Maintenance Keeps Your HM-100 A Star


Every racing engine needs regular maintanance and the Tecumseh Star is no different. Basically, the needs of this boomer are no different than the needs of a Briggs stocker.

Every regularly scheduled maintenance session should begin with a complete tear down, cleaning and inspection. Look at the condition of the cylinder, the bearing bosses, the lifters and valves, the piston and rod. Make sure to carefully examine the condition of the crankshaft. While this is a beefy unit, it never hurts to take an extra minute and make sure the unit is free from cracks.

Next, clean all of the engine parts. A good practice is to clean everything first with solvent and then clean your parts with a good hot bath of water and Dawn dish washing detergent. Blow dry your parts and spray a light coat of WD-40 on the steel and iron surfaces. Remember you can’t be too rich and your engine can’t be too clean.

This next step is the messiest. You will need to break the glaze on your cylinder wall. The best way to do this would be with a Sunnen mini hone and a torque plate torqued to 12.5 foot pounds. Use a fine stone and hone just enough to rough up the cylinder to a dull shine. Ideally, you should try for a 30 to 45 degree cross hatch pattern. Try to start with rough stones, wash the cyclinder clean with solvent and then use fine stones to finish the job. If you don’t have fine stones, use 400 grade wet and dry wrapped around your hone. If you don’t have a Sunnen unit, you can use a Flex Hone or Lisle Glaze Breaker hone if you are certain your cylinder is fairly straight. Don’t go beserk here. Just lightly rough up the cyliner walls. Wipe your cylinder with a rag and measure the bore at 90 degree angles top and bottom (to check for squareness). Record the dimension in your engine log book.

The valve seats need a light re-cutting. Use the Serdi Micro Cutter or Neway valve seat cutter and place a light cut on the seats. It is helpful to paint the seats with Dykem to aid in seeing the extent of the cut. Following the cut, the next step is to lap in the valves using a high quality lapping compound. Check the seal of the valves by shining a bright light over the top of the valve and looking up into the port to see if any light shines through. Wash the block again and spray a light coat of WD 40 on the iron and steel components. Assemble the camshaft, lifters and valves into the block with the side cover on. Check the valve lash and set, by grinding the valve stem, to the spec called out by your camshaft grinder. If you are in doubt, try for between .006 to .010.

Assemble the valves, springs and keepers. Since the Star used a split ring keeper, this operation takes a bit of time. This operation can be made easy with a small engine C-clamp, which is available from many tool dealers. The best advice one could give on this is to lay the motor on its side and use a small blade screwdriver with a dab of grease on the tip to hold the keeper while you manuever it into place. It is a good idea to use new valve springs since you have it apart.

Within the side cover, turn the crank shaft bearing keepers and remove the bearing. It would be wise, at this point to replace the bearing. check the fit of the bearing on the crank shaft. Make sure the bearing can slip on and off, yet remain fairly snug. New cranks need to be polished so that the bearing will slip on.

The piston uses three rings. Lightly side lap the rings on a surface plate. Place the rings in the cylinder bore and check the end gap. Fit the rings to a gap of .006-.008. Remove five coils from the oil ring expander. Using a ring expander, place the oil ring on the bottom ring land. The second ring, the one with the cut on the inside goes next, followed by the top ring, installed with the notch facing up.

Insert the lubricated crankshaft into the block . Torque the rod bolts to 120 inch pounds, alternately increasing the torque by 20 inch pound increments. IMPORTANT-replace the rod bolts with news ones! Be sure to match the arrows on the rod and the cap. Find the timing marks on both the cam and crankshaft and install the cam. Coat the cam and lifters liberally with a moly based assembly lube.

Install the ring and wrist pin. Orient the rod arrows toward the valves, along with the arrow on the piston. Oil the cylinder with your racing oil. Place the piston assembly in a ring compressor and guide the piston into the barrel. Take caution here to not hit the crank with the rod. Using a moly based lubricant, coat the lifters and insert into the engine. Lubricate the camshaft and insert into the engine.

Fit the side cover on the engine with a thin coat of gasket sealer and measure the side play of crank. Using shims and gaskets, acheive a side play between .005 and .015. Torque the side cover bolts to 100 inch pounds.

Bolt on the ignition and install the flywheel and crank with the stock timing key. Torque the flywheel to 40-50 foot pounds. Never use an impact gun on the crank.
Put a new head gasket on and torque head to 120 inch pounds.
Put the fan shroud on.
Put on the breather cover and the gasket.

At this point rebuild your carburetor and replace all the gaskets. Set the pop-off to 10 pounds and the lever to .040.

Congratulations, you have just finished your Star rebuild and should now be looking forward to flying around your local race track.


  • Rotate the engine several times by hand to assure there are not suprises and there is good compression and no valve leaks.
  • fill crankcase with 21 ounces of the synthetic karting oil of your choice.
  • Do Not idle engine. The crankcase wil fill with methanol willrapid wear to the piston, rod, crank, cam and cylinder will result.
  • Do not hammer or excessively impact the end of the crankshaft to remove the flywheel.
  • Use caution when installing the crank not to damage the main seals.

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