tappet clearance advise

[mgcjag]

I thought Peter Crespin gave the answer above.....

[Hugo]

Can we, PLEASE, end these long running sagas and just accept that Jaguar changed the clearances.
Unless we can drag someone back from their grave to get their reasoning we will just have to accept it !!!

Mark started this off by asking for advice. He has followed it and reset the clearances. Hopefully he is happy with the result and will have a nice, quiet engine. Geoff

We know that they did increase the clearance, but what we don't know, or I don't know, is why. I'm intrigued by this apparent mystery, and I'd like to find out what I'm missing. Just trying to add to the sum of human knowledge here

[mgcjag]

You have been told why and its also in the link i posted.....if you want it directly from Jaguar then good luck..

[Hugo]

I thought Peter Crespin gave the answer above.....

No he didn't, as my response makes clear. Unless I have part of his text missing.

[Hugo]

You have been told why and its also in the link i posted.....if you want it directly from Jaguar then good luck..

No I haven't "been told why". Unless I am missing a post somewhere. Your link gives lots of basic information about camshafts, but doesn't even mention the issue at stake. I read it twice.

[mgcjag]

Did you not see this bit
"In 1968, Jaguar changed the lobe pattern of the camshafts. This was not an emission change but rather a change to smooth out the tappet noise. The lobe engaged the tappet more gently rather than slapping. These cams are identified by the two bolt front and a groove cut around the front flange.

[Hugo]

Yes I did. But my question was, and remains, why a change of camshaft necessitated an alteration in valve clearance, when the factors that dictate the clearance remain unaltered.

[PeterCrespin]

Geoff, if it takes a few more iterations to establish technical understanding, what's the problem? Stop reading the thread if it bugs you, no?

Hugo wrote: "...my question was, and remains, why a change of camshaft necessitated an alteration in valve clearance, when the factors that dictate the clearance remain unaltered.."

Because you're ignoring a key factor Hugo; cam design.

I'm on an iPhone so am not sure of the sequence, but here are I think a few answers in order:

You seem to accept now that the XK clearances open up when hot, not close up. Great. You also understand that head expansion is important etc. etc. So far so good on those factors which do indeed remain largely unaltered.

I am, however, not sure why you can't also see that if all these factors remain identical in two identical engines, the design of a cam can also change the required gap for otherwise identical examples?

It's a bit like when you install performance cams and want to plot the timing diagram - the cam maker usually specifies a wider than normal gap for setting up the cams. This is to avoid bogus cam timing errors on where the exact flank to base circle starts. The transition can vary a few degrees when turning the cam to measure lift at such low levels, same as it's hard to get TDC perfect when turning the crank and try to get the exact nul point. That's why the double stop ignition timing method is better and why, say, a 50 thou valve clearance is useful for checking the cam diagram. When running, the greater clearance likewise makes the very subtle quietening ramps feasible.

Chris: The idea of running tighter clearances and thereby squeezing a tiny bit more area under the lift curve is largely a chimera too. The 'extra' lift would not be at maximum lift (the cams being as good as identical at max lift). The extra opening from using tighter clearances would be spread out over quite a few degrees at either tail of the curve. Numerically it might look on paper that you've 'now got quite a sporty lift and overlap graph, but the extra durations of opening are at miniscule lift. On the minus side, although the very low quietening ramp lift gives no real extra cylinder filling, the extra degrees of opening can leak gas and spoil the compression stroke when the inlet closes later; spoil the power stroke when the exhaust opens sooner and spoils the vacuum and idle too.

I'm sure some people tinker with compromise settings, and I like the S3 cams run with 12 thou clearances, but just accept that although the thermodynamic fundamentsls of the engines don't change, the maker of the cam gets to specify the designed clearances and there's no point trying to put up a different spec just because we're not aware why clearances may need to change. All good fun anyway...

[Hugo]

