This engine is a replica of the first model engine made by Irwin Ohlsson. Designed and constructed in 1934 by a team comprising Irwin Ohlsson, Victor Savage, Erwin Schwartz and HR Barney, the plans for this engine later appeared in the Popular Mechanics, circa 1937.
My reproduction was made from one of Roger Schroeder's kits and served as my introduction to what a great guy he is. I'd carefully machined all the inside of the crankcase cavity, bored for the bushing and was parting off the case from its chucking stub. As the parting blade broke thru, the case dropped gently onto the cross slide, bounced into the air and leapt into the gap between the saddle and the rotating chuck. Result? Myfords: one; castings: nil. I wrote to Roger confessing all and asked him how much for a replacement casting. My "standard" MS Word template for letters had my email address included, so I was pleasantly surprised a few days later to receive an email along the lines of ".. replacement on the way to you, no charge. Don't let your funny English lathe eat this one!"
Roger's reproduction is faithful to the original. A full construction article on this engine, including excerpts of the original plans, supplemented by new drawings appeared in SIC magazine, issues #6 through 12, with a supplement on improving performance in issue #14. It is your conventional, long stroke, side port, ignition engine; essentially the same as the first commercial Ohlsson engines, except for size. The Original Ohlsson is has a displacement of only 0.12 cu in, while the first mass produced Ohlsson and Rice engines were 0.56's. I'd guess the manufacturing and handling characteristics were improved by going to the larger size.
If the spark plug in my version looks enormous, that's because it is! I used a 10mm NKG plug, while the plans called for a rare and expensive 3/8" 24TPI plug - but that's only 1/32" (.8mm) difference, so my plug is not that much out. While it may look very klugy, in this case there's no doubt which came first, the chicken or the egg, as Ohlsson's engine is virtually the progenitor of all the sparkie model engines that followed in the pre-WWII years and certainly pre-dates the appearance of miniature spark plugs.
The only other feature really worthy of note on this engine is the black finish I've applied to the fins. This was totally unanticipated! The front bypass cover and rear inlet tubes were silver brazed to the cylinder as ignition (gas) model engines run hot, far too hot to use any "soft" solder, even high temperature silver solder. When doing this type of work, I use a boron based flux paste and a propane gas torch, the combination of which, to my initial surprise, produced a uniform black finish! As polishing out the fins would be too damn difficult, I decided to turn it into a feature and just buff up the area below the fins. I kinda think the result looks good.
My Original Ohlsson has had one result-free running session. Although it has a good piston liner fit and the internal dimensions are very close to the plans, the compression is just about zero. I put this down to the large, extra cylinder volume introduced by the space around the insulator in the NKG plug. A cure is practical: either make a new head that decreases the internal volume, or make a replica spark plug that does not increasae the cylinder volume. Someday. Real Soon Now...
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