Valve clearance

R

Randy Lausch

Member
Putting a new head gasket on a 4 cylinder Model A I decided to check the valve clearance . Two of the intake valves had 8000 clearance, I wonder if anyone knows how critical that clearance is. I believe they are supposed to be 1300. It seems to run good, before I tore it down. Should I tip the valve stems or leave them be. Thanks Randy
 
0.013 inch would be a good clearance. 0.080 is too much. Does the car have adjustable valve lifters? If not then you are in for some work. The only way I know of to reduce the valve clearance from 0.080 to 0.013 for original type valves is to install new valves and then grind the ends to get the right clearance. The ends must be ground flat and perpendicular to the stem. There was a special tool available to do that job and some Model A mechanics may have one.
 
Randy Lausch said:
Putting a new head gasket on a 4 cylinder Model A I decided to check the valve clearance . Two of the intake valves had 8000 clearance, I wonder if anyone knows how critical that clearance is. I believe they are supposed to be 1300. It seems to run good, before I tore it down. Should I tip the valve stems or leave them be. Thanks Randy
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nkaminar said:
0.013 inch would be a good clearance. 0.080 is too much.
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First off let us start off by using the correct vernacular just so we are all clear. By 8000 to you mean .008" or .080"? 1300?

.008" is a little on the tight side while .080", as nkaminar suggests, is WAY too loose.
 
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Yep, we need to know just what the clearances are. What are they on intake vs exhaust ? 8000 and 1300 mean nothing. What were they measured with ?

Like said, are these adjustable or not ?
 
.008" on an intake may be OK but definitely not enough on an exhaust. Either way I would give them more clearance. I recommend .012" on intakes and .014" on exhausts. Others may or may not agree.
 
Jim Brierley said:
.008" on an intake may be OK but definitely not enough on an exhaust. Either way I would give them more clearance. I recommend .012" on intakes and .014" on exhausts. Others may or may not agree.
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.012 and .014 are good round numbers that will work fine.
Remember, model A cams have no clearance ramps.
 
Pete, please explain "clearance ramps" ... cam profile, heel and toe I understand.
Also, do you recognize the old school term , "valve lash" for the clearance?

Joe B
 
JoeCB said:
Pete, please explain "clearance ramps" ... cam profile, heel and toe I understand.
Also, do you recognize the old school term , "valve lash" for the clearance?

Joe B
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Clearance ramps are the area just past the concentric base circle that take up the valve lash slowly. This essentialy lets the valve up and down easier than if there were no ramps as in early day cam designs which were called "3 arc".
3 arc had a base circle arc, a lift ramp arc and a toe arc. All done with a compass and ruler. No fancy computer programs....lol
The only reason they got away with it was, low rpm engines and excellent materials and heat treat.

"Valve lash" is a common term in use today.
 
Got it, thanks Pete. Think of Clearance ramps as Acceleration and de-celeration ramps, lessen the impact on the valve stem/ tappet.

Joe B
 
From one of the best engine rebuilders I ever knew, Gordon Fischer (now deceased):

"For a show car that I want as quiet as possible I set them at i-.011 and e-.013. For a driver I set them to i-.010 and e- .015."
 
Warning: Technical Discussion:

Cam design has come a long way since the Model A days. Modern cam design takes into consideration the acceleration of the valves which is the first derivative of velocity, dv/dt. This allows minimum force to start to move the valves, a smooth transition from moving up to moving down, and a smooth closing of the valve. The design of the cam may also take into consideration the second derivative of velocity to help with the smooth movement. All this allows the maximum lift of the valves in a modern engine without excessive force and wear.
 
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I have a new engine, and it has the Stipe IB-330 cam. I was surprised at how much more lift that cam has when I compared it to stock A specs.
 
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