One of the most distinctive features of the Offy is its crankcase side breathers. These are highlighted in the photos below.
On the early engines these were quite low, over time, they were raised and additional internal baffling was added.
Only the right side of the engine has these baffles so I start out making the left side since it is simpler. A 1/8″ ball end mill is used for the final finishing of the fins.
Below is the roughing pass on the right side breather plate. I left too much material on the original work piece. A tool caught it on the finishing pass and snapped, a little bit scary. The finishing tool path wasn’t aware of the big chuck of material and made an approach through it. I did not include the oversized work piece in my tool path simulation, otherwise I would have identified the issue before the run. Fortunately the part was not ruined, just the end mill and my pride.
Below is the finishing pass, again with the 1/8″ ball end mill running a .007″ stepover. The finishing pass took about an hour.
Then it is on to the breathers themselves, below is the roughing pass on the bottom of the part. I have used 2 flute end files for ever on my CNC router, but recently I have switched to three flute end mills and have been pleasantly surprised how well they work.
Below is the finishing pass on the top side of the breathers.
the breathers are mounted in the crankcase side plates at a 10 degree angle. I made a set of custom angle fixtures with the 3D printer. On a small part as this with very light cutting forces, they work really well and are quick and easy to gen up.
The machining was done on the manual mill.
I went for a tight press fit. There will be a set screw coming in from behind the cover plate to secure the breathers. Below is a test fit.
Then the other side.
Below is the finished part along side the 3D printed fixtures.
The crankcase side breathers installed on the engine. I think they look cool.
