Old OS 61 FS: An Exhaustive Experiment.
03-02-2015, 10:29 PM
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Old OS 61 FS: An Exhaustive Experiment.
I am in the process of selling my house and retiring to rural France. I devoted yesterday afternoon to running up five engines. They had not been run for a long time or they had just been rebuilt. Having run them, the plan was to oil them up and put them into store.
Among these was an old OS 61FS which I had not started in years. This was the first of the OS four-stokes to have enclosed valve gear. It's also physically much bigger and heavier than the later FL 70 which I also test-ran yesterday afternoon. The exhaust looks for all the world like a brake pipe union with a bit of brake pipe sticking out. Mine has the optional extra of something which looks like a small silencer which is held on with grub screws. Apparently its function is simply to provide exhaust pressure to the fuel tank. I had lost one of the grub screws and substituted a cheese headed screw which worked but didn't look right! With the complete "brake-pipe exhaust" fitted, it flashed up straight away and ran beautifully. However, whilst fooling about with exhausts from my other OS engines in the range 40-52 cu.ins, I discovered that the interior thread in the OS 61's head matched the thread on the exhaust pipes of the smaller engines. I bought an extra exhaust pipe with two clamping nuts from Just Engines and fitted a suitable straight-through silencer from a job-lot I'd bought off eBay. The OS 61FS is a very quietly timed engine. It's a bit livelier than the well-sort-after OS 60 FS with the exposed rockers but it's in quite a low state of tune. I fitted the other exhaust feeling that it was such a docile engine that I thought it would make little difference. Besides actual exhaust pipe was much wider and shouldn't restrict gas flow. Blow me but I couldn't get it to run at all! I refitted the "brake-pipe exhaust" and the engine ran straight away! Then I removed the pressure line to the tank and the engine gained several hundred rpm. So much so in fact that it blew the little silencer off and gained a few more rpms. Even without any kind of silencer, I didn't think that the engine was that noisy. This one is destined for a Majestic Major!
Views of the cognoscenti and OS 61 FS aficionados appreciated.
Pictures attached showing all of the exhaust arrangements. Sorry about the Heath Robinson prop retaining arrangement.
Among these was an old OS 61FS which I had not started in years. This was the first of the OS four-stokes to have enclosed valve gear. It's also physically much bigger and heavier than the later FL 70 which I also test-ran yesterday afternoon. The exhaust looks for all the world like a brake pipe union with a bit of brake pipe sticking out. Mine has the optional extra of something which looks like a small silencer which is held on with grub screws. Apparently its function is simply to provide exhaust pressure to the fuel tank. I had lost one of the grub screws and substituted a cheese headed screw which worked but didn't look right! With the complete "brake-pipe exhaust" fitted, it flashed up straight away and ran beautifully. However, whilst fooling about with exhausts from my other OS engines in the range 40-52 cu.ins, I discovered that the interior thread in the OS 61's head matched the thread on the exhaust pipes of the smaller engines. I bought an extra exhaust pipe with two clamping nuts from Just Engines and fitted a suitable straight-through silencer from a job-lot I'd bought off eBay. The OS 61FS is a very quietly timed engine. It's a bit livelier than the well-sort-after OS 60 FS with the exposed rockers but it's in quite a low state of tune. I fitted the other exhaust feeling that it was such a docile engine that I thought it would make little difference. Besides actual exhaust pipe was much wider and shouldn't restrict gas flow. Blow me but I couldn't get it to run at all! I refitted the "brake-pipe exhaust" and the engine ran straight away! Then I removed the pressure line to the tank and the engine gained several hundred rpm. So much so in fact that it blew the little silencer off and gained a few more rpms. Even without any kind of silencer, I didn't think that the engine was that noisy. This one is destined for a Majestic Major!
Views of the cognoscenti and OS 61 FS aficionados appreciated.
Pictures attached showing all of the exhaust arrangements. Sorry about the Heath Robinson prop retaining arrangement.
