sure that all the bearing surfaces are round and aligned with each other. If they are not, then a procedure called align-boring is performed. In many cases, machine shops will out-source this particular job because the machines required for the job can be large and expensive. The process of 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.

The crankshaft should also be taken to the machine shop before using it in a rebuild. The crank is put through a process called magnafluxing, which can isolate and identify microscopic cracks that may lead to failure at a later date. In general, the 911 cranks are very strong, and aren't normally susceptible to cracking. But it's better to be safe than sorry.

If a bearing spun in your 911 motor, your machine shop might recommend that you regrind the crankshaft. In general, I recommend that you find a good used standard crank that doesn't need to be reground. The grinding process removes the hardened surface on the crank and makes them wear out faster.

It's also a wise idea to get the crank polished. The bearing surfaces of the cranks require a smooth surface in order to properly create a thin oil film to ride upon. If the surface is a bit 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 lift.

The connecting rods are important parts that need to be reconditioned at the machine shop as well. New wrist pin bushings should be placed at the rod's end. There is also a procedure called resizing that makes sure the size of the rod bearing that fits around the crankshaft is the correct size. Over the life of the motor, rods sometimes stretch, causing the rod bearing surface to become slightly out-of-round. In order to correct this, the smaller half of the rod is removed and a small amount of material is removed from the mating surface. Then the rod is then re-mated with it's other half, and the bearing surface is machined to the original factory specifications. Removing the small amount of material from the smaller rod half is common, and doesn't affect the strength or reliability of the rod.

It may also be necessary to regrind your camshaft. In general, the 911 motors do not exhibit large amounts of wear on the cam shafts, but a re-grind and polish close to original specifications may be necessary. If the camshaft is pitted, then 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.

The most valuable tool in the machine shop is your machinist. Make sure that you choose one who takes pride in their work, is more than willing to answer your questions, and offer to show you around the shop. Remember that all the attention to detail that you may take rebuilding your motor could be useless if your parts are not machined correctly.

It all begins here with the bead blaster.
Figure 1

It all begins here with the bead blaster. A bit better than sand blasting, the bead blaster is kinder to the surface of the metal when it is used. Paint, oil, dirt and grime are no match for the bead blaster. It is simply amazing how parts that are completely covered with grime exit the blaster looking like they came out of new Porsche factory boxes. In order to properly assess the condition of cylinder heads, they must be completely cleaned in the bead-blaster.

You can often assess the condition of the valve guides, simply by placing a good valve in them and seeing how much it wobbles.
Figure 2

You can often assess the condition of the valve guides, simply by placing a good valve in them and seeing how much it wobbles. If it doesn't move around too much, then further measuring of the valve guide wear will be required. In most cases, the valve will wobble, indicating that the guides are worn, and should be replaced.

The valve guides are removed from the heads by threading them, and then inserting a cap screw into them.
Figure 3

The valve guides are removed from the heads by threading them, and then inserting a cap screw into them. The screw and valve guide can then be easily pulled from the head.

The old and new valve guides are placed side by side here for comparison.
Figure 4

The old and new valve guides are placed side by side here for comparison. In many cases, the valve guide design and materials used over the years has improved. In particular, the valve guides from the 2.7 liter motors used from 1974-77 were often known to wear out after about 60,000 miles. Newer advances in materials can increase that wear time up to 250,000 miles or more.

The heads are machined on a special jig that is aligned to cut the valve seats to match the valves exactly.
Figure 5

The heads are machined on a special jig that is aligned to cut the valve seats to match the valves exactly. A special cutting tool cuts the angle of the seats while the machine holds the heads aligned to the inner bore of the valve guide.

The tool used for grinding the valve seats is made of special tool steel, and is ground to reflect the desired profile and angle of the seats.
Figure 6

The tool used for grinding the valve seats is made of special tool steel, and is ground to reflect the desired profile and angle of the seats.

Used valves can usually be reused if there is enough material on the edge for a regrind.
Figure 7

Used valves can usually be reused if there is enough material on the edge for a regrind. The valve on the left is a brand new one, the one on the right doesn't have enough material left on its edge for another regrind. Intake valves can usually be used again with no problems, but the exhaust valves should only be used once, unless they are the more expensive sodium-filled ones. These exhaust valves dissipate heat much better than plain stainless steel ones, and thus have a longer life.

Here are two valves who have seen better days.
Figure 8

Here are two valves who have seen better days. The valve on the left has been ground so thin that its edge has cracked off. The valve on the right came from an engine that exhibited signs of the valve getting too hot. If the seat and the valve don't meet and mount perfectly, then hot spots will build up in the valve, causing cracks like the ones in this valve.

If there is enough clearance left on the valve, then it can be reground to match the valve seats.
Figure 9

If there is enough clearance left on the valve, then it can be reground to match the valve seats. The process is performed on a special valve grinder that can be set to match the angle of the valve to the angle of the valve seat.

The honing machine places a crisscross pattern on the inside of the cylinders, helps the rings seat, and also helps keep the compression high.
Figure 10

The honing machine places a crisscross pattern on the inside of the cylinders, helps the rings seat, and also helps keep the compression high.

Your best asset in rebuilding your engine is your machinist.
Figure 11

Your best asset in rebuilding your engine is your machinist. Look for one that doesn't mind answering your questions. Beware of ones that insist that they know what's best without explaining to you why. Sometimes the cheapest machine shop isn't always the best bet either. Try to find one that takes pride in their work. Ask around, as stories of bad shops tend to spread quite easily from the mouths of disgruntled customers.

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Comments and Suggestions:
theweaver Comments: I have posted my problem on the site, and have not yet recd a satisfactory answer regarding my thoughts about honing alusil barrels. The wear on my barrels is so minimal!
I understand that they have a coating, but cannot establish who can do it, or whether enthusiasts have had success honing and replacing rings without the coating, nor what will happen to the barrels if they are not coated etc. I was also told that there are articles on the site which can point me to the problems and solutions. All I can get is advertisements when I do a search on this site.

Ultimately, if there is someone in the Uk, preferably in or near to the Staffordshire area this would be great.
No one seems to know what this mysterious coating is. I feel sure that this is something which when found out is quite straight forward. From a purchasing background within a large factory which I have, it would be quite easy to source if we knew what is is!!
My local auto machinists treat Porche around here like the plague! Should you wish to look at my topic 911 head bolts, you perhaps could see mine and others' dilemma.


Kind regards Geoff Degg
August 15, 2012
  Followup from the Pelican Staff: I have heard of people doing this by hand and having success, however I would not do it myself unless I had a local machine shop with Alusil machining strips.

Once the silicon is ground with normal maching methods, you lose lubrication quality by grinding the silicon off the cylinder wall.

Here is thread discussing it: http://forums.pelicanparts.com/porsche-911-technical-forum/591676-honing-alusil-cylinders.html - Nick at Pelican Parts
 

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