cams :tappet clearance importance

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.

Basic misconceptions in bolton performance

Onto more serious stuff ..

Morons always reverse and park their cars facing outwards

I'm trying to find a parking spot in a busy downtown shopping mall.
I could see some spots  open but then what's up ?whats holding up the queue ahead ?
Its the smart ass trying to park his car facing the right way!!
      I just dont get it.. whats the point ?
     It is tougher to park in reverse, it takes up more time, it holds up people and blocks both lanes.
While parking into the slot : (1)The driver will 1st go forward and into the opposing lane instead of simply turning into the slot.This will hold the opposite lane as well as your own lane. Nice job smart guy.(2) Now the driver will reverse into the slot, its slower and tougher to back into a slot going backwards u stupid moron.Reversing into an open area is easier then reversing into a limited parking slot..get it ?
So by now car is parked. (3)Here are the additional disadvantage I can see - if you have a wall then your boot is now close to it..try and load stuff into your boot after you come back from shopping.
But hey who cares this guy will load stuff into his boot while holding up traffic again..
While getting out of the slot :
(1)Its the same while getting out. Since the steerable wheels are now facing outside the turning radius is reduced. The car will have to travel slightly further to get the tail end of the car out of the slot before being able to turn the steering to full lock and drive away.(2)So now this guy will go into the opposing lane and then start turning which will take more space.
Morons dont get it ? If you face the road doesn't mean its faster to get in or out.
Its like that Ostrich thingy where these birds think they are safe by burying their head in the sand.. vaguely similar analogy - these retards face the exit so they think its faster/convenient ? or fun ?whatever..
Other technique used by 99% of the normal people - find a slot, simply drive into it.To leave : back out till you can get your front wheels turned just enough to face the right way. Now drive off.


    

Blood group or Octane rating ?

Really how much of petrol content do you have in your blood ?
Not much ?quite a lot or you actually have an Octane rating instead of a Blood group to be relevant?
Do you get all upset if the engine didnt feel right today ? was it the heat was it the bad fuel or was it the ignition or the tappets ?
Love the way the exhaust sounds and not one opportunity is missed to hear that exhaust growl or lazily burble/pop on the over run?
Crazy enough to do a "down-shift syncro rev match" in city driving...?
Gear shifts are timed perfectly with tacho or the feel of the car..
Missing a doctor's apointment or a meeting is ok..hey shit happens. BUT never a car servicing schedule.
If the oil is a .5 mm higher than the upper mark on the dip stick its a crime ?
Who cares how long the destination is or how beautful the scenery is as long as the engine remains in the power band..
Pinging/knocking will trigger a heart attack.. and worse if its your own engine ..
You know what your clutch smells like and of course burnt rubber ..
Nothing sounds better than an overlapping cam trying to breathe at idle..
Statistics like rod-stroke ratio, gear ratio, drive ratio, air-fuel ratio mean more then what mileage can be derived from tankful then u are on the right track..
If you can still relate to stuff here and are guilty of some or all of the above u know its too late ..you are addicted. The car Gods will be happy..