Motobecane porting the right way

Porting your cylinder according to the old french magazine specs (Motobecane porting will get you a quick, easy boost without any thinking, but it is suboptimal because it usually kills your low end, and even if you start with a "fast" cylinder, there are a still LOT of different variations in port geometry that Motobecane put out stock. Following these instructions here, you can achieve a much better result with just a little foresight and caution.

Porting a Motobecane AV7 Cylinder, the Right Way

Here are instructions to do a mild port on a stock AV7 cylinder. If you are thinking of trying it out, please read all of it first and post questions. This port job in the pictures and instruction is a mild job which will work very well with a SHA15:15 or SHA16:16 or perhaps a PHBG 16 or 17.5. A performance pipe will definitely wake up the motor after this port job but it will also have good performance gains with the stock blunderbus exhaust if the muffler portion is opened up. You would also need to retard the timing from 24 degrees (2mm) to 18 degrees (1.5mm).

Typical stock event duration is:

  • Exhaust - 130 degrees
  • Transfers - 100 degrees
  • Intake - 106 degrees.

This is the number of degrees that the port is open during a full revolution of the engine. Porting the engine will increase the duration of the port opening to gain more power from each charge.

Target event duration is:

  • Exhaust - 145 degrees
  • Transfers - 100 degrees
  • Intake - 130 degrees.


Rule of thumb is to never guess a measurement, recheck your marks or numbers if you are unsure, you can always recut bigger but you will not be successful trying to replace the material removed. Either cutting by hand or with a power tool, you will be rough cutting the exhause from the outside into the cylinder. You will need to wear safety glasses and a mask, aluminium shards in the eye can only be removed at the clinic by picking out the piece as it is not magnetic. Inhaling aluminium and chromium dust is definately not a great idea. A rotory cutting tool (Dremel) will speed the cutting along, always and only use carbide tips and not stones for cutting aluminium. Stones are porous and can load up with aluminium which expands at a different rate then the stone and cause it to fracture. A stone fracturing at 10 or 20,000 is explosive. I only use files and rifflers to port motors. Machining aluminium is not a long process and I feel safer using hand tools on soft metal then speeding through it. You will need to use files and perhaps rifflers to finish off the roughing out of the port. You would need flat, round and rounded (one side flat one round) shapes. Medium cut files are the best, though a course file will produce quicker results it may cause flaking of the chrome at the edge of the port. Always cut the cylinder when it is off the engine, only when firmly held in a vise. Protect the work area and the rest of the motor by placing a magnet under the cylinder standing in the vise. Though the aluminium is not magnetic the chrome is, and so is the file and the steel wool. Always was off the cylinder with kerosene and a rag and check with clean hands if there is no leftover shards before returning the cylinder to the case. Even a bit of soft aluminium can gall on a bearing or the cylinder and ruin your build.


  • Degree wheel
  • Straight edge
  • Coat hangar
  • 1/2 copper tube
  • Washers
  • Paper and pen for notes
  • Dremel and carbide bits
  • Various shaped files
  • Various shaped riflers
  • Medium and fine steel wool
  • Magnet
  • penetrating oil
  • alcohol or brake cleaner
  • kerosene or varsol
  • rags
  • magnet
  • General purpose grease
  • plasticine/putty clay
  • Safety glasses and mask


Cut 2 each 1 1/2 inch pieces of copper tubes to hold the cylinder to the case Use two washers and two original nuts to hold the cylinder down cross corner Keep the studs well lubricated with light oil as the nuts will be on and off a number of times

Print a degree wheel and glue it to a cd Slightly enlarge the center hole of the cd to match the protrusion in the novi nut Remove the flywheel and attach the cd to the cam using the novi nut Measure02.JPG

Cut a 6 inch long piece of wire coat hangar to make a pointer Put in a loop on the end so the top front nut & bolt of the case can hold it into position Put a right angle bend in the end so it will point to the graduations on the degree wheel

When turning the crank you will need to turn it from the clutch side so you do not move the degree wheel and put your measurements off. I would also suggest to always triple check your measurements and verify that your calibration is correct by rechecking the TDC point. The degree wheel does not move locations by itself, however, you do not want to take a chance and have a rather expensive ornament rather then a nicely ported motor.

Calibrate the degree wheel by putting the engine top dead center - close as you can Rotate the degree wheel so the pointer shows TDC Turn the engine forward and place a straight edge across the cylinder bore Turn the engine until the top of the piston dome just touches the bottom of the straight edge and note the degrees [example 22degrees] Turn the engine in the opposite direction until the top of the piston dome just touches the bottom of the straight edge and note the degrees [example 26degrees] By the examples given of 22degrees one side and 26 the other the degree wheel is off by 4 degrees total as both sides should be equal. Adjust the degree wheel by 1/2 of the difference and then double check the readings to ensure the wheel is centered properly. Take care the setting does not move as you tighten the Novi nut to hold the degree wheel secure. Always double check the setting after to ensure the calibration is correct.

