{"id":12,"date":"2018-08-09T13:34:43","date_gmt":"2018-08-09T13:34:43","guid":{"rendered":"http:\/\/commons.princeton.edu\/62-tiger-cub\/?page_id=12"},"modified":"2018-08-15T13:31:05","modified_gmt":"2018-08-15T13:31:05","slug":"cub-engine","status":"publish","type":"page","link":"https:\/\/commons.princeton.edu\/62-tiger-cub\/cub-engine\/","title":{"rendered":"Cub Engine"},"content":{"rendered":"

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Here is a good look at the engine as it appeared when we began working on it.<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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When we first started to disassemble the bike, the engine was one of the things that we focused on.\u00a0 Its importance to the success of the project is clear.\u00a0 The bike had been parked for several years prior to the time that it was purchased for the class.\u00a0 The picture gives a good idea of what kind of condition the exterior of the engine was in.\u00a0 The cylinder was rusty and some pieces were missing or had been replaced with nonstandard parts.\u00a0 However, it was a good starting point for restoring this bike.<\/p>\n

The Triumph Tiger Cub has a 4 cycle engine with one cylinder. \u00a0This means that one out of every four piston movements is the combustion of the gas and air mixture. \u00a0The cycles are as follows:<\/p>\n

1. Intake- the piston moves downward while the intake valve opens, creating suction which draws the fuel\/air mixture into the cylinder.<\/p>\n

2. Compression- the intake valve closes and the piston begins moving upwards again, compressing the gas\/air mixture. \u00a0This compression also heats the mixture. \u00a0The amount of combustion is determined by the shape of the piston, which can be changed to alter compression ratios. \u00a0A higher compression ratio puts more stress on the engine, and requires a higher octane gasoline, while also increasing engine performance. \u00a0In our bike, we opted to use a piston with mid-level compression ratio, rather than Glenn’s 10.5 to one piston which would have required either leaded gasoline or extremely high octane fuel (higher than premium gasoline) to run properly in our system.<\/p>\n

3. Power- the spark plug fires approximately when the piston is approximately at its highest point (top dead center) initiating combustion. \u00a0The gas rapidly burns and increases pressure in the cylinder, pushing the piston back downwards.<\/p>\n

4. Exhaust- the exhaust valve opens, and the gases remaining from combustion are pushed out by the pressure from the cylinder moving upward<\/p>\n

The interaction of the piston and the valves determines max RPM, since at extremely high RPM the piston moves up and down so quickly in the cylinder that the valves cannot open and shut quickly enough.<\/p>\n<\/div>\n<\/div>\n

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\u00bb\u00a0<\/span>An animation of a four cycle engine<\/a><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Cylinder and Piston<\/h3>\n

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Crankshaft Assembly<\/h3>\n
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Piston and camshaft of a later Triumph Tiger Cub<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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These two exploded views from the Triumph parts manual give a sense of the assemblies that compose the upper half of the engine.\u00a0 Some of the operation of the engine can also be divined.\u00a0 These diagrams correspond to the version of the Tiger Cub that succeeded the 1962 Cub.\u00a0 However, the proper ones will be posted once they can be accessed.\u00a0 The engine is a 200 cc displacement single-cylinder four-stroke.\u00a0 The engine also has overhead valves.\u00a0 This configuration was fairly advanced at the time of the motorcycle’s production.\u00a0 As can be seen from the images, every two rotations of the crankshaft corresponds to four one way cycles of the piston. The pinion gear on the crankshaft drives a corresponding gear on the camshaft.\u00a0 The step from 25 to 50 teeth causes the camshaft to rotate at half the speed of the crankshaft, so every rotation corresponds to one power stroke of the piston.\u00a0 The lobes on the camshaft push against the tappets which in turn push the valve pushrods.\u00a0 The pushrods rotate the rocker arms in the cylinder head.\u00a0 The rocker arms alternately open and close the intake and exhaust valves.\u00a0 These valves supply the engine with fuel and air for a power stroke and evacuate the exhaust on the following stroke. \u00a0Since our bike is a high performance model, the intake valve is larger than the exhaust valve, so that more fuel and air can be drawn into the cylinder on the intake stroke.<\/p>\n

Also in this diagram is the piston, which is used to create both high and low pressure (depending on the cycle) in the cylinder. \u00a0The piston has three rings around it, with the top and bottom being used exclusively to create pressure, while the middle one is also used to distribute oil. \u00a0The piston has a slight taper (the top end is skinnier) so that it fits well within the cylinder when hot from combustion. \u00a0This is why a cold engine will generally not run as well as an engine that has already been heated up- once the engine is heated up the piston creates an optimal seal within the cylinder.\u00a0 The heat produced by combustion also influences the design of the cylinder.\u00a0 The cast iron cylinder has fins to dissipate the heat of combustion into the air flowing past the motorcycle.\u00a0 On the ’62 cub, the the fins were oval in shape while earlier models had round ones and later models featured square ones as shown in the drawing above.\u00a0 The aluminum cylinder head is also finned to transport heat away from the inside of the combustion chamber.<\/p>\n<\/div>\n<\/div>\n

