Group 7 - Lawn Mower Engine - Critical Design Review
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The reassembly procedure has been planned to be a multifaceted procedure that requires many steps and sub-steps. The steps to re-assembly are as follows:
The following procedure is rated on a difficulty level of 1-5. 1 being very simple, usually completed by hand. Levels become progressively more difficult through 5. A degree of difficulty of 5 would be constituted by rigorous effort, take a long time, and require many tools and attempts to complete. High levels of difficulty are from parts being damaged or dented or designed to be replaced by professionals or with special tools. Note: this scale reflects the degree of difficulty for this particular assembly.
- Replace Valves and valve springs. Using a small regular screwdriver, compress each spring to lift valves and remove tension from retaining plates. Using a second screwdriver, use it to strategically turn retaining plate until latches with valve. Slowly release tension from springs and remove upper and lower retaining plates, spring, and valve by hand. (Degree of difficulty:4) See Fig 1.13b, Fig 1.13c, and Fig 1.13d
- Replace Valve assembly cover Do so my attaching two 8mm bolts to cover gasket. See Fig 1.13a (Degree of difficulty:2)
- Replace Valve lifters. Do so by hand.(Degree of difficulty:1)
- Assemble breather
- Replace filter into bowl. Do so by hand. (Degree of difficulty:1)
- Attach hose. Do so by clipping from engine block by hand.(Degree of difficulty:1)
- Replace filter mount. Do so by replacing two 8mm bolts from engine block.(Degree of difficulty:1) See Fig 1.12
- Assemble Piston Do so by replacing the rings by hand. Be sure to place the oil ring in the correct slot, while making sure the remaining two rings are replaced with the chamfered edges up.
(Total degree of difficulty:2)
- Replace retaining pins. Using needle-nose pliers, insert the two retaining pins in the piston head after placing the wrist pin and connecting rod by hand in the correct positions(Degree of difficulty:2)
- Insert piston into cylinder head Do so by using a piston ring compressor or to use 2 small flat head screwdrivers to slowly work around the rings and work them into the cylinder head.
(Degree of difficulty:4)
- Replace Cylinder Head Place gasket in place by hand and replace 8 13 mm bolts. (Degree of Difficulty: 2)
See Fig 1.5a, Fig 1.5b and Fig 1.5c.
- Governor assembly
- Replace gear assembly. This is done by hand.(Degree of difficulty:1)
- Replace the lower retaining ring. Do so using a precision regular head screwdriver or a small pair of needle-nose pliers.(Degree of difficulty:3)
- Replace spool by hand.(Degree of difficulty:1)
- Replace upper retaining ring. Do so using the same procedure used to remove upper ring.(Degree of difficulty:3)
- Replace Cam Shaft Put balance pin and shaft in place by hand and insert into engine block. (Degree of Difficulty: 1)See Fig 1.8
- Insert Crank Shaft Insert into engine block by hand. Place spacer on end of shaft and insert key to lock in place using a hammer to lightly tap in place. Degree of difficulty:3) Note: During disassembly, the threads on the crank shaft were damaged, so prior to reassembly, the threads were re-tapped using a 7/16 in. 20 Tap. (Difficulty:2)
- Replace Key and spacer. Do so by placing the space by ahdn and using a hammer to lightly tap the key into the keyway until it is completely inserted (Degree of difficulty: 2)
- Replace Connecting rod cap. Do so by inserting 2 6.5mm nuts where the rod meets the crankshaft. This must be done while the piston and shaft are aligned properly, otherwise the screws will not fully tighen and cause potentially major damage to the engine upon operation. (Degree of difficulty:2)
- Replace electromagnet assembly. Do so by affixing two 11mm Bolts from on top of the engine block. An electrical wire is held in place by a clip mounted on the engine block. The clip and hose are both replacedby hand. See Fig 1.10 (Degree of difficulty:2)
- Replace Flywheel Do so by replaceing 11/16" nut and preventing flywheel from spinning using a screwdriver to hold flywheel against engine block. (Degree of Difficulty:3)See Fig 1.9a
- Replace Crank Case Cover Do so by affixing 6, 10mm bolts.(Degree of difficulty:2)
- Notes
- The Crank Case Cover is designed to slide onto the Cam Shaft with a slight push by hand. However, it seemed apparent that the shaft was damaged and had a gouge in it which resulted in the cover catching in the gouge. It took rigorous but careful hammering and prying with a screwdriver to replace the cover. See Fig 1.7a and Fig 1.7b.
- Notes
Replace Gear Pulley and Blade Mount Do so by placing the mount and gear by hand. Use a hammer to tap the key into the keyway until fastened securely. Fig 1.6 <i>(Degree of difficulty:4)
- Replace Starter Assembly.
