Typically the exhaust side is most “at risk”, because of the higher temps.
I’m sure someone has an example of an intake-side tappet that’s come loose, but if I were a betting man, you’ve probably got a 9-in-10 chance of the issue being on the exhaust side.
I began the process of measuring valve clearance. I then realized there are two different specs for clearance given, one for the early and one for the later motors. As I have a 67 Series 1 4.2, and the engine was rebuilt by GTJ, I couldn’t be sure which cams I have and thus the correct spec.
After research, I have 2 bolt cams. I’m not sure if mine have the “groove” on the front flange that is said to ID the “mid” cam (which uses the later specs) vs. the early cam.
If the cam sprocket mates to the face of the cam flange, leaving the whole edge of the cam flange exposed, then I have the early cam. But if the sprocket overlaps and “hides” the groove, then I can’t tell.
Then, I discovered that I might need “bent” feeler gauges to measure, since the nature of the cam galley is such that I cannot get a straight-on shot to slide a feeler gauge in without bending it. I’ve always understood that a straight on insertion of the gauge is the way to do it (as on Mini A-series and Datsun L series motors.)
This is all the more important if I have the early cams - my thinnest gauge is only .006", and is made of brass so it can deform easily. So off to find some appropriate feeler gauges. Unbelievable if, in the Seattle regional area, I have to get these over the Internet.
I find so far that using the starter to rotate the cams is very easy. I just used the car starter button with the coil disconnected. A short press rotates them neatly a few degrees. I can see the exhaust side easily even from the passenger compartment, and I’ll just lower the top to see the intake side. Without the car raised, I couldn’t even clearly see the starter connection for a remote starter let alone hook it up.
Dave
I watched a few YouTube vids on XK cam clearance measurement and they just use a straight blade feeler gauge and bend it in to measure.
If that’s OK I can just get some straight blade gauges, more common and more likely to have the thin .004 and .006" gauges I need (not sure if any of the bent gauge sets locally available have those thicknesses.)
Dave
when replacing the cam covers a good idea to check the copper washers under the nuts.If they look old anneal them or replace with new ones.or you WILL get weeps from around them ,even when theyre overtightened.Also check the o ring under the oil cap has not gone hard,if it has it will be the source of another weep on a long run JOHN
Would one of t he symptoms be that the ‘rattle’ sometimes disappears; especially when dropping back to idle?
I suppose in the early stages of one coming loose, the noise could be intermittent. Once you reach a certain point of “looseness”, I would imagine the noise would become pretty consistent.
It makes sense (to me) that the noise would be most prominent when the engine is working hardest, and temps are the highest.
The noise from the loose guide is caused by the can hitting the guide, which is nearly certain to damage either the cam lobe or guide. Also, the loose guide will be visibly higher in the head than the others. I can’t imagine there would be ANY force that could possible cause the guide to go back down based on temperature.
as Ray said…the tappet guide lift, IF that happens, causes the cam lobe to strike its edge…very visible on the guide edge, very noisy. A rattle is likely elsewhere, loose timing chain, maybe valve clearances, even in the damper or fan. Bent feelers do work the best, check clearances very carefully, with go and no go…, write down what you find on a chart…by intake, exh, cyl number…next ot your found clearances, write the clearance you want, next to that the difference-more or less clearance than what you found that is needed to meet your spec…then later if you decide to adjust and pull cams, you will be writing what shim you find existing…and then what shim you will need to replace it with.
In the archives look up valve adjustment procedures by P Saltwick., …look for tiny cam part numbers…that will help in ID…these are not the larger MC numbers that may appear which as casting numbers.
Nick
I have an early cam and set mine at the .004 and .006”. They have since tightened a bit. When I get back to the car (in storage this summer, 
, I’ll reset them an extra .002” wider. I can stand a little noise better than a burned valve
As I feared, no one I could find in the Seattle region had a bent-type feeler gauge (with .004-.006 sizes) in stock. Hopefully one will appear at NAPA tomorrow.
