Tappet gap... How much does a few thou really matter?

The reason I posted this originally is, I wonder if anyone knows how a setting of .005 or .007 will change the behavior of the valve. How much is the duration(s) changed? How many degrees of cam rotation go to waste if the clearance is set .001 or .002 too large?

Let’s face it, a little wear on the end of the valve, the shim, the cam tip… all make even the most fastidious setting change after 10 thousand miles, no?

Knock thyself out! :wink:


Yea, nice theory. You know it was never done that way by the average mechanic at ACD, and certainly never in later years.

Especially because one had to remove the through-the-fender exhausts, then the damn fender!

Yea… thanks, but any treatise that shows hydraulic lifters isn’t going to have anything useful for us. At least not without suffering eyestrain anyway.

I’ll bite. Nothing. There is very little or no flow across the valve seat at very low lift. You need to consider when the valve begins to open in relation to the movement of the piston. It’s a very dynamic system. Just because the intake valve opens, doesn’t mean that the dynamics are favorable to induce flow. For this reason, most cam grinders quote cam duration at 0.050" of lift. They know that nothing significant occurs in the first 0.050".

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So tolerances are more to insure the valve closes, but doesn’t have much to do with the time it’s closed, cooling considerations. That will be taken care of by the cam’s specific grind.


Greetings All,

To me, it woukd seem that if the manufacturer wants you to measure something using thousandths of an inch…its important.

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Hooray…“if the manufacturer wants you to measure something using thousandths of an inch…its important”
.Bingo ! and direct to my questions and point…talkin just a few thousandths…and in the low range of em…004 to 006 or 008…so a little is a lot. I will just say…tho Dick did…SET TO THE SPEC.
That makes it quite simple…and not a question at all. Nick

I love this gang…:grimacing:

Tighter clearance means the valve will get to .050"(or any other lift amount) sooner as it relates to the piston position, meaning slightly more duration…very slightly more.

Valve timing on the twin cams is set “with special valve timing clearance of 0.010” (0.25 mm.) inlet and exhaust", see page B.44 of Service Manual.

For the earlier pushrod engines, valve timing is done “to the special valve timing clearance of 0.20” (.51 mm )", see page B.44 of the Mark V Service Manual (yes, same page number).

The larger “special” valve clearance in both cases allows the cam to be measured for functional degree range when open further than a couple thousandths. It also moves the cam to where the measurable change with a gauge will have less degree uncertainty in the measurement.

Increased duration with no flow is of no benefit. The velocity of the piston is of much greater importance. You could open the valve at bottom dead center, but there would be no flow across the valve seat into the cylinder. This is why the intake valve opens a few degrees before TDC. Not because you can achieve any significant flow at that point, but so that it is out of the dead band as the piston begins to move down the cylinder in earnest. You can gain some inertial benefits at sustained hight rpm, but that is race car stuff, not street driving.


A salient point, often overlooked, and one where overactive “mind dynos” tends to extrapolate, maybe hopefully, to a situation where differences of a thousandth simply isn’t perceivable, nor helpful or harmful.

Street v. track: two very different performance envelopes.

One dyno-supported example is the valve lash on a Formula Vee engine: stock value was 0.004", cold.

We’d set them to zero lash–even slightly preloaded–cold, and on a dyno, there would be slight increase in the torque band. Slight, as in 1(ish) ft-lbs.

Meaningful in a racing class where we’d look for any advantage, anywhere, and on an engine whose peak torque was in double digits.
NOT meaningful on a street engine whose torque numbers are in the triple digits.

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Someone once asked NASCAR driver Daryl Waltrip why he didn’t put a coolant temperature gauge in his car: “Why? I’m gonna run it till it blows up.”

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In Skip Barber cars I found the weight of the driver had a significant affect on acceleration out of the corners. Barber Series drivers tended to fall into two categories: skinny, talented, flyweight teenagers and 40-50-something guys who were less than svelt. If you weigh in at 215 and 13 year old wunderkind Gabbi Chavez weighs in at 85 lbs, you are dead meat on acceleration. (actual example from Road America one year)

S’why I liked FV: 1025, with driver…:wink:

oft said…in the old days…racing tyres were skinny…and racing drivers wide…now it is the opposite way round. Tho…Nuvolari was slim.
NASCAR now…they do consider temp…when in the draft…they can only run nose to tail for a little while before it will overheat…but in the last few laps…as said…run til it blows up. An interesting move…is when there is paper or plastic debris over the radiator intake air path…and they move up to a cars tail…and in the vacuum…the debris is sucked off…works almost every time. At Darlington last Sun…plastic was on the center of the splitter…it really affected the airflow and handling…

Yep, the cars are VERY aero-dependent…tape is applied or removed to change handling and control temps.