Place tension spring on a large flat surface and using palm of hand to hold in place, use other hand to wind spring as tightly as possible. Without letting go of spring, grip with needle nose pliers to hold shape. Attach retaining clip to inner hook and attach outer hook to starter housing both by hand. Make sure the Spring lies flat in the housing and slowly release tension with pliers until the housing holds spring in place. (Degree of Difficulty: 5)
- Place gear mount and clip on back, both by hand. . (Degree of Difficulty: 2)
- Slide housing onto mounting bracket. (Degree of Difficulty: 2)
- Screw 8mm, reverse thread hub screw into mounting bracket to affix the housing. . (Degree of Difficulty: 2)
- Place Starter housing cover onto housing and attach with 2 #1 Phillips head screw driver. (Degree of Difficulty: 2)
See Fig 1.4a, Fig 1.4b and Fig 1.4c
- Replace the Carburetor Assembly and connected Manifold.
- Replace Throttle mount Mount an 8mm bolt from engine block. See Fig 1.3a (Degree of difficulty:2)
- Replace Fuel Fitting Do so by hand.(Degree of difficulty:1)
- Replace Throttle Assembly Do so by hand (Degree of difficulty:1)
- Replace Primer Bulb. Do so using a small regular head screwdriver to push bulb into correct position. (Degree of difficulty:1)
- Replace Shutter Do so with a #0 Phillips Head Screwdriver.(Degree of difficulty:1)
- Replace Float Pin, and Spring Clip Do so with needle-nose pliers.(Degree of difficulty:1)
- Replace Float Bowl. Requires the mounting of a 10 mm.(Degree of difficulty:1)
- Assembling the manifold and carburetor requires replacement of two 7mm Hex Head screws that fit a #1 Phillips head screwdriver. See Fig 1.3a and Fig 1.3b (Degree of difficulty:2)
- To mount the manifold to the carburetor assembly, remove two additional screws of the same size.
- Replace Engine Shroud. This step requires affixing of 2, 8mm Hex Head Screws, with lock washers, and 2, #2 Phillips head screws. See Fig 1.2 (Degree of difficulty:2)
- To replace the oil fill spout from the shroud, mount two 8mm Hex Head screws.(Degree of difficulty:2)
- Replace the Gas Tank. This requires placement of two 8mm Hex Head Screws. This also requires that the oil dipstick be removed (by hand) to allow clearance of the tank over the fill spout. Using a pair of needle-nose pliers, replace the clamp holding the fuel line to the gas tank. The handle of the starter assembly will also need to be replaced using a pair of needle-nose pliers pull the cord through the handle and then by hand, tie into a knot. See Fig 1.1a, Fig 1.1b and Fig 1.1c (Degree of difficulty:3)
This completes Reassembly.
DIAGRAMS COPYRIGHT OF TECUMSEH INC. 1998.
Product Status
This product never worked, either before we began analysis of it or after it was put back together. Before and during disassembly broken pieces were continually found throughout the product, and provided further evidence that this product would not work. See the Initial Product Assessment for a brief overview of initial issues. Some of the misplaced parts were returned to their intended locations, and other parts, such as the valve rings were slightly damaged during product dissection. All in all, the engine was put back together better than originally received, but even this was not enough to resuscitate this product.
The group attempted to fiz and correct the many problems at hand but were unable to successfully complete the process and therfore were not able to get the engine to run. The diassembly process was different than the assembly process in a few ways. In one area, removal of teh flywheel was a very difficult task and took almost twenty minutes tom complete. During reassembly, the process only took about three minutes. In another area, the tension spring was removed in about five seconds but took well over an hour to complete. Once the spring was placed inside, it was determined that the coil was in the wrong direction and had to be switched which took an additional twenty minutes.
Overall the same tools were used in both processes with the exception of the optional tools. We did not need the flywheel puller in reassembly and the Piston ring compressor was not used in the dissection. The use of these tools is generally required but we used neither in any of our steps due to the inability to obtain them. On a good note, the group succesfully completed the assembly of the engine and restored it to a condition better than that received though it still down not function as there is still an issue with the starter engaging once mounted. It works when it is not attatched, which it did not do during dissection.
The biggest challenges faced are evident in the overall difficulty ratings of particular steps. In general, the starter assembly, notably the tension spring took well over half of teh assembly time and the piston replacement took a good deal of time as well. There didn't seem to be many other issues in assembly as the group documented the dissection well and was able to replace components without a problem or error. the use of teh technicians handbook again assisted in the process.
