Tappet clearance is a very important part of performance of an engine.
What is it and why is it so important ?
Taking example of a typical engine with a valve train having a SOHC, rocker arms, 4 valves..
To make the basics clear in this scenario : the cam follower i.e the rocker arm contacts the cam surface at one end and the valve at the other. The rocker is pivoted like a see-saw in the middle.
Since in the SOHC design there is a distance which has to be bridged between the cam lobe and the valve the rocker is usually long to account for this.
When the cam rotates the profile pushes the rocker arm up, this translates into the other end pushing the valve down.
Now there is a slight gap/clearance between the cam's surface and the follower.
Why this gap ?
Simple answer : To account for expansion of the valves while getting heated. If the valve expands it would not seat properly, it would be partially open
Due to expansion the valve end in contact with the rocker arm whose other end should be ideally touching the base circle of the cam would be partially pushed in. This is wrong as the valve should be closed and seated perfectly sealing the port during the time the follower is on the base circle of the cam.
But due to expansion due to heat this is not perfectly seated...its being pushed in and therefore partially open.
Now if we add some clearance between the follower and the cam(base circle portion) we will be able to allow this mechanism to expand for a few mm.
This simple problem was solved by a simple solution of providing a small clearance.. so far so good.
BUT doesn't this create a certain amount of wasted rotation of the camshaft to bring the surface of the lobe in contact with the cam follower ?
In more technical terms this gap would create some amount of delay in the time required for the follower to contact the flank.Here flank is that portion of the cam which provied the actual lift for a specific duration(note ..I always say duration and lift together)
Due to this clearance we introduced there will be a reduction of the time the follower would have spent in contact with the cam flank.
Just to exaggerate a bit to make this clear : say 30 degrees of the camshaft were spent in rotating the cam in bringing the flank closer to the follower and bridge the gap.
Since 30 degree of rotation was used up the follower gets a little less time in contact with the cam.
This could mean lesser time spent in opening the valve
So if we designed a cam for x degrees to keep the valve open , it would be (x-30) degrees now due to this delay.
To account for this clearance the cams are designed accordingly - their base circle is chopped of slightly by = tappet clearance distance.
This way the flank begins to open a bit sooner instead of waiting to be contacted by the follower.
Now we understand that tappet clearance is very critical to the cam performance.
The best setting for clarance is specified by the manufacturer, if set perfectly the valve will be seated during the time the base circle of the cam is in contact and will open the valve as per the caculated duration just at the right time.
Too much clearance = reduction in opening of the valves,valves open late, the rocker arm with the follower comes in contact a bit later and in turn opens the valve later .Also it will create the characteristic noisy clatter after heating up almost similar to a diesel engine.
Too less clearance = valves remaining open for a longer time,loss of power.
Extremely tight = valves not closing completely during the base circle contact and allowing the hot gas to keep flowing and burning the valves.Intake valves can also get burnt like the Exhaust valves.
The behaviour/feel of the engine and the exhaust note will clarly indicate if you have got it right ..
A good test would be to let the car idle till it gets hot.
After getting hot this clearance begins to play a more significant role.
During idle:
-If the idle remains constant and smooth you are good to go.
-If the engine begins to stumble or the idle becomes rough it means you didnt leave enough clearance
During driving:
If the idle is smooth and there is no roughness it means we can now drive off and do another check.
-If there is loss of power felt then it means tappet is still a bit to tight, try for the next bigger (+ .0xx mm) gauge.
-If there is that noise again then it means you are still loose ; go for the next lower setting (-0.xx mm) gauge
I suggest you go for the setting in 2 stages :1st make sure the feeler gauge just fits and slides a bit tight, if this does not work then go for the next .00x setting but this time let it slide smoothly.
Also always remember to inspect with gauge again after tightening the locknuts just to make sure it didn't shift.
Once you get it right - note it down and try to set it to same.
Not necessary it will work on all cars with same setting but its a good place to start as a refernce point.
Each cars cam/rocker follower/tappet wear out differently and diff levels of carbon build ups therefore each engine may not respond in same way.
