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Head milling

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Milling the head refers to the process of removing a very thin layer of material from the side of the cylinder head that seals with the cylinder. This process slightly decreases the volume above the piston at TDC and increases the compression of the engine.

Head Milling a Laura M48

This information was written for the Batavus Laura M48 engine, but it can also apply to other aluminum head engines. It is important to remember that the surface of the head where it attaches to the cylinder needs to be as perfectly flat as possible, free of oils and grease, and smooth. The head and cylinder work together to cool the engine. Do NOT use any type of gasket material.

If you are unsure of your ability to do this type of work, don't. A running low-powered moped is better than a dead moped. Expect this to take 2 or more hours to complete. Milling the head increases the compression and gives the engine more power without disturbing the port timing. You will need to do exhaust and intake work to achieve the best results.

Tools and supplies needed:

  • A very flat piece of (any of the following):
    • The type of board used under flooring on a flat concrete surface called underlayment.
    • A discarded piece of marble counter top.
    • A precision measuring surface used with height gauges and such. Commonly a big piece of granite.
  • Milling tools:
    • A good quality metal file.
    • A wire brush to clean the file.
    • A flat metal ruler (like a machinists pocket rule)
    • A spare piece of metal to practice your filing techniques on.
    • Sandpaper in the following grits:
      • 80 (5 sheets).
      • 150 (10 sheets).
      • 400 (wet/dry) (10 sheets).
    • light machine oil.
    • A can of WD-40.
  • A spare spark plug.
  • A good socket set/torque wrench.

Removing the Head

Remove the spark plug. To remove the head, keep in mind that the head bolts are tight and have probably been on for a long time. Use some WD-40 or other commercial spray oil to help prevent breaking the bolts. In an X pattern, loosen each nut about a quarter turn. Then, remove each one in turn. Reassembly is the opposite of this procedure. Check or ask in the forums what the correct torque is when re-installing the head. For the M48, this is 11 foot pounds of torque. You might want to increase this some because there will be a lot more compression when you are done. I used 17 foot pounds.

Once the head is off, examine the surface of the cylinder and the head and how they match up. If there are any ridges, scratches or nicks, they should be sanded smooth. Install the spare spark plug. This will allow you to gauge how much material you have removed and help prevent the piston from striking the spark plug when you are done.

Place the head in a good bench vise using the spark plug to hold it in place. The cooling fins on the head are not a good way to clamp it! They can break. Once the head is in place, make sure it is level. This will help you keep it level during the milling process.

Milling the head with a file. Note technique and how the head is held in the vise.
Milling the head with a file. Note technique and how the head is held in the vise.

STEP ONE -- Filing

Using your file, assuming your filing technique is good (if not use a practice piece of metal first! Develop good technique), begin filing across the entire surface of the head. Use long strokes with the file, stopping often to:

1. Clean the file with the wire brush. This is because the aluminum will ball up on the file and cause scratches on the head.

2. Check the work with the pocket ruler. Place the edge of the ruler on the surface, and check several areas with a strong light behind the ruler. Any gaps under the ruler indicate a low spot. Be careful when fixing these. You can make the surface uneven real easy.

Continue these steps until you have milled off an amount you are comfortable with or you are about .050" from the spark plug.

Check the gap from the plane of the head surface to the tip of the spark plug. Measure with a feeler gauge.
Check the gap from the plane of the head surface to the tip of the spark plug. Measure with a feeler gauge.

STEP TWO -- First Sanding

Once you are sure that the filing is good (level surface), remove the head from the vise. Starting with the 80 grit sandpaper taped to the flat surface, move the head in a figure 8 pattern. Keep at it until the you have removed all the file marks. Check your work often. You will see differences in the surface that you are sanding. Once it is even across the surface, move to the next step.

STEP THREE -- Second Sanding

Continue the sanding process with the 150 grit sand paper. Use as many sheets as it takes. Keep checking your work. This is the most important step - here you will be making the surface of the head as smooth as possible. Take your time.

STEP FOUR -- Third Sanding

Start the sanding process again with the 400 grit sand paper. Use the first sheet dry. After that, use small amounts of the machine oil while sanding in the figure 8 pattern. Continue sanding until the metal is almost mirror smooth.

Final Very Important Steps

Using the WD-40 or carburetor cleaner (spray type), clean the entire head. Any shavings can ruin your piston. Reassemble the head reversing the steps above.

When starting the moped for the first time, let it run for a few minutes. Then, let it cool and remove the head. Now look at the surfaces where the head and cylinder match up. If there are oil streaks going to the outside of the head, you have a low spot and need to sand more. If not and/or the oil is in a circular pattern, you are done. Reassemble and ride.

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