LawnMowers and SnowBlowers

Engine reassembly

Once you have performed whatever magic is required to repair or replace broken or damaged parts, here are the steps that will transform your pile of parts into a (hopefully) working engine.

If any filing, sanding, or grinding was involved, make sure all traces of abrasives have been removed from every part. The best approach is to clean with soap and water or mild detergent and dry thoroughly. Then immediately coat all ferrous parts with engine oil to prevent rust.

Where the internal moving parts are involved, liberal use of fresh engine oil will also make things to go together smoothly and help protect the surfaces from damage due to initial lack of lubrication.

• Oil seals: If you removed the oil seal(s), clean the inside surfaces where the seals go and install new ones by pressing them in straight and square with a block of wood and rubber mallet or better yet, use a drill press or arbor press. Make sure you get the correct side facing out! Installing the new oil seals after the crankshaft has been replaced may be easier. There is a special tool for this but a piece of pipe that just fits over the crankshaft cut off square will work just as well. Remove any burrs on the crankshaft to prevent damage to the new seal and take care that any rubber lip on the seal does not get folded over.

• Breather: If this was removed, replace valve plate, spring, gasket, and cover. However, this is probably already assembled.

• Valves: Use a valve spring compressor to fully compress the spring for the intake valve and install the valve, any washers, and retaining clips. Do the same for the exhaust valve. Install the valve cover. <>
• Piston rings: Replace any that were removed. Use a piston ring expander if available or your hands to expand the rings and slip them over the piston and into their proper grooves. Note orientation and position! Avoid scratching the relatively soft piston. Do not expand more than needed - the rings are fragile.

  Note the typical arrangement (from top to bottom):

  • Compression ring (solid).
  • Compression ring (solid).
  • Oil ring (slotted with internal expander spring).

  But, you drew a diagram, right?

  Note: if new rings are installed, you should deglaze the cylinder wall with fine emery cloth in a cross-hatch pattern (diagonal strokes). This is needed break in the new rings. Then very thoroughly clean the cylinder to remove all traces of abrasive residue.

• Piston pin and connecting rod: Put a few drops of engine oil on the pin, position the rod, and then slide the pin into place. Use a press if it is a tight fit. Use new circlips to secure the pin. Make sure you get the orientation of both the rod correct! It is also desirable to install the pin in the same orientation as it was originally. If the pin was never entirely removed, this should not be a problem.

For the following, position the crankcase flywheel/magneto side down on some wooden blocks so that when the crankshaft is installed, it's end will be clear of the table-top.

• Crankshaft: Using a gentle rotating-while-inserting, place the crankshaft into the flywheel/magneto-end bearing. Use engine oil to prevent scratches. Take care not to bend over the lip of the oil seal.

• Piston into cylinder: Coat the piston and cylinder wall with engine oil. Orient the rings around the piston so that the gaps are staggered by 90 degrees and not above the pin location. Suggest from top to bottom: 45, 135, 225 degrees. Use a piston ring compressor (commercial or home-made). Tighten until the rings are fully compressed and then release just a hair. Position the piston in the correct orientation - rod with respect to crankshaft - and gently tap into cylinder using a wood block and rubber mallet. If it hangs up, the compressor is too loose. If it does not move at all, the compressor is too tight.

  CAUTION: Do not use a metal hammer - there is a good chance you will crack the fragile aluminum piston.

  CAUTION: Don't let the bottom of the rod or rod bolts hit the crankshaft! Put a wad of rag inside to prevent this.

• Rod and cap to crankshaft: Coat the crank pin journal with engine oil. Position the crank pin journal and rod bearing so that they are in contact. Place the rod cap in position - noting match marks. Using a new lock plate, lock washers, or rob bolts, as appropriate, hand thread the nuts or bolts on as far as they will go. Jiggle the cap to adjust and then tighten some more by hand.

  CAUTION: double check that you have the match marks aligned. If correct, the bearing formed by the rod end and cap will fit the crank pin journal perfectly - seated fully - with no free play even when only finger tight. If you attempt to fully tighten the rod nuts or bolts and the cap is backwards, you may ruin the rod and cap by distorting the soft metal.

  Now, use a torque wrench to tighten the nuts or bolts to the proper torque as listed in your engine manual. Alternate between the two nuts or bolts tightening in small equal increments until the proper torque is reached. Where a range is specified, aim for the middle.

  Where a lock plate is involved, torque to the middle of the acceptable range and then tighten the nuts or bolts just enough further to align a flat with the edge of the plate. Then, bend the plate over to lock it in place. DO NOT reuse an old lock plate.

  It may be a tight fit to get a torque wrench inside the crankcase. Here are a couple of comments:

  • I use a basic 3/8" deflecting beam type torque wrench - nothing fancy. To this, I add a 3/8" to 1/4 inch adapter (short) and a 1/4" socket.

  • Position the piston/crankshaft to provide the most clearance for each nut or bolt. These will differ.

  • This can also be done with an open end wrench and spring scale but the torque wrench is so much easier!

  It is just possible to get both the clearance and angle to use the torque wrench effectively. With a ratcheting torque wrench it would be easier but this is not essential.

  THIS ASSEMBLY IS MOST CRITICAL and is probably the single most important place to get the torque just right. Too tight and (especially for aluminum alloy rods/caps) you will strip the threads and/or distort the precision fit. Too loose and the bolts will eventually work their way out. You really don't want the cap to pop off while the engine is running at full power!

• Valve lifters: Install the valve lifters in their respective holes.