I am familiar with the concept of using a bigger gap for timing purposes, but I don't think it's a good idea at all. Good old Gardner diesels used to give you a nice diagram giving the opening & closing points of the valves, so you could just take the mean point between the two & get it spot on.
But some years ago I had to rebuild an AEC diesel and could find no info at all on valve timing. By then AEC had been taken over by Leyland who in turn had been taken over by Volvo. So it was impossible to find anybody who knew anything about these old AEC engines. I did eventually get the following advice; set the valve clearances to .021" (instead of the running clearance of .014") and set the cam timing so that the pushrod on no. 1 cyl "is just beginning to get stiff to turn with your fingers" at x degrees.
I spent some time scratching my head to figure out what this all meant. I defy anybody to determine the point at which a pushrod "is just beginning to get stiff to turn with your fingers". I found that this could mean anything within about an eighth of a turn of crank rotation.
Then, in a flash of inspiration, I realised what they meant; this was the point at which the valve was just beginning to open when the clearances were set back to normal. So I set the gaps back to .014", stuck a dial gauge on top of the valve & waited till it moved. Bingo! That was the sweetest engine you ever heard.
But none of this addresses my original question; given that the ideal clearance is nil at running temperature, and given that the manufacturers set the specified clearance to cater for all the different expansion rates of the various items involved and still leave a safe margin, why would they increase that margin when none of those variables have changed?
There must be a simple reason out there that I am missing. I accept that a gentler ramp will tolerate a bigger clearance without clattering, but that implies that XK engines must have been running with insufficient valve clearance for the first twenty years of their life. If the gap wasn't insufficient to begin with, why did they make it bigger?

[Geoff Green]

Hugo,

You need time to cool the valve by leaving it on the seat for a determined time. With manufacture differences that could be additive a gap is specified. When emissions increased the temp the seat parking time increased to allow more heat transfer to keep the valve from burning which is done by increasing the gap.

Cam profile changes will also change the gap ( clearance).

Because part manufacture may be completed at different times for a system, country laws occur at different times and emission cams work with 3 SU setup an overlap may happen that 40 years we may think is incorrect work.

[Hugo]

That's an interesting take - that the change is due to emissions controls causing the engine to run hotter. Harley engines had a negative valve overlap for political reasons - something to do with emissions but I'm not sure which aspect of it.

But my question then becomes this; why did they not design the revised timing into the cam itself, rather than by the rather crude method of leaving a bigger gap to keep the valve on its seat for longer?
Also, I seem to recall reading on the link Steve sent that the cam change was NOT due to emissions regs?

[M4rk]

Hi All,

If anybody is interested Just thought I would give you an update following the clearance changes.

No additional noise from the tappets in fact very quiet (Seats and guides were replace/cut during rebuild)
Engine seems to idle smoother
Timing has changed 1 degree from 9 to 8 btdc

I am impressed with the knowledge base on this forum, would be interesting to know exactly why the clearances changed, I don't know whether the cam profile changed but would it be similar reasons as to why performance cams have larger clearances?

Mark

[mgcjag]

Hi Mark...yes we are intetested to hear that you have sorted your problem....thanks for the update.. Steve

[Hugo]

Hi All,

I am impressed with the knowledge base on this forum, would be interesting to know exactly why the clearances changed, I don't know whether the cam profile changed but would it be similar reasons as to why performance cams have larger clearances?

Mark

That's the question that's bugging me too. Yes, the cam profiles changed, but we are talking about the clearance on the back of the cam, when the pointy bit is pointing away from the tappet. So the tappet & valve stem assembly at that point don't know or care what cam profile you are using. None of the relevant bits (valve or tappet) have changed, but the clearances have changed.
Given that Jaguar engineers are probably not complete idiots, they would presumably have had a sound reason for trebling the valve clearance, but I'm hanged if I can figure out what it is.

[Series1 Stu]

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

[Hugo]

I'm afraid you've left me behind with this one. Why would the contact point be nearer the centre of the tappet?

[PeterCrespin]

Hugo, the higher the lift the taller the lobe and the further towards the edge of the tappet it makes first contact. Hence for full race cams the lobe would actually strike the tappet guide unless oversize tappets and guide are used.

This is related to your prior comment that tappet clearance only relates to the space under the base circle. It doesn't. Clearly if you have too large a gap it won't make contact until the camshaft has turned several more degrees ( and the cam contact has moved closer to the centre of the cam (where it touches at max lift). Being symmetrical, with a bigger gap the valve will close earlier as well as open later - i.e. it will reduce cam duration/overlap.

That said, the reason Jag changed the clearance was not to alter the cam timing, it was to preserve 'normal timing' when they changed to quieter parabolic cams with revised opening and closing ramps for the reasons stated in previous posts.

[Hugo]

Ok, gotcha. BUT - what you appear to be saying is that when Jaguar revised the cam design, they made the timing wrong by increasing the duration. They then 'fixed' this problem they had thus created by opening up the gaps to get the timing back where it should be.
In which case, why didn't they just design the cams properly, to give the correct duration (with the revised ramps) in the first place? Seems a bit Heath-Robinson to me. Especially since their motivation was to reduce valve noise, then they negate that by opening up the gap?

[PeterCrespin]

That's not what I said or implied at any point Hugo, sorry. I'm done on the subject.

[Hugo]

I apologise if I have mis-understood you - I would be grateful if you could put me right. I'm just tryng to understand all this.