Title piston

A cyber friend of mine, Andrew, who happens to also own a Honda CB160 asked if I would be interested in performing a piston modification for his Ducati 160 Monza Jr. He had sent me a document that Tom Bailey wrote outlining changes that could be made to a stock piston for these engines.

The stock 160 piston came with an extra oil ring at the bottom of the skirt. I guess Ducati figured they needed a bit more control. In order to reduce friction and drag some owners opt to cut the bottom oil ring groove off. Once the skirt is trimmed the sides, just below the pin bosses, get a 1.5” radius machined into them in order to lighten the unit up. Once the machining is done the piston then gets polished in order to relieve any stress caused by nicks and scratches.

Since I am not a production machine shop I am not usually set up to perform custom modifications. I am, however, always intrigued by the challenge of figuring out how to accomplish the task. I agreed to give it a go and told Andrew that if I screw it up he’ll be on the hook for a new piston. I guess he figured it was worth the risk because he sent me the piston.

I am happy to say that the minor procedure worked out fine. No money mistakes were made and the piston is on its way back to Andrew.

The following is picture book format of the procedure. It’s nothing too exciting however it involves a piston, metal, and machining so how can that not be cool to look at!

Stock piston

Stock piston from the Ducati 160 Monza Jr. You can see the 4th oil ring groove on the lower skirt.

Trimmed skirt

Trimmed off the lower part of the skirt just to the point were the oil ring groove disappeared. Note how thick the casting is.

Trimmed pistion casting

Trimmed off the thick section of extra casting. I measured the stock piston and then machined the skirt to the identical dimensions.

Piston holding fixture

I built this fixture to mount to my rotary table in order to secure the piston perfectly in place for machining a 1.5″ radius into the skirt.

Skirt radius

Here the piston is mounted and one side cut.

The following is a quick clip showing the milling machine set up for radiusing the piston. The fixture was designed specifically for a 1.5″ radius.

Completed piston machining

Finished rough machining. Both side radius dimensions are identical.

Piston B4 polishing

Final step is to polish the piston up to relieve any stress fractures. This is the stock piston before polishing.

Piston clean up on lathe

I set the piston up on the lathe and give it a quick cleaning before I move onto a 3 stage buffing process using the polishing compounds and the buffing wheels.

Completed piston bottom

Sorry, no pictures of the buffing. Here is the completed piston underside.

Completed piston top

Completion stage, looks awesome. I need to blow this picture up and frame it. I could stare at this stuff all day.

Comments
  1. Fab says:

    Gord, that piston reminds me of my custom made forged race pistons for my M10 BMW engine in my racing days… except not as shiny! Looks awesome and brings back some memories!! Great Stuf!

    Fab

  2. The Stu says:

    That last picture is money, Gord. Awesome job as always.

  3. chris says:

    Great job. Look forward to reading about new projects

  4. […] the bottom oil ring on the piston, per Tom's direction in his book. The posting can be found here: I Like A Piston With A Short Skirt | GordsGarage Blog Mine was found in a dumpster about 35 years ago and left in a backyard until last year. Heavy […]

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