Motorcycle Cylinder Head – Porting and Polishing

Carrying on with the self taught approach to the rebuilding of this cb650′s engine, I decided to port and polish the cylinder head myself . Having a spare head to rely on in case I screw it up I dove right on in. My budget doesn’t cover the cost for a new air compressor that can run a die grinder so I’m using my Dremel to tackle this project. For the first while I was using grinding stones to remove the material, and it wasn’t until I read a tutorial that I realized I needed to use a carbide cutter. This sped the process up immensely. Now my process of porting is as follows:

  1. Carbide cutter for heavy material removal
  2. 60 grit sandpaper on a half inch and quarter inch drum sander
  3. 100 grit sandpaper on a half inch and quarter inch drum sander – Stop there for the intake ports (explanation below)
  4. Continue with gradually finer grits till 320 grit
  5. Using a tapered and drum polishing tip with black emery polishing bar
  6. Pat on the back for job well done.

I’ve read quite a few articles about porting and polishing and like everything else, people have differing opinions on this subject.My take on this is to reduce the amount of drag and turbulence causing artifacts inside the head. The material around the valve guides will be ground down and smoothed out. All of the casting flaws and flash will be removed. Where some opinions differ is to polish the intake ports or leave a little rough. I opted to leave the intake ports rough because of the theory of air movement like the dimples of a golf ball. I did polish exhaust ports to get the air the hell out of there.

As well the combustion chambers had work done to them as well. There were sharp edges where the bowl meets the deck so that was smoothed out. The reason for that was to prevent pre-detonation. As well the area around the valve seats were smoothed where necessary to have the best possible air movement. The combustion chamber bowls were polished to help in the prevention of carbon buildup.

Enough yapping, lets get to the pics.

Intake port using a grinding stone. About midway up the side of the intake port you can see the cast flashing. The idea is to get the walls nice and smooth. The carbide cutting tip is way, way faster and worth the money. Get that first and save your brain.

Intake port grinding

Here you can see the different stages from left to right; untouched, rough cut, polished. After this shot I got the carbide cutter and redid the polished exhaust port because it wasn’t as smooth as I wanted it. And as it turned out, once I was finished with this head and was about to lap the final valve it was then I realized that the valve guide was toast. Explained why the engine wouldn’t run well regardless of what we did to it. Fraking hell.


When grinding material near the valve seat, there may be a divot near the port and the hardened material, just take your time and grind this smooth so the transition is perfect. Be careful here so that you don’t accidentally hit the valve seat, then your done and will have to get it ground professionally and that doesn’t keep this build cheap.

Intake port smoothing

Here it is with a finished transition.

Perfect transition

There is a small amount of flash along the edge of the combustion bowl that needs to be removed. This could aid in pre-detonation if it gets too hot. It’ll have small radius put on there.

Small lip at edge of bowl

Here is an almost finished intake port, everything has been smoothed down and you can see the combustion chamber has been polished as well.

Finished intake port

Here is the finished exhaust port, all polished up.

Polished exhaust port

That’s that for this head because I was using the wrong equipment it took quite a few hours about 20. This head isn’t usable because of the valve guide problem so I’ve started on the 2nd head and it’s already looking better and moving along much faster. I’ll have it done in half the time.

That’s it for now

Here’s all the images in one collection

One thought on “Motorcycle Cylinder Head – Porting and Polishing

  1. I really like your website I have been porting heads all my life on some of the fastest race bikes in the world

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