Motobecane porting

Porting a Motobecane AV7 cylinder

This method follows a simple design laid out in various French tuning manuals. It gives a good boost to power and top-end but with a loss off the line. A more detailed and theoretically sound AV7 porting tutorial is available here: Motobecane porting the right way

Using the following diagram Motobecanecylinderport.jpg

Using this method requires that you start with the "fast" cylinder. This cylinder has oval shaped port windows.

  • ECH refers to the exhaust port.
    • raise the exhaust port by 3mm
  • T refers to the transfer ports
    • this diagram suggests not changing the transfer ports, but many people widen (do not raise or lower) these ports by as much as 2mm per side.
  • AD refers to the intake port
    • widen the intake port by 2mm per side, then lower the port by 3mm

it is important that the amount the intake port is lowered equals the amount that the exhaust port is raised. If you do one, do the other.

    • It is also common to widen all of the inner passages to each port window. Working the Transfer ports can be particularly beneficial.
  • finishing work
    • after all work is done it is important to use a fine file or sanding bit to polish port window edges. This is to avoid ring catching.
    • carefully remove all metal shavings

Original Pamphlet

Prepa av7 page01.jpg Prepa av7 page02.jpg

Modifying a Motobecane Piston

Using the following diagram Motobecanepiston.jpg

  • Remove all material directly below the window in the piston skirt
  • There are two windows in the piston skirt. Remove the material below each of them

Modifying a Motobecane Cylinder head

Using the following diagram Motobecanecylinderhead.jpg

  • shave 2mm off of the entire cylinder head
    • no note has yet been made, whether a head gasket should be used or not.
  • change the angle of the crown by cutting at a 15 degree angle
    • this is similar to the stock head on a Motobecane 7 (these heads will not mount on a 50 frame) Because of the shape of the 7 Head, there is no room to shave it down.

Porting a Motobecane AV7 Crankcase

The transfer ports of a stock cylinder are larger on the cylinder then when they meet to the case.

  • with the cylinder off, the marks will be clear where the two do not match. It is in the points of the transfers, or the "devil horns."
  • it is highly recommended to take the case apart before removing this material. Any small metal shavings can quickly destroy main bearings and oil seals.
  • some people will carefully stuff the case with rags or tape and then clean the shavings out to save time. this is to be done at your own risk

Common Tools

  • the most common tool for port work is a dremel rotary tool with a size assortment of carbide cutting bits.
  • a fine grit sanding bit is important for polishing the edges of port windows and inside passages.