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Machine Shop 101

Pelican Technical Article:

Machine Shop 101


2 hours - 2 days






Professional engine machine shop with a very skilled engine builder

Applicable Models:

BMW E30 325i Coupe/Conv (1987-93)
BMW E30 325i/iX Sedan (1987-93)
BMW E30 325is/iX Coupe (1987-93)
BMW E36 323i Convertible (1998-99)
BMW E36 323is Coupe (1998-99)
BMW E36 325i Convertible (1992-95)
BMW E36 325i Sedan (1992-95)
BMW E36 325is Coupe (1992-95)
BMW E36 328i Convertible (1996-99)
BMW E36 328i Sedan (1996-99)
BMW E36 328is Coupe (1996-99)
BMW E36 M3 Coupe (1995)
BMW E36 Z3 Convertible (1997-02)
BMW E36 Z3 Coupe (1999-02)
BMW E46 323Ci Coupe/Conv (1999-00)
BMW E46 323i Sedan/Wagon (1999-00)
BMW E46 325Ci Coupe/Conv (2001-06)
BMW E46 325i/xi Sedan/Wagon (2001-06)
BMW E46 328Ci Coupe (1999-00)
BMW E46 328i Sedan (1999-00)
BMW E46 330Ci Coupe/Conv (2001-06)
BMW E46 330i/xi Sedan (2001-06)

Parts Required:

Nothing to all engine internal components depending upon condition of engine

Performance Gain:

An engine that runs like it's new

Complementary Modification:

Rebuild the transmission
101 Performance Projects for Your BMW 3 Series

This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your BMW 3 Series. The book contains 272 pages of full color projects detailing everything from performance mods to timing the camshafts. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any 3 Series owner's collection. The book was released in August 2006, and is available for ordering now. See The Official Book Website for more details.

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.

If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs. If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one. Your continued support directly affects the expansion and existence of this site and technical articles like this one. As always, if you have any questions or comments about this helpful article, please drop us a line.

Figure 1

A bit better than sandblasting, the bead blaster is kinder to the surface of the metal. Paint, oil, dirt, and grime are no match for the bead blaster. Parts that are completely covered with grime exit the blaster looking like they came out of new BMW factory boxes. In order to properly assess the condition of cylinder heads, they must be completely cleaned in the bead blaster.

Figure 2

Twenty-four valves look most impressive when they are laid out on the table like this. Each valve will be inspected and reconditioned to ensure proper sealing with the cylinder head.

Figure 3

Cylinder heads look amazing after they have been cleaned and blasted. The inset shows what the combustion chamber looked like before blasting. Cleaning the head is essential to finding and fixing cracks (see our head gasket replacement article).

Figure 4

The heads are machined on a special jig 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.

Figure 5

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

Figure 6

Valves can typically be reused if there is enough material on the edge for a regrind. The valve on the left is brand-new; the one on the right doesn't have enough material left on its edge for another regrind. Intake valves can often be reused without problems, but exhaust valves should only be used once, unless they are the more-expensive sodium-filled valves. Sodium-filled exhaust valves dissipate heat much better than plain stainless-steel valves, and thus have longer lives.

Figure 7

These two valves have seen better days. The valve on the left has been ground so thin its edge has cracked off. The valve on the right shows signs of getting too hot. If the seat and valve don't meet and mount perfectly, hot spots will build up, causing cracks like the ones in this valve.

Figure 8

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

Figure 9

The Magnaflux process is quite interesting to watch. A crankshaft (or any metal object) is magnetized by the large electromagnetic coil shown on the left. Then, a magnetic powder that can only be seen in ultraviolet (UV) light is sprinkled on the crankshaft. The powder is then blown off the surfaces. It will find its way into any hidden cracks on the surface and can be seen when the UV light shines on the crankshaft. The VW Type IV oil pump gear on the right has a crack near the base of one of the gear teeth. The crack shows up as a green line under the UV light (white arrow).

Figure 10

Your best asset in rebuilding your engine is a competent machinist. As chief technical writer for, I often ask my machinist for his opinion on engines I'm working on. Look for a machinist who doesn't mind answering your questions. Beware of machinists who insist they know what's best without explaining to you why. Very often, the cheapest machine shop isn't always the best bet, either. Try to find one that takes pride in its work. Ask around, as stories of bad shops spread easily from the mouths of disgruntled customers.

Comments and Suggestions:
Dave Comments: I'm getting some increased oil consumption and cloud of grey smoke with difficulty starting on my 2004 3.0 X5 with 200,000 miles. Compression is good. Is the likely culprit then my valve guides? and can I change them without pulling the head? Would it be best to go ahead and pull the head and take to machine shop?
November 14, 2017
  Followup from the Pelican Staff: I would confirm the crankcase breather is Ok. if so, you could have have bad guides / seals. the head will have to be removed to repair it. I would remove, send to machine shop. - Nick at Pelican Parts  
Markos Comments: I have a M52 with 250K and I'm thinking about a turbo build on the stock motor with few new parts higher horse power bearings new oil pump and gaskets. Question is the head do I get it bead blasted then resurfaced?
August 31, 2017
  Followup from the Pelican Staff: You will have to remove the cylinder head, then send to a machine shop that offers those services. - Nick at Pelican Parts  
Bravo0267 Comments: A future project- Mls or Oem head gasket , cylinder head and/or timing chain cover need to be resurface for even mating? What sealant would you use for this mating area? Thanks brent
June 18, 2017
  Followup from the Pelican Staff: I would use all OE parts if it is in your budget.

