Chainsaw Repair

That is the most detailed port mapping I have ever seen.  Well done and Thanks for showing us.

it would be very good idea to look at much better sources for information then what i have shown.

depending on what you are trying to do, (or using) would think some of these old school measurements may be needed.

in the last two photos.  ..not trying to measure width there.  better ways to do this.

hope it helps
-joe

I've used mm ruled graph paper (1.25mm sq.) for measuring the port map area. Tedious to count, but seemed to be fairly accurate

that method works well.

but when the port shape is an oval (ellipse), i find doing it that way gets tough.  probably an easier way to do ovals is to get the measurement of the port height. 

the paper needs to be wrapped around the piston, or stuffed in around the bore to get all port widths.  (pic above).

those measurements can be plugged into an online area calculator.  (or done by hand).

lots of these online calculators.  never used this particular one.  example:  http://www.asknumbers.com/area_calculator.aspx

-joe

Looked up the Staedtler plastic templates and still made in the USA!!!   Cool deal. 

the old gordon jennings book and even the macdizzy site's explanations can be confusing.

both of them go to great detail on how to measure widths of ports....and then go on to show the port map completed on graph paper.

hope this will help some folks who have never done this, (at least this is what works for me).

i don't use graph paper when making the port tracing.  if i put graph paper curved inside the cylinder bore for the tracing, then just took it out to count the squares, the port area would be wrong. (larger than it actually is).

why? because the port width now appears larger......the graph paper squares that were once curved, then get full size now that the paper is laid flat for square counting.

i just use plain paper to get the trace. then measure the port widths and heights as shown in pics, and write them down.

if i need to count squares, just redraw the ports using templates. using the measured heights and widths (obtained by calipers).

Now I'm confused!!!! Lol.   Actual width would vary if tracing and transposing onto another piece of flat paper because of the curvature?   Will appear larger because of the curve. (Longer distance)

Rubber mold of the port would be pretty accurate.  Cut the mold flush (perpendicular ) at apex of curve and measure.