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

I maintain that a 0.002" extra gap will not cause any of those issues. It is good to get all valves, on a side, to within 0.001", for best results.

On that, I was fastidious.

IIRC, none less than @Dick_Maury sets them at those tolerances.

Again, for the hundreds I set like that, never did I see any issues.

which is it.? .it matters…001 for best results…or it does not matter…? is 002 of no matter? if exhaust at 006…002 extra which is 33.3 % difference is of no consequence? or if at 008…? so when then is there a consequence? Do exhaust valves never burn, do bucket guides never become loose? Nick

Strawman: I never said they don’t. I said it wont happen because of an extra 0.002" clearance, certainly not on a street engine.

On a given side, it matters. Read what I wrote.

It all depends on the camshaft. The regular 3/8 lift cams set .004 and .006 for inlet and exhaust. Same cam lobes but different on each side. It obviously does not hurt to run .002 wider on the exhaust than the same lobe on the inlet. For racing Jaguar recommends going even wider. There is a fine line between to tight and to loose. To tight and you get valves that can burn because they are not on the seat long enough to cool. To wide and they make noise. The later Parabolic cams engage the tappet more gently and over a wider duration. Clearances were specified .012-.014 for both sides. I have found that the engines like the wider range of clearance and do not make more noise. Setting them at .014 will help produce a smoother idle. Very noticeable on the later Fuel injection motors as they run leaner and are much more sensitive to valve adjustment. So unless you are a Camshaft designer or engineer smarter than the ones that built the engine, I recommend setting the valves as close to spec as possible.

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Long ago, when we spoke on the phone, before you rebuilt Tweety’s head, you agreed wrt to the wider specs, which is why I felt good about doing so, as I had done in prior years, in my business.

Ceteris paribus, it also generally lengthens the time in between valve adjustments.

In fact, Tweety had run ~160,000 miles, on the wider specs, and that was as a result of a discussion Dad had with Ed Iskenderian, who knew a thing or two about cams.


FWIW, MG pushrod engines from 1.3L to 1.8L call for .012 to .018 (depending on engine and intake/exhaust) set while HOT.

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I always preferred to set valves when smokin’ hot…:persevere:

Theoretically, the Auburn was supposed to be adjusted while idling.

A flathead, where the tappet box covers were on the same side as the intakes and exhaust.


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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.