Only the cylinder may need machining. Have it checked. - Nick at Pelican Parts
andje Comments: j,aimerais reparer un moteur de bmw528ipouvez vous m'aider svp
March 25, 2016
  Followup from the Pelican Staff: Sorry, I can only speak English. - Nick at Pelican Parts  
Cowboy Comments: I have a 91 318IS, just purchased a reconditioned head and it came with Valve old cracked head doesn't haveSprings...only double cam and hydraulic do I proceed?
August 24, 2015
  Followup from the Pelican Staff: Your old engine uses valve springs as well. - Nick at Pelican Parts  
sean Comments: Thank you for talking about machine shops' parts cleaning services. I know that I often just hand clean my parts to the best of my abilities, but I am not always satisfied with how clean they are. $100 is definitely worth it for the deep clean you get from the machine shop.
April 9, 2015
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
Nate Dawg Comments: 1995 bmw 525i e34 m50 motor, my baby has 300,00 miles and running pretty good. i have black soot in the tailpipe and i can't pass california smog, could my fuel injectors be delivering to much fuel ?
January 28, 2015
  Followup from the Pelican Staff: They could be, could also be too high of fuel pressure or oil burning. I would check fuel control and monitor the exhaust for visible smoke. - Nick at Pelican Parts  
memo Comments: Am getting ready to work on my 2003 bmw 325i 6 cylinder, apparently has a blown head gasket, I will like to buy the book for this project, which one you recommend.

June 23, 2014
  Followup from the Pelican Staff: I would grab the Bentley manual. ?command=show_part_page&please_wait=N&make=BMW&model=218§ion=TOLBKS&page=1&bookmark=0&part_number=PEL-B305 - Nick at Pelican Parts  
Aviator Comments: My BMW Chilton manual dedicated to years 1989-1991, and dedicated to 318-M3 and 525-M5 says on page 3-47, note 24 specific to S14: "BMW does not recommend machining the head." I am original owner of the 1989 E30 S14 and I am in the middle of a 1st time head gasket replacement due to engine overheating. All the machinist I've spoken with want to resurface the deck according to standard procedure, but the book specifically says it is not recommended for S14. Another reputable 2000hp race boat engine builder friend of mine straight edged the deck and said he doesn't see that machining of the deck is needed. Costa Mesa machining says 1 mil per cylinder of warpage is allowed on BMW heads, which I am going to recheck to see if it is in spec, i.e. 4 mils for the S14. What is Pelican parts' opinion for this S14 E30 head surface?
May 27, 2014
  Followup from the Pelican Staff: Check the date on the manual, there may be more current information from BMW on the subject. The head can be machined if a thicker head gasket is available. Give our parts specialists a call at 1-888-280-7799 and they can help figure out which part or repair kit you need.
- Nick at Pelican Parts
Ned Comments: already spent a much cash and need some help to repair the cylinder head, making a sputtering sound on take off and fumes/smoke are given off - pls help making holes in my pocket
March 17, 2014
  Followup from the Pelican Staff: Sounds like an engine misfire. What color is the smoke? - Nick at Pelican Parts  
Livewire Comments: I have a 95 M3 that was showing signs that the valve guides were worn. I removed the head, took to a BMW recommended machine shop that does all of their head work and got it rebuilt Cams removed, planed, valve guides, and valves groundI didnt remove the oil check valve on bottom of head before I took it to them and when I got it back, I removed it and it was full of bead blast material. I cleaned it ball worked fine and the passage best I could and reinstalled. My engine runs fine but cant get the lifters to stop ticking and I cant see any oil moving around through the head when looking through the oil fill hole. Oil pressure is good.I am also sure that the new head gasket is on correct.
Any thoughts before I remove the head again?
October 28, 2013
  Followup from the Pelican Staff: I sounds like there is not enough oil pressure getting to the cams and lifters. The oil comes up through the block and cylinder head. The passages in the head might be plugged or there could be a problem with the head gasket. It looks like you are going to have to start taking it apart to find the problem.
- Nick at Pelican Parts
Adam H. Comments: I'm curious...if I only wanted to resurface the head, do the valves need to be removed in addition to the cams? Thanks in advance.
January 7, 2010
  Followup from the Pelican Staff: The camshafts have to be removed at the minimum. The rest depends onthe machine shop. I prefer to remove valves when I have a head resurfaced so I can inspect the valve seats and valves. However, some machine shops will vacuum test the cylinder head for you, this requires the valves remain in the head.- Nick at Pelican Parts  

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Page last updated: Wed 2/21/2018 02:04:39 AM