• Camshaft/camgear: Carefully rotate the crankshaft until the timing mark faces the camshaft bearing location. Slip the camgear in place so that the timing marks exactly align (or if your engine is one of those exceptions, so they are off by one tooth - see your engine manual if in doubt). For most Tecumseh's:
  The camgear timing mark (if not marked) is in line with the center of the hobbing hole (small hole in the face of the gear).
  • The corresponding timing mark on the crankshaft is either a beveled tooth on its gear or in line with the keyway.

  • If the engine has a Craftsman type (float) carburetor, advance camshaft 1 tooth clockwise (except for the Craftsman variable governed fuel system, whatever that is!).

• Oil pump: Install the two piece oil pump. The flat must face out.

• Crankcase/oil sump cover (temporary): Install the washer(s) that go on the crankshaft. Without using a gasket, install the crankcase/oil sump cover. Use a gentle twisting motion and take care to avoid damaging the oil seal. Slip the ball end of the oil pump plunger into its cavity in the cover before it is fully lowered. With a little jiggling, the cover should seat properly. Thread a couple of the mounting bolts in hand tight to hold it in place.

• Test for free rotation: Use the blade adapter and key as a means of grasping the crankshaft and rotate it through two complete revolutions. There should be no binding of parts though you will feel the resistance of the piston as it moves up and down in the cylinder and the valves as they are lifted once in every two revolutions (which you will also see from above).

• Remove the cover. Double check that the mating surfaces are free of old gasket material and dirt. Use a new gasket. Again, take care not to damage the oil seal and line up the ball on the end of the oil pump plunger so that it fits in its cavity in the cover as it is slid into place. Install all the bolts and tighten in a staggered order incrementally to the proper torque.

• Cylinder head: Position the cylinder head and new head gasket and install all the head bolts finger tight. If any are of a different size, make sure they go in the proper locations so that they do not bottom out or engage too few threads. (Tecumseh bolts seem to be all the same size.) Tighten the head bolts in the recommended sequence in 3 or 4 equal increments to the torque specified for your engine.

  Engine repair manuals always recommend using a new head gasket. The old one has shaped itself to the texture and imperfections of the head and cylinder and you could never match this up perfectly upon reassembly. The result can be leakage of hot combustion gases and ultimate failure of the gasket and possible damage to the mating surfaces.

  However, if you have not done anything to the head or cylinder surfaces and the gasket is in essentially perfect condition, you can risk reusing it but I won't guarantee long term reliability! My general recommendation is that you use a new head gasket once you are sure that everything works properly and thus there will be no need to remove the head again. Unless the old one is damaged, it will work fine for testing purposes.

  A typical Tecumseh bolt tightening sequence is shown below (except 8 HP). Check your service manual for the specific recommended procedure for your particular model engine.

             Flywheel/magneto end
              ___________________
            /| | | | | | | |  8  |
           |5| | | |3| | | |=|=O |
           | O=|=|=|=O=|=| | | | |
          /| | | | | | | |  _  | |
         |1| | | | | | | | |O| |2|__
         | O | | | |  Spark Plug O  |
         | | | | | | | | | | | | |  |
          \|7| | | |4| | | | | | |  |
           | O=|=|=|=O=|=| | |6| |  |
           /\| | | | | | | |=|=O |__|
      ___/____\|_|_|_|_|_|_|_|_|_|___\___
      -----------------------------------
               PTO/Blade end
  
  

For the following steps, position the engine on wooden blocks blade/PTO side down.

• Ignition: Install the components (if any) that go under the flywheel (e.g., points, condenser, cam). Install the magneto coil or electronic ignition module. Temporarily position it so that it is as far away as possible from where the flywheel will go. Tighten the bolts.

• Set the point gap and ignition timing (point type ignitions systems only). See the section: Setting the point gap and ignition timing.

• Flywheel: Place any inside spacers proper side up onto the crankshaft. Position the flywheel key in the keyway and then install the flywheel onto the shaft. Jiggle it a little to seat solidly. It should not now move from side-to-side at all. Add the washers, starter cup, and flywheel nut. Screw the nut on by hand and then tighten securely (but not to full torque necessarily at this time) using a socket wrench. Torque to specifications once the engine is mounted as this will be a lot easier.

• Set the flywheel magnet-magneto gap (if you have not done this already): Place an appropriate spacer (e.g., .015 inches) between the flywheel magnet and magneto pole pieces. Loosen the magneto coil mounting bolts. The magnet will draw the pole pieces tight against the spacer. Tighten the bolts to the recommended torque.

• Install the spark plug with a new washer (and a dab of anti-seize compound). First, thread the plug in by hand to get it started and then tighten to specifications (15 to 30 ft-lbs typical).

• Install any electric starting components.

• Install the muffler. A dab of anti-seize compound will make removal of exhaust system components much easier at a later time should the need arise.

• Carburetor: Position the carburetor assembly in its proper location.

• Reinstall the throttle and governor linkages: Where there is no speed adjustment or idle position, the direct governor linkage goes in the hole closest to the engine and the spring hooks onto a fixed vertical metal strip with only one hole at one end and the lower hole in the governor lever at the other. Thus, in operation, the spring attempts to keep the throttle open and the governor pulls on the throttle to close it. Increased spring tension results in higher speed. Don't get these backwards when you go to reinstall the carburetor on the engine!!!

• Reattach the primer tube, if you removed it at the carburetor end.

• Reattach the stop switch wire, if any.

• Install the carburetor onto the engine with a new gasket if needed. Tighten securely to the proper torque (6 to 8 ft-lbs).

• Double-check that the throttle linkage and governor spring are in the proper holes and nothing is binding - you should be able to move the throttle back and forth without any sticking or tightness. It should return to the full counterclockwise position instantly as a result of the governor spring tension.