Vintage Tecumseh Engine Build
Article by Mark Trotta
After sitting for decades with a seized piston, this rusty old Tecumseh was restored to it's former glory. The fact that it was rebuildable after 50+ years is a testament as to how well these small engines were originally made.
Before and After Tecumseh H25 Engine Build
Engine Disassembly
Before starting engine teardown, the entire motor was sprayed with WD-40 and let it soak for a over a week. Occasionally, I would walk over to it and spray it several more times.
The longer it soaks, the better the penetrating fluid will work.
Starting with the blower housing, the motor was disassembled, one part at a time.
I kept track of the disassembly steps by taking step-by-step pictures, and also wrote down notes in a notebook. This proved to be invaluable later on, while reinstalling the governor and carburetor linkages.
After removing the cylinder head, this is what I found.
Most of the cleaning was done with cans of brake parts cleaner, compressed air, and shop towels.
Once the side cover was removed, I found this nasty sludge build up--no wonder the motor stopped working!
Engines that have been run on non-detergent oil will often have a thick sludge build-up.
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Frozen Piston
The piston was stuck solid in the bore and would not move, because the rings were "frozen" to the cylinder bore.
Read: Remove Stuck Piston
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Valves and Seats
After removing the valves, I turned my attention to the valve seats, which were corroded. On small-frame Tecumseh engines, valve seats are not replaceable!
Read: Integral Valve Seat Repair
The exhaust valve had very little wear and was reusable after being cleaned up with a wire wheel. The intake valve was too corroded and had to be replaced.
Fortunately, intake valves for the Tecumseh H25 were still available (exhaust valves are obsolete). New valve springs were also available and also replaced.
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Block Inspection and Cleaning
Once the motor was completely disassembled, the entire engine block was sanded down. Smaller parts (valves, bolts, etc) were wire-wheeled. The bolt threads were cleaned and threaded bolt holes were tapped.
With the exception of the broken or missing ones, I was able to re-use the 50 year-old nuts, bolts, and washers.
In non-ball bearing small engines, the crankshaft rides directly in the aluminum block and side cover. These surfaces must be smooth and round.
If the side cover bushing is badly worn or damaged, the cover can be replaced. If the engine case bushing needs replacing, consider having a machine shop do this, or finding another block.
Next up was checking and measuring the crankshaft and connecting rod for wear and replacement.
Read: Tecumseh Crank and Connecting Rod Inspection
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Engine Reassembly
On Tecumseh H25 engines, the crankshaft gear is pressed on. If it has severe pitting or missing teeth, it needs to be replaced.
When installing crank gear, the dot faces out so you can line up the timing marks.
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Read: Tecumseh Points Ignition
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Install Piston Rings
Instead of a small engine ring compressor, I used an automotive hose clamp to install rings on this small-bore Tecumseh.
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Blower Housing And Engine Tin Restoration
Most old Tecumseh engines are painted either red, white, or black. This one was originally white, so that's what color I re-painted it.
Read: How To Restore Engine Tin
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The last part of the engine build was finding a proper carburetor, adjusting the governor, and finally, engine start-up.
Read: Tecumseh Carburetor Replacement
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