Select your language

In accordance with most instructions for diesel walk-behind tractors, valve clearances are recommended to be checked and adjusted every 500 hours or at least once a year. Also, this operation must be performed if the assembly has been disassembled. For example, when replacing the valves themselves, springs, rocker arms, pushers, camshaft, as well as in cases where the cylinder head was removed.

The instructions presented here are a translated excerpt from the maintenance and operation manual for Japanese Yanmar engines. L48N, L70N and L100N. Chinese diesel engines 170F, 173F, 178F, 180F, 186F, 188F and 192F are copies of them. On other engines from walk-behind tractors, valve clearances are adjusted according to identical principles. Only the ways to search for TDC differ.

Instructions: how to adjust the valves on a walk-behind tractor


The valve clearances on the walk-behind tractor are adjusted only when the piston is in the following position:

  1. At or near top dead center.
  2. On the compression stroke.

Only in this position, both valves are completely closed, and the rocker arms are loose and do not put pressure on them.

This is a 4-cycle diesel engine. The piston passes through TDC 2 times, however, before the intake stroke, the intake valve is already ajar (1st figure). On the compression stroke, both valves are closed (3rd figure).
This is a 4-cycle diesel engine. The piston passes through TDC 2 times, however, before the intake stroke, the intake valve is already ajar (1st figure). On the compression stroke, both valves are closed (3rd figure).

To set the piston to the desired position, the easiest way is to navigate by the marks that are applied to the flywheel and the cooling fin on the cylinder. There is also an easy way to find the correct position without marks. This is done as follows:

  1. Open the valve cover to see how they move.
  2. Slowly turn the flywheel with the recoil starter until the exhaust valve opens and closes (goes down and back).
  3. Continue turning the flywheel until the intake valve also opens and closes.
  4. After the intake valve has closed, rotate the flywheel an additional 1/2-1/4 turn.
  5. In this position, the piston will be at or near top dead center on the compression stroke.

Possible errors that are made when searching for the desired piston position for adjusting valves on a walk-behind tractor are discussed below.

To check valve clearances and adjust if necessary:

  1. Set the piston to top dead center on the compression stroke.
  2. Check the gaps with a feeler gauge.
  3. If the probe moves freely, the clearance is too large.
  4. If the probe cannot be inserted into the gap, it is either too small or missing altogether.
  5. To adjust the clearance, loosen the rocker lock nut.
  6. Turn the adjusting screw until the feeler gauge in the gap begins to move with a slightly perceptible resistance.
  7. While holding the adjusting screw, tighten the locknut.
  8. Rotate the flywheel to find the desired piston position again.
  9. Recheck the gaps.
  10. If the probe moves with little resistance, the gaps are set correctly.
Motoblock valve clearance adjustment scheme
Motoblock valve clearance adjustment scheme

For the engines listed above, the standard clearance for both the intake and exhaust valves is 0.15 mm (±0.05 mm).

Possible errors when adjusting valve clearances


Always check gaps one at a time, not all at the same time.
Always check gaps one at a time, not all at the same time.

Despite the simplicity of the operation, the adjustment of the motor block engine valves is often performed with the following errors:

  • Clearances are adjusted on a warm engine. In this state, due to the fact that the parts of the valve mechanism are warm, there may be no gaps at all. As a result, if you increase them on a warm engine, after it cools down, they will become noticeably more than normal.
  • The piston is at TDC but not on the compression stroke. In a four-stroke diesel engine, the piston is at top dead center twice per cycle - during compression, and also between the exhaust and intake strokes. In the second case, when passing through the TDC, the exhaust valve is completely closed, but the intake valve is already slightly ajar. Accordingly, there can be no gap on it.
  • A feeler gauge is not used to measure gaps. Shaving blades, homemade plates, pieces of copper wire and other materials at hand should not be used for this operation. The correct probe not only has the exact and desired thickness over the entire plane, but is also made of abrasion-resistant hard steel.
  • When looking for TDC, the flywheel rotates in the wrong direction. Although the diesel engine can be started in either direction, it must be rotated as it rotates during normal operation when finding the TDC and compression stroke.
  • The intake and exhaust valves are incorrectly defined. It is very easy to distinguish between them - the inlet is located on the side of the air filter, the outlet is on the side of the exhaust pipe.
  • Adjustment with over-tightening of the probe. After tightening the locknut, the valve clearance tends to decrease. If during rechecking it turned out that the probe is difficult to enter, set a slightly larger gap. When tightening the locknut, it will decrease.
  • Tightening the locknut without locking the adjusting screw. When tightening the rocker lock nut, there is a chance that the adjusting screw will spontaneously turn, due to which the set clearance will fail. To prevent this from happening, hold the screw while tightening the locknut.
  • The set clearances are not rechecked. Adjusted valve clearances should always be rechecked in two steps. First, immediately after tightening the locknuts. Secondly, by turning the flywheel two full turns (setting the piston back to TDC on the compression stroke).
Gap feeler set
Gap feeler set

If you know of any other mistakes that can be made when adjusting valves on a walk-behind tractor, share them in the comments below the article. Sensible options will definitely be added to this list.

Why are valve clearances needed?


The gap between the rocker arm and the valve support plate is needed to compensate for the thermal expansion of the assembly parts when the engine reaches operating temperature. That is, when the engine warms up, correctly set gaps disappear.

If the thermal gaps are too large:

  1. On a cold engine, the valve guides may fly out.
  2. When the engine is warm, a characteristic clattering sound may be heard.
  3. The valves will open too late and close too early, which will adversely affect the power and economy of the engine.
  4. The wear of the working part of the rocker arms will accelerate.

If the thermal gaps are too small or missing:

  1. The valves will open early and close late, which will negatively affect the power and economy of the engine.
  2. On a warm engine, the valves will remain ajar on the compression stroke, which will adversely affect compression.
  3. It will be difficult to start the engine (especially if the cylinder-piston group is far from new).
  4. The fuel mixture will not burn out completely, which will lead to engine smoke and frequent clogging of the nozzle spray.

That is, properly installed thermal valve clearances on a walk-behind tractor have a positive effect on the resource, power and efficiency of the engine, and also simplify its start.

VIDEO: adjust the valves on a walk-behind tractor