This article discusses about the importance that tappet clearance holds in 4 stroke engines and how we adjust it.
Tappet clearance is of utmost importance in the 4 stroke internal combustion engines. In this article we will discuss deeply about this concept and will also know as how to adjust it.
What is Tappet Clearance and its Importance
Tappet clearance is the clearance between the rocker arms and the point where they rest on the valves. Usually both the surfaces are flat surfaces. In four stroke engines, rockers arms arrangement opens and closes the inlet and outlet valves. But there is no direct connection between the two. In fact rocker just sits on the valves. And then push rods move the rocker arms, and thus the valves. In the image below you can see the arrangement.
When this clearance increases or decreases, there is an adverse effect on the timing for opening and closing of the valve. If the clearance increases- The valve will open late and close early If the clearance decreases- The valve will open early and close late
The general clearances are somewhere between .30mm to 1.5 mm depending upon the manufacture and the valve (Air or Exhaust). A clever engineer will always keep an eye on it and will adjust them timely.
How to take the tappet clearance
Taking tappet clearance is a skill, same as the adjusting it. First of all you need make sure that the unit you are going to take tappet clearance should be in top dead center. This is clearly marked on the flywheel. You can use turning gear to turn that particular unit to TDC.
Note: Some engine makers want it to be done when engine is warm and some wants when engine is idle of cold. So if they ask for warm engine then just run the engine for 5-10 minutes.
But usually there are 6-8 units in a four stroke engines. So at same time, flywheel will show two units at TDC. To make sure that particular unit is at the compression stroke and not at the power stroke, you can turn the push rods of both units by hands to check. The one with free push rods is the one which is at compression stroke and the one with tight push rods is at power stroke. So avoid the one which is at power stroke.
Now you need good filler gauges of different thickness. Image below show them. Now gently try to push these filler gauges between the clearance of valve and rocker arm. You may have to try different thickness.
Just check in the manufacturer manual as how much standard clearance should be.
How to adjust tappet clearance
Now you know that your clearance is less or more. But now comes the adjustment part. This is quite important though. You need to open the nut on the rocker arm as shown below. After opening, you can adjust the clearance.
Note: Keep the filler gauge all the time between them, to get the perfect clearance.
Tip: When you move filler gauges while adjusting for particular clearance, you should be able to slide it smoothly (not too tight and not too loose). After adjustment, tight the nut with given torque.
Keep the record for future use. Hope this will be useful for you. This was all from this article on tappet clearance.