Vintage Tecumseh Engine Build

Article by Mark Trotta

After sitting for decades with a seized piston, this rusty old Tecumseh was able to be restored to it's former glory. The fact that it was still rebuildable after 50+ years is a testament as to how well these motors were originally made.

1968 Tecumseh H25 engine restored
Before and After Tecumseh Engine Build
how to rebuild a flathead Tecumseh engine

Engine Disassembly

I started the engine rebuild by spraying the entire motor with WD-40 and letting 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.

Vintage Tecumseh engine build

Starting with the blower housing, I disassembled the motor, one part at a time.

rebuild vintage Tecumseh engine

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.

rebuild tecumseh flathead engine

Lots of cleaning followed, mostly using disposable shop towels and brake parts cleaner.

rebuild Tecumseh flathead engine

Once the side cover was removed, I saw 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.

rebuild tecumseh flathead engine

The piston was stuck solid in the bore and would not move, because the rings were "frozen" to the cylinder bore.

remove stuck piston in small engine

Read: Remove Stuck Piston

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!

small engine valve spring compressor

Read: Integral Valve Seat Repair

replace valves on small engines

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.

replace valves on small engines

Fortunately, intake valves for the Tecumseh H25 are still available (exhaust valves are obsolete). New valve springs are also available and were also replaced.

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.

how to rebuild a Tecumseh flathead engine

With the exception of the broken or missing ones, I was able to re-use the nuts, bolts, and washers.

vintage small engine build

Block Inspection

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.

small engine piston removal

Read: Tecumseh Crank and Connecting Rod Inspection

Tecumseh magneto repair

Read: Tecumseh Points Ignition

Tecumseh flathead engine rebuild

Install Piston Rings

A typical small engine piston ring compressor will work with 2-1/8" to 5" pistons. The smallest piston ring compressor will work with 1-3/4" to 3-1/2" pistons.

Here's a simple hack for a piston ring compressor - this hose clamp allowed me to install rings on this small bore Tecumseh.

small engine tools

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 a new crank gear, the dot faces out so you can line up timing marks.

tecumseh h25 crankshaft

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.

restore vintage small engine

Read: How To Restore Engine Tin

The last part of the Tecumseh flathead build was finding a proper carburetor, adjusting the governor, and finally, engine start-up.

old school go kart project


Related Articles:

Vintage Tecumseh Engine Identification
Tecumseh Crank and Connecting Rod Inspection
Integral Valve Seat Repair
Tecumseh Carburetor Replacement
Tecumseh Points Ignition Repair
Remove Stuck Piston From Cylinder Bore
How To Restore Engine Tin