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\u00bb\u00a0<\/span>A good introduction to the difference between two and four-stroke engines.<\/a><\/div>\n
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Disassembly<\/h3>\n
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Disassembly of the cylinder head assembly and removal of the rocker arms<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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After the engine unit was removed from the frame, the cylinder head was unbolted and lifted off.\u00a0 The cylinder and piston were also removed from the lower end of the engine.\u00a0 Disassembely of the cylinder head began with the removal of the rocker covers.\u00a0 Next, the rocker arms were removed as can be seen in the picture.\u00a0 The picture also gives a better idea of the condition of the cylinder head.\u00a0 The valves were removed by compressing the valve springs with a C-clamp and a special tool while the clips holding the spring in place could be removed.\u00a0 This was a tricky operation, as the clips are small, but putting them back was much more difficult.\u00a0 With the valves removed, the top end of the engine was totally disassembled and restoration work could begin.<\/p>\n<\/div>\n<\/div>\n

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Completely Dissasembled Cylinder and Head<\/h4>\n

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View of the completely dissassembled cylinder and head showing all components<\/div>\n
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Restoration Work<\/h3>\n

Leaving it in Better Shape than We Found it in.\"\"<\/h4>\n
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Trying to clean the cylinder head in the parts cleaner.<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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With the engine apart cleaning, the first step of restoration, could begin.\u00a0 Trying to clean the head in the parts cleaner was ineffective.\u00a0 Blasting it with an abrasive glass bead media proved to be much more effective and cleaned it up pretty nicely.\u00a0 Most of the the other parts that were reused were simply wiped down prior to the start of reassembly.\u00a0 The piston had to discarded because the diameter of the cylinder bore was too great for the piston.\"\"<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n

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Apparatus for refinishing valves<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Repair work on the top end of the engine includes resurfacing of the valves, cutting of new valve seats, and boring the cylinder.\u00a0 The valve surfaces needed to be refinished in order to properly seal the cylinder and yield optimum performance.\u00a0 The valves were reworked using a grinder and a lathe as in the picture.\u00a0 New valve seats were also cut into the cylinder head.\u00a0 Instead of a single 45 degree angle as would have been usual, the new valve seats were cut to 31, 46, and 61 degrees.\u00a0 This three-angle valve job results in a smaller contact area between the valve and the seat.\u00a0 This characteristic results in a better performing and more durable arrangement.\u00a0 The valve seats were cut by hand using a kit designed especially for the purpose.\u00a0 After the proper dimensions were obtained, the seats were lapped by placing grinding compound on the valve and rotating it against the seat to even out the surfaces and provide an even tighter seal.\u00a0 The valves were then replaced. \u00a0The purpose of this work was to create a good seat for the valves, which now contact their seats in small, thin circles. \u00a0In determining how much of the valve hits the seat, we struck a happy medium between two competing ideas. \u00a0A very small contact area would have high performance for a short amount of time, since it would seal very well but would have poor heat dissipation. \u00a0On the other hand, a larger contact area would have better heat dissipation but would make a worse seal.<\/p>\n

The next step in the overhaul will be to bore the cylinder to 0.060 in. over standard. \u00a0This step is necessary because the engine was poorly bored previously. \u00a0When it was bored previously, the person who did the work cut it to exactly .040 over and didn’t hone it to remove the boring marks. \u00a0As the engine was run, the piston gradually wore down the small grooves from the previous boring, making the cylinder significantly larger and robbing the engine of power. \u00a0This is extremely unfortunate, because it means that we will be required to bore the engine to .060 over, which is the maximum amount that can be done, since the quality of metal in the engine deteriorates the further we bore from the center. \u00a0The engine will need to be replaced if it requires any more boring. \u00a0A new 0.060 over piston with a 9:1 compression ratio has been ordered.\u00a0 With this step, the overhaul of the top end of the engine will be nearly complete.\u00a0 Stay tuned for updates as the project progresses.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

Here is a good look at the engine as it appeared when we began working on it. When we first started to disassemble the bike, the engine was one of the things that we focused on.\u00a0 Its importance to the success of the project is clear.\u00a0 The bike had been parked for several years prior … Continue reading “Cub Engine”<\/span><\/a><\/p>\n","protected":false},"author":5,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-12","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/commons.princeton.edu\/62-tiger-cub\/wp-json\/wp\/v2\/pages\/12","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/commons.princeton.edu\/62-tiger-cub\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/commons.princeton.edu\/62-tiger-cub\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/commons.princeton.edu\/62-tiger-cub\/wp-json\/wp\/v2\/users\/5"}],"replies":[{"embeddable":true,"href":"https:\/\/commons.princeton.edu\/62-tiger-cub\/wp-json\/wp\/v2\/comments?post=12"}],"version-history":[{"count":3,"href":"https:\/\/commons.princeton.edu\/62-tiger-cub\/wp-json\/wp\/v2\/pages\/12\/revisions"}],"predecessor-version":[{"id":128,"href":"https:\/\/commons.princeton.edu\/62-tiger-cub\/wp-json\/wp\/v2\/pages\/12\/revisions\/128"}],"wp:attachment":[{"href":"https:\/\/commons.princeton.edu\/62-tiger-cub\/wp-json\/wp\/v2\/media?parent=12"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}