tappet clearance advise

[Hugo]

The only reason I can think of for this is to ensure that the cam contacts the tappet closer to the centerline of the tappet, thus reducing the extent of the offset load on the valve gear and it's tendency to rock. This might possibly reduce the noise levels too.

Regards

I'm pretty sure this premise is incorrect. The contact point on the cam-follower will remain constant, regardless of the position of the cam. It doesn't start at the edge and sweep inwards, as you seem to imply. Altering the valve clearance will not alter the point at which it makes contact. It will affect the position of the cam when it hits, but not the point on the cam-follower that it hits. This will be constant - slightly offset from the centre at all points of the camshaft compass (slightly offset to make sure the cam-followers rotate). Another theory bites the dust?

[Series1 Stu]

Hugo, sometimes you talk dross! I don't want to go into this too deeply but you really should research things before setting out your thoughts.

I've prepared a couple of sketches to demonstrate my point. These are pretty much to scale, although I had to guess at the cam lobe base diameter. In these sketches, I have greatly exaggerated the clearances in order to clearly demonstrate the point I was making.

This first sketch shows the situation with the smaller clearance.



This second sketch is with the clearance increased.



You can clearly see that, everything else being equal, the initial contact point moves further inboard on the tappet as the valve clearance is increased. Do you see this now?

I don't believe that my suggestion implied that the cam starts at the edge and sweeps inward, these things are mutually exclusive. Although having looked at the Jaguar cut away illustrations of the cams and valves the cams do appear to sweep the whole diameter of the tappet.

None of this explains why the valve clearances were increased though. Perhaps it was to ensure that the resultant force of the action of the cam on the tappet is in the correct direction, ensuring the valve spring can be overcome and avoiding locking the assembly?

Regards

[Hugo]

Ok, that looks pretty convincing. But your cam has very straight sides, and I was envisaging a cam with quietening ramps, and these would be the first part of the cam to press on the cam-follower, and by definition the point of first contact would then be closer to the centre of the cam-follower than in your illustration. I instinctively feel that it should be possible to design a cam lobe that behaves in the way I described.
But until I can prove this, I will concede the point and I am grateful to you for taking the trouble to illustrate it!
I'm not sure why you would say "I don't believe that my suggestion implied that the cam starts at the edge and sweeps inward, these things are mutually exclusive." as this is precisely what your illustration shows, isn't it? It starts at the edge, and at maximum lift, the point of the cam will be in the centre of the cam-follower. What I should have added, of course, is that the cam then sweeps across to the other edge on its way up.
So - we still don't know why they opened the clearance up.
Your theory in your last sentence could more easily be achieved by getting the cam profile right. Every suggestion so far boils down to the theory that the increased clearance is to compensate for a flaw in the profile of the new cam. There has to be a better reason.

[johnney]

Hugo, the original cams have straight sides the parabolic cams are not straight leading to the valves starting to open sooner and close later (Increased duration). This means that maintaining the same gap as the original cams leads to an increase in duration. Increasing the gap has the effect of reducing the cams duration back to the design ideal.

[Hugo]

Yes, that is a logical explanation, but it falls into the category I described earlier - that they increased the clearance to compensate for a flaw in the cam profile. Why didn't they just design the cams to give the proper duration to start with?

[christopher storey]

There is no "flaw in the cam profile". To achieve the wanted effect the cams are not straight sided but are "shouldered" . The effect of that, as has already been explained is that the parabolic cams start to open the valves earlier in their angular travel than did the straight sided ones . That is not a flaw , but an inescapable consequence . So, to avoid increasing the duration ( and overlap ) above the designed 252 degree duration, the clearance is increased

[Hugo]

Yes, I understand all that. But it doesn't negate my point that the later cams, as designed, give too long a duration. My question remains - why did they not just design the cam profile to give the correct duration in the first place instead of messing about with the gap to get the duration right, and thereby at least partially negating the quietening effect ?

[chrisfell]

Jaguar introduced the quieter opening cams some 20 or more years after the XK was first fitted to a car. There was no flaw in the design. Nor was there a flaw in the parabolic cams that needed a wider gap to compensate. So why was the design changed?

Jaguar seem not to have left us any clues as to their reason or reasons. I'm happy with my view that the increased clearance specs from a very specific 0.004"/0.006" to the more variable 0.012" to 0.014" had as one of their causes a relaxation of the service interval for valve checks. Now, if someone has the official service intervals for checking the valve gaps pre and post the changeover, this theory could be proved correct or wrong.......... I can only find intervals for checking timing chains.

