Tecumseh Crankshaft And Connecting Rod Inspection
Article by Mark Trotta
As seen in the picture, the connecting rod was damaged and needed to be replaced. Although I had no previous history of the motor, the damage was likely due to dirty oil and sludge build-up.
Tecumseh Connecting Rods
Unlike car engines, most connecting rods in small engines do not have a bearing insert. The rod is made of softer metal (aluminum) than the crank (steel), so the rod will fail first. If the bearing surface on the rod is damaged, the rod should be replaced.
Tecumseh H25 models had either one of two different types of connecting rods. They will be either stud and nut (early) or threaded bolt (late), and are interchangeable with each other.
This engine had the stud and nut-style connecting rod. I found that the studs got in the way of connecting rod removal, so I replaced it with the threaded bolt version. Most automotive engines use the threaded bolt design.
If the crankshaft is still in the block, check the end play. This is the movement on it's axis, or back and forth motion. A dial indicator is needed for this, but an experienced mechanic can feel it by hand.
If there is excessive crank end play, a washer may have been forgotten, or one can be added. However, in very worn engines, the crank may need to be replaced. Service manual states .005" to .027" is acceptable.
Once the crankshaft is removed, check the journals for pitting or scoring. There was a small amount of aluminum on the center journal that had spun off the connecting rod. I was able to remove these galled-up aluminum deposits with fine Emory cloth.
You can't tell crank straightness by eye, you need to measure the journals with a micrometer. This should be done on several places to make sure the crank journals have not gotten an oval or "egg" shape.
A bent crankshaft is more common on vertical-shaft engines than a horizontal-shaft engines. If the side cover came off easily, it's probably not bent.
Measure Crank Journals
Crankshaft journals need to be within specified tolerance, otherwise your engine won't last very long. If you don't have a digital micrometer, you can at least measure the center journal with a piece of green Plastigage. Lay a piece of the Plastigage thread across the journal, then put on the rod cap and torque it to spec. Remove the rod cap and measure the squashed Plastigage against the packet. After the measurement is taken, remember to thoroughly clean the journal off.
- Center main bearing diameter measured 0.8735" - out of spec by 0.002".
- (Tecumseh service specs - 0.8755" to 0.8760")
- Outer Main journal specs measured 0.8735" - within tolerance
- (Tecumseh service specs - 0.8735" to 0.8740")