When the degree wheel is calibrated then you can start to check the stock numbers of the cylinder. Use a strip of plain printer paper 1/4 inch x 6 inch as a snag gauge for the pinch point of the ports. Rotate the crankshaft until the exhause port opens, place the end if the paper into the open port and start to close the port until it just grabs the paper in the pinch point between the roof of the port and the piston crown. Record the number of degrees from BTDC as indicated by the degree wheel. Measure01.JPG

Do the same process for the transfer port.

For the intake using the degrees from TDC with the pinch point between the piston skirt and the floor of the intake. You check this measurement from outside the engine.


  • Exhaust 65
  • Transfers 50
  • Intake 53

From the opening of the ports we can get the duration of each engine event by doubling these numbers. Exhause (65x2) 130 degrees, transfers (50x2) 100 degrees, Intake (53x2) 106 degrees. The engine I am working on just needs to be "warmed up" a little so I will be using a proven event timing sequence for my target. Exhause 145 degrees, transfers 100 degrees, intake 130 degrees. To dial in these numbers on the engine you first need to do the math and divide the duration by 2 to determine the opening point of each event. Exhause (145/2) 73 degrees, transfers (100/2) 50 degrees, intake (130/2) 65 degrees.


Turn the engine and observe when it reaches 73 degrees before bottom dead center, this is where the start of the exhaust event occurs. Using a fine tip permanent marker mark a line around the cylinder where the piston crown rests. This line will be the height of the new ceiling of the exhaust port. Exhause00.JPG

The width of the port and shape of the port will remain basically the same and we will just be raising the ceiling of the port. Use your permanent marker to draw vertical lines for the width so you have a guide to follow while opening up the ports. You need to keep the exhause port smooth with a straight ceiling, straight sides and rounded corners between the sides and ceiling or floor. Because you are merging to a round exhause header, yourport will need to end up slightly trapizoidal with the top a little narrower then the bottom. Keep this final shape in mind as you plan out the cutting. Our new event plan calls for 3mm being cut and the finished size of the port will be ample to use with a 16mm carb on the intake side. Rough it out with the rotary tool or files, cut most but not all of the way to begin with and check the measurements and redraw the lines if necessary. Clean the cylinder well before checking measurements using the case and piston. Once you are darn close you should switch to gentler files or rifflers and finally polish the port with medium then fine steel wool.



The transfers are good at their original event duration so there is no need to mark those. If you have a more agressive porting plan then you would need to perform the same process for the transfer port ceiling height as you did with the exhause port, but using the new transfer numbers. Transfers are a pain to open up and you will need rifflers to get around the corners as you will need to work on machining from the inside of the cylinder. If you need to widen the transfers only take material from the trailing edge of the port (closest to the exhause side of the cylinder) or you will damage the scavenging flow. Because transfers are so difficult to reach and important to flow it's best to leave them as-is.


The intake is quite different. First you need to ensure the port is capable of handling the volume. Compare the dimension of the port window cross section area (width x height) to the dimension of the carburator bore cross section area (PI x radius squaired). To save on the maths here is the rounded numbers. 12mm carb = (3.14 x 6 x 6) 113mm 13mm carb = (3.14 x 6.5 x 6.5) 133mm 14mm carb = (3.14 x 7 x 7) 156mm 15mm carb = (3.14 x 7.5 x 7.5) 177mm 16mm carb = (3.14 x 8 x 8) 201mm The stock intake on the engine was 21.2 wide and 8.5 high (180mm) which is good for up to 15mm carb. To introduce a 16mm carb you would first need to add volume to the intake cross section area. As there is a good difference between the stock and target timing it is safe to raise the ceiling of the intake 0.5mm (to 9mm total) and widen the intake 0.7mm on each side (to 22.5mm) which will give you 203mm.

When measuring the intake you need to measure the port width and height inside the port. One method to accurately get the measurement inside the tract is to press a ball of putty against the port from inside the cylinder and measure the imprint of the clay which went into the port. Measure03.JPG

When cutting material from the cylinder intake you will first need to match the outside of the port to the intake from the carb. Make sure that the cross section area of the port will be equal to the cross section area of the carb. From that point, blend the shape of the outside of the cylinder's intake port to meet the inside port window. After this preparation work you would then proceed to modifying the intake port timing event. You would use the degree wheel to find 65 degrees before top dead center and mark a line on the piston around the entire intake port. You then need to remove the piston from the engine. Extend the vertical lines (sides of the intake port) down to the edge of the piston skirt. Measure the width of the port marked on the piston and locate the center of the port floor. Mark an arc between the points where the vertical lines meet the edge of the skirt, using the port floor mark as the center of the arc.


The material within the arc is the material which requires removing. Using a file or the rotary cutter you then need to cut the skirt of the piston. Finish off the cut by rounding off the end of the arc where it meets the skirt. The rounded corner will help to prevent the corner from damaging the cylinder wall. Once the arc has been cut you need to slightly bevel the bottom of the piston and the edge where you have removed material. The bevel edge is crutial to prevent the piston to act like a scraper and remove oil from the cylinder wall.


Make sure all the edges of the ports are smooth and chamfered and clean the cylinder and cases very very thoroughly to get any and all metal shavings out. Reassemble the engine as before with the new matched carburetor. You will need to upjet, so tune accordingly. A performance exhaust or derestricted stock will enhance your performance gains.

Original instructions with pictures here: [1]