[Hugo]

That's an interesting idea - but I feel that if Jaguar just wanted more flexibility in the settings to accommodate longer service intervals, they would not have trebled the gap - just added a couple of thou or so.
One question not yet adressed is why they did away with the differential between inlet & exhaust settings?
But bang on cue, and just to complicate matters further, I just came across an old article online that states that they upped the gaps to 14 AND 16 thou, not 14-16, so maybe they kept the differential after all and everybody has just been reading it wrong for the last fifty years?
The more we learn, the less we know!

[mgcjag]

Really.....everyone has had it wrong for the last 50 years but you have now found the answer....anyway its 12-14 and not 14-16.....you really do believe too much you read online..... Steve

[Hugo]

I did not say I had "found the answer" - I said I had found information which contradicts the previous information. It's on here, page 5; http://vintage.mitchell1.com/PClubData/ ... 735167.pdf
It won't let me copy & paste, but it states quite clearly that the clearance on engines after xyz are "intake .012" and exhaust .014".
You may judge for yourself, but this looks to me like a pretty professional document, printed by an outfit in California for the motor trade. In fact it looks to me for all the world like a re-print from a Jaguar manual.
So no, I don't believe everything I read on the internet. As you should know by now, I question everything.
The point I was trying to make was that I have now uncovered further information which, instead of leading us to the answer, has led us away from it.
The question now remains as I stated it - why did Jaguar do away with the differential, or did they in fact keep it and everybody has just been mis-reading the numbers all these years? Stranger things have happened you know!
"intake .012" and exhaust .014" is absolutely unambiguous, but "12-14 thou" is less so - it can be read both ways (and indeed it was by an earlier contributors to this debate).
I would very much like to find out which is correct, and also why they trebled the clearance.

[mgcjag]

Mitchel manuals...a poor second to a Haynes....both are well known to have inacurate information.
From a Jaguar manual

[Hugo]

Second to Haynes? Is that possible? Haynes go into great detail telling you how to open the bonnet, chock the wheels and disconnect the battery, then when you want to know something useful they say "refer to main dealer".
I'd never heard of Mitchell manuals.

[Hugo]

The plot thickens. I just found this on an earlier post; -
"The cam clearances are stamped on the original vin plate as 12. And 14 .
So this came from the factory and was not a later owner installed item.
I put the car on xke data site. 880108 with a photo of the vin plate. The cams have two holes on the ends."

[johnetype]

Hugo, the plot doesn't thicken, you're just making a right Baldrick out of it. Contemporary XK engines of the day left the factory with a sticker on the side of the camshaft cover by the oil filler cap that said:

"TAPPET CLEARANCE INLET AND EXHAUST 012 INS TO 014 INS"

It couldn't be more straightforward, from the horse's mouth so to speak, the clearances for both inlet and exhaust are both the same with a 2 thou tolerance which is the nearest you're practically going to be able to set them up to.

John

[Hugo]

Oh if only it were that simple - the data plate said quite clearly inlet .012", exhaust .014". There was a picture of it - it was on this forum somewhere a couple of years ago. And it was an early car, which nobody could figure out!
I did just read a possible plausible explanation for the whole thing - emissions. Is it possible that the re-design of the cams co-incided with some EPA ruling or something, and that Jaguar opened up the gap to REDUCE the duration and thereby comply with the new regs? Harley Davidson did something similar by incorporating a cam with negative overlap.
If that is the case, is there an argument for running the later cams with the smaller gaps?
I'll be putting my (1971) engine back together shortly, so I'll see how the duration measures with both earlier and later clearances.
15 and 57 degrees on both cams if I recall was the original spec?

[Series1 Stu]

I can't imagine that emissions were a consideration when 880108 was built, that's a very early 3.8 car.

[Hugo]

That one's an oddity - I was thinking of 1968 or whenever they changed cams?

[Geoff Green]

Here you go. RTFB. No need to over think things. All the info is there.

[Hugo]

That's great - very useful, thanks. The earlier cams were 15 & 57 degrees, weren't they? (That's from memory). So it seems they increased the duration by a couple of degrees each side? I wonder why they did that after twenty years of production? Or maybe the new cams just had slower ramps so the nett effect was negligible to zero?
But none of this explains why they saw fit to treble the clearances with the new cams, which was my original question. I wonder what the duration would be with the later cams and the original clearance? Too much, probably.