Troy-Bilt 78 User Manual

MTD uses a splash lube system for its horizontal shaft 

engines. The connecting rod has a dipper on it that 
“splashes” oil around the inside of the engine.                 
See Figure 5.5.

NOTE: The cam and the balance shaft were 

removed for a better view of the lubrication 
system.

The splashing action will create an oil mist that 

reaches the cylinder head. There are two oil passages that 
run along the engine cylinder. The one on the top side of 
the engine is the oil supply passage. The oil mist will flow 
through this passage to the cylinder head. See Figure 5.6.

The second oil passage runs along the bottom side of 

the cylinder. This is the oil return passage. As the name 
implies, it allows the oil collecting in the cylinder head to 
return to the sump. The return passage is the tiny hole that 
is in between the two tappet passages.

NOTE: Because these engines use splash lubrica-

tion, the type of oil and the oil level is critical 
for proper operation of the engine. If the oil 
level is too low, the dipper on the connecting 
rod cannot splash the oil into the engine. If 
the oil level is too high, the oil will not change 
into a mist to reach the upper side of the engine.

Dipper

Oil supply passage

Oil return

Tappet

 passages

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