So I got a straight blade gauge and shoved them in, bending them to slide in. I tried to use consistent force, which was a tight fit requiring some effort to push it through, I came up with this:
Intake 1 - .004
Intake 2 - .002 - .003 (possibly a .003 could have fit, but the blade couldn’t take more force
Intake 3 - .004
Intake 4 - .002 - .003 (as above)
Intake 5 - .005
Intake 6 - .002 (even .the 002 didn’t really fit, it was too thin to take more pressure)
Exhaust 1 - .010
Exhaust 2 - .006
Exhaust 3 - .008
Exhaust 4 - .006
Exhaust 5 - .008
Exhaust 6 - .006
Clearly exhaust #1 is way off, and intake 2-4-6 concern me. I’ll recheck those when I get the bent feeler gauge which may be more accurate since they’re pre-bent and may not need as much force.
There was no nicking, scuffing, or scratches of the tappet guide perimeters. All tappets rotate freely. The blemish on the one cam lobe I pictured is by far the most prominent on all lobes.
I always thought the ticking sound seemed to come from the exhaust side - could the .010 on exhaust #1 be the culprit? It’s a very rhythmic tick, I cannot characterize it as a rattle.
Dave
Now you’re heading down the rabbit hole… the one where you think re-shimming the valves is a matter of third grade math [e.g. (old shim size + (.010 - .006) = new shim size].
Good luck.
I actually have no desire to adjust my valve clearances, but if things are way off I’d rather bite the bullet than have a problem later. If they were just somewhat off, or if people tell me don’t worry be happy, I’ll gladly cork it all back up.
For one thing, I don’t know how to safety wire…
Dave
I always set them at 6/8, for that reason, and they never sounded all that much different, to me.
The metal in the head in that area is less than 1/4" thick. So if you don’t go through and you tap the hole for a 1/4" setscrew you’ve got maybe 3 or 4 threads thread, which isn’t really a good hold.
I accept that you’ve done what you describe, and it hasn’t caused you any problems and you’re happy with it.
I do what I do for paying customers. Pragmatically, and ethically, we want what we do to work for them, and we also don’t want to have to fix things if they break.
At the end of the day there comes a point with internet discussions where we have to agree to disagree. You’ve done what you’ve done and you’re happy with it. I would do it differently. It’s up to the reader to make a decision about what they would want for their motor.
It has been a while since I did this, but I am certain that the screws supplied with my kit were of a length that didn’t require a through hole.
The supplied screws were also a cheap, coarse thread, self-tapping affair that I replaced with high-quality, fine thread machine screws.
So, what I did was substantially better than what the suppliers of these kits expect to do the job, and what I’m sure a vast majority of owners are living with. I’ve never once heard of one of those kits coming loose, so I have total confidence in the work I did.
I remember it being substantially more than 3-4 threads with my fine thread screws, but I’ll give the benefit of the doubt on that. But, ultimately, you have to ask yourself what these “hold downs” are expected to do. It’s not providing hundreds of pounds of clamp load, like a bearing cap.
I can understand the use of fine thread screws in aluminum on items sun
Ch as the tach gen, where there is little tensional force. But I’d worry about it here where I think that that force could be significant over time. Just my 2 cents.
Where would this force come from? The tappet guides only see sliding force on them, and that isn’t much.
These are the screws that get supplied with many of these ‘hold down’ kits.
They are designed to resist the upward drift of a loose tappet guide. Something on the order of ‘tens’ of pounds would accomplish this.
Also, if this was something that required significant force, you’d want a plate that picks up on more than about 20% of the circumference of the guide.
But that’s enough to push them out on occasion, which if the hold down is installed would translate to tension on the screws. You’re correct in that the force may be small, but over time in a frequently repeated motion of thousands of times per minute, would that cause an issue? 
