Thanks for the interest and comments everyone.
Chuck: For the hall effect device I am using a SS441R in the flat TO92 package by Honeywell for my pickup.
http://www.alldatasheet.com/datasheet-pdf/pdf/345668/HONEYWELL/SS441R.htmlThis not what the CDI module distributor supplies but is equivalent. It has good magnetic sensitivity so magnet spacing and strength requirements are not very critical. They are a little more robust than some of the other manufacturers products. They are available from many distributors but I get mine from Mouser as they are generally a little cheaper there. Besides they are in your and my neck of the woods. Price is about US$ 1.70 each but they drop to just under a dollar each if you buy ten.
George: You are right that both of us like small toys. Really no harder to build once you get used to it.
Now what good is a cylinder mount with out cylinders?
A few changes were made to Brians's design (of course you say) to suit my style. Same basic geometry.
First off I increased the thickness of the outboard spigot so I could fit an oil cup. Briqan mixes oil with his fuel and I don't so I have to lubricate the cylinder. Typically on engines this size one drop of oil in an oil cup is good for about an hour of running. A little more for the first couple of hours when breaking in. This extra thickness then needed to be cut away with a counterbore spotface for the cylinder mounting bolts.
Second I shortened the spigot that goes into the cylinder mount to clear the valve rather than cut a relief into it. I will lengthen the piston by about 0.014 inch to bring the compression ratio back up. This means that the piston will go beyond the end of the cylinder into the combustion chamber. This won't hurt anything.
A little more care was necessary in finishing the bore because of the smaller size and I will be using a lapped piston cylinder with no ring.
Started by turning one end of a bar of Class 40 cast iron to the outside shape with one spigot on the end and finishing the fins. After cutting off I machined the spigot on the second end and bored it to about 0.3745 to finish to 0.375 by lapping. Boring was done with a solid carbide boring bar.
To remove the high spots from boring I ran a fine grit Flexhone into the cylinder about 6 strokes with the Flexhone in a hand electric drill motor. This brought the bore diameter up about 0.0003 inch. Finishing was done with a adjustable brass Acro lap charged with 10 micron diamond compound. A very good cleaning followed, first with solvent and then with hot water and detergent. A spray of WD40 followed as the iron would start to rust within the hour with no oil protection.
Gail in NM