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

Asking an analog question in a digital world here… I’ve Googled this and I’m not finding it, maybe someone here knows.

Not Jag engines, but in general… What is the actual specific effect of missing the tappet gap by 1 or 2 thousandths, aside from the paper-thin difference in valve lift? (Assuming it’s not so tight that the valve can’t close)

The quick answer is:
too much clearance = valve clatter
too little clearance = valves not closing right

Long answer:
The engineers who specify these things have worked out by trial and error how much would lead to valve clatter on a very cold engine, sub-zero F. Undesirable in a Mark VII which is expected to be whisper quiet, at least when new.
Also by slide rule calculation how much the valve stems expand in length when very hot, such as racing an XK120, which they expect customers to do, leading to valve heads not quite seating at the moment of detonation, leading to heads heating up and contact faces burning off.
So they pick gaps to specify, 004 and 006 for Mark VII and 006 and 008 for XK120, and they allow for the chance that small independent shops and home mechanics may not hit these gaps accurately with their feelers, but that 001 either way will not bring the customer back squawking.

Brings to mind this post from several years go.

I’ve also seen the sound described as “a well oiled sewing machine”.

Why were clearances the same for XK120SE cams with their additional sixteenth inch lift?

I recently had the tappet clearances reset to 4 and 6 thou on my late 120 FHC after I noticed what I described as ‘excessive valve train noise’.
The tappet most out of spec was an inlet at .007" which for an earlier 120 engine would only have been .001 thou out however .003" for my later 120 engine’s specs.
This was quite noticeable in regards to valve train noise to the point I suspected something worse (loose tappet guide or similar)
I’m happy to report my engine now has the consistent tappet noise I would have expected and I no longer have one or two louder ‘tapping’ sounds I previously could hear from within the cabin (FHC).
Regards, Graham

Not much: I always rn later Jags at 5/8, and the audible difference was not that noticeable.

I set the Rover valves to +3 thou, b/c lash pads are really dear, and though a wee tickety, at speed it doesn’t bother me.

I look at it this way…the Factory established specs…for whatever reasons…to be “optimal” The factory revised the specs…for whatever reasons…many think customer complaint of noise…but we do not know the “why” for sure. IF…just noise…then I like the first specs…making the assumption that they were optimal…except for noise. But who knows. All that said…the specs are in just a few thousandths…not much. So a little difference is actually a lot of difference. Try to hit spot on. Especially with exhaust where not enough lift of the exhaust valve…and staying closed too long …can mean too much heat and a burned exhaust valve…or…a loose bucket guide due to heat. (they are a heat/fit) . I know it is frustrating.and .tedious…to measure it all…mechanical measurements…all set…then…put it together and find it a thou or 2 off…seems it always is…a mystery of the universe, …then do it another time. But I did…ended up spot on…and sleep well. I used the 006 008…on my 53 …but…it has an early Mk7 replacement head…thus the 5/16 cams…the original SE 3/8 were not transferred across by the DPO. …Nick

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.