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#1
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Piston in Tohatsu
This is about a vintage air-cooled Tohatsu 5 hp outboard, from seventies, I
think. I am currently putting back together pieces of my outboard. When inspecting the piston I started to wonder an extra hole (about about halfways up the piston side, a little below the rings. Piston has an arrow on top, which has quite definitely previously been pointed to the exhaust port. When assembled this way, the hole on the piston side never opens to anything. If the piston is turned 180 degrees this hole would open to the exhaust port, when the piston is at the TDC. That would mean there is a momentary straight passage from the crank case to the exhaust, maybe about 10 -20 degrees around the TDC. Other cuts in the piston apron are fine both ways for the mixture to flow. I'm used to think the manufacturer would not put a hole in the piston side or any other details just for fun. And there is a quite high probability someone has opened this engine before and possibly could have assembled the piston incorrectly. And, of course, there is a faint possibility I have messed with the disassembly and those holes have been aligned all the time. So now I am not certain what to do. The reason for this opening to exhaust would obviously be some kind of crank case filling enhancement but I have not stumbled on this kind of construction before. Have to admit I have had very little to do with engines in practice. Would you please give me your opinion, or preferably solid facts, if anybody has them. Which way to put the piston? -- Vesa Helanti |
#2
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Vesa Helanti wrote: I am currently putting back together pieces of my outboard. When inspecting the piston I started to wonder an extra hole (about about halfways up the piston side, a little below the rings. Two-stroke manufacturers started putting those "extra" holes in the rear side of pistons in the 1960's to help the engine breathe better. At some point, they allowed fresh fuel/air mixture to pass from the crankcase into a 5th, or "boost" port... I first saw those holes in the 1967 Yamaha YDS-5 and TD-1C 250cc 2-strokes. Prior Yamaha 250cc models had two transfer ports on each side, but the 5-port Yamahas had a 5th transfer port in the rear of the cylinder. It was fed by a hole in the rear of the piston... The 5th port blew fresh mixture across the top of the combustion chamber, assisting to clear the cylinder of exhaust products... Yamaha was learning from the hot-rodders here in the USA. There was no room for a 5th transfer port in older model Yamahas, so the racers had been grinding a sort of "ditch" up the rear of the cylinder wall to allow the extra boost port mixture into the combustion chamber... If your piston hole truly never lines up with any of the transfer ports, and only has 4 ports, not 5, it is likely that somebody couldn't find a Tohatsu piston and used a Yamaha piston instead... Piston has an arrow on top, which has quite definitely previously been pointed to the exhaust port. When assembled this way, the hole on the piston side never opens to anything. If the piston is turned 180 degrees this hole would open to the exhaust port, when the piston is at the TDC. That would mean there is a momentary straight passage from the crank case to the exhaust, maybe about 10 -20 degrees around the TDC. The arrow on top of the piston always points toward the exhaust port. You would never want to have hot exhaust gasses intentionally pass into the crankcase, you might get a crankcase explosion, or you might scorch the thin film of lubricating oil off the tiny needle bearings in the rods... Also notice that the piston will have two wire pins in the ring grooves. The pins keep the piston rings from rotating around so the ends of the piston rings can never snag in the exhaust port and break. If you turn the piston around and it snags a ring during operation, you might wind up buying a new motor... (I remember a guy who thought he knew more than the mechanics at the Greeves factory. He looked at the L-shaped Dykes-pattern piston ring, and he told me that the "idiots" at the factory had installed the piston ring upside down. So he turned the ring over and tried to slide the cylinder down over the piston ring. It wouldn't go back together that way, the ring stuck out too far...) Other cuts in the piston apron are fine both ways for the mixture to flow. Instead of raising and lowering the intake, exhaust and transfer ports, speed tuners would commonly cut the rear skirts of the pistons off and file the crown of the pistons to effectively open the exhaust and transfer ports early... I read a funny story once about some famous American flat track racers who were being considered by Yamaha International to be members of their factory roadracing team, at least for American Motorcycle Association races. They knew that they could get more power out of the TD-1 racers that they were loaned for the performance tests, so two racers were working on one piston with a hacksaw. One was sawing, the other was holding the piston down on the bench. The guy with the hacksaw was sawing furiously and cut the other guy's thumb... Yamaha was so happy with the flat track racers' riding, they hired all of them for the team... I'm used to think the manufacturer would not put a hole in the piston side or any other details just for fun. And there is a quite high probability someone has opened this engine before and possibly could have assembled the piston incorrectly. And, of course, there is a faint possibility I have messed with the disassembly and those holes have been aligned all the time. Consider the possibility that I offered above: it's a *Yamaha* piston. In the early 1960's, a young rider once proudly told me that he rode a Tohatsu motorbike and how wonderful it was, better than my Honda. But I have never even *seen* a Tohatsu motorbike, I have only read about them... The reason for this opening to exhaust would obviously be some kind of crank case filling enhancement but I have not stumbled on this kind of construction before. Have to admit I have had very little to do with engines in practice. I been studying 2-stroke engines for 40 years now, and have never seen a motorcycle 2-stroke that had a hole in the *front* of the piston, only in the rear of the piston... |
#3
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"Vesa Helanti" wrote in message ... Snip Piston has an arrow on top, which has quite definitely previously been pointed to the exhaust port. Vesa Helanti Most, if not all "small" 2 strokers with an arrow on the piston top will need the arrow to point toward the flywheel end of the crankshaft. BTW, I lived in Tokyo in 1962-1963 and worked with a Tohatsu factory rider, I was invited to visit the Tohatsu R&D lab and saw a 125cc 2 stroke turn 27K RPM and produce 47HP on a dyno , in 63 that was an unheard of output Aubrey in big D 96 Virago 535 78 Suzuki GS750E |
#4
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ufo wrote: Most, if not all "small" 2 strokers with an arrow on the piston top will need the arrow to point toward the flywheel end of the crankshaft. The arrows on top of every Yamaha and Suzuki piston I ever saw pointed forward, toward the exhaust port. The OP's engine could not possibly be assembled with the arrow pointing *sideways* because the connecting rod would also have to be twisted sideways... |
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