Valve shim position

Sorry, next question - I have fitted new valves, springs, collets and valve caps and am about to start the valve clearance shimming process. The head has had new valve seat inserts.
On some of the valves, the shim is not well retained in the top of the valve cap, sitting on top of the valve stem rather than sitting down in the recess in the spring cap. It looks as if these shims could easily slide sideways under the large inverted tappet. Is this normal, or do I have a problem with the valve stem lengths? Does this mean a trip back to the machine shop to get the valve stem lengths reduced?

It has to sit on the valve stem. It can’t have more than 0.006” of air at any time because then the tappet would hit the cam. Therefore if it sits in the recess even just by a hair it can’t get out.
There’s a measurement for the distance between valve stem and camshaft, without tappet or shim. .320” or so.

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Thanks David - yes, thinking about it, if it doesn’t sit on the valve stem it’s not going to work the valve accurately.
I’ve gone through all the old parts from dismantling the engine and none of the shims are retained by the recess in the valve cap, all of them can slip sideways - but none have, judging by the wear marks and polished areas they have all stayed in the centre of the valve cap, centred on the valve stem. I can’t see what holds them there - the valve cap recess certainly doesn’t, they don’t even touch it - but if it works, it works.
I found the 0.032" dimension, but couldn’t see a sensible way of measuring this without a huge stack of feeler gauges.

The shim 35 should sit in the recess in collar 31, and touch the end of valve 22.


Write down all your shim thicknesses.
Then you put on 34 tappet cup.
In theory if you had a wide depth gauge you could measure from the tappet face to the head flat face, i.e. the center of the cam, and calculate your shims. That is probably how the factory did it.
But most of us put in the cam and measure the maximum gap, cam to tappet.
Then recalculate your new shim thicknesses.
You only have to do this once, then you should be good for 100k miles.
Unless you start getting valve seat erosion.
Or run it dry and get cam wear. :scream:

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Thanks Rob,
I’m having to do it from scratch as all the old components are very badly worn. I have new valve seat inserts, valves, springs, caps, collets, spring seats, stem seals, tappetts and shims, so apart from the original cams everything is new.
My concern was that the shim 35 sits above the recess in 31 when placed on the valve stem and can move around inside tappet 34 as it’s not located. However going back to the old components and dry-assembling those, they are the same - and none have moved, so presumably the spring pressure holds everything in place.

No, the spring pressure doesn’t hold it in place.

In a way it does by making the tappet follow the cam but there is no spring pressure on the tappet while the valve is closed.

Even then it can’t hop out: If you put a shim on the edge of the collar 31 and put the valvetrain together, the valve would be open slightly no matter how the cam sits, and you would have (less than) zero clearance.
That in turn means the shim can’t hop out of the collar because, while not always touching the collar, it cannot escape past the tappet to collar gap because that gap will never be big enough.
Also, the distances are determined by the groove in which the collets sit, and the valves are machined to decent tolerances and the stems must not be ground. That means you can’t do anything wrong, just set the valve clearances and have fun! (After… although I enjoy spending the time).

I put shims in, measure, and write my measurements onto the head where the valve cover would sit. I tend to do this in the middle of the night and get confused, that’s why. Then I calculate the correct shim, look for it (I have access to a good collection) and hope for the best. Usually I need to go back in again but then it’s good. Remember, only one cam at a time! I think I would measure the .340” with a stack or make up a bit of rod and maybe try to hold that in, go/no go. But it should be good.

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You mean the 35 shim is taller than the 31 collar, sits above. That is correct.

If you mean the 35 shim can spin, that is correct.
It sits in the pocket of the 31 collar, but has no room to pop out. It’s too tall.

Just for info…tha cam lobe does not strike the tappet centrally…but just off center…so the tappet actually rotates …you can see the wear pattern on one of your old tappets if you look…Steve

On one or two of the exhausts (haven’t got around to the inlets yet), the underside of the shim 35 sits on the valve stem above the top of the collar 31, so the shim can simply slide off the valve sideways. It’s the same in most of the old ones I still have. There appears to be no lateral location at all.
Steve, I think all tappets rotate. The flat tappets in my US V8s all have a very slightly domed underside which causes them to rotate, as do the ones in my Cooper S.

There’s no way it can slide off if the distance of the valve stem above the groove for the collets is correct :thinking:

One of mine didn’t always rotate (I guess?) - but you can also see that it did at other times -

I had one shim that felt like it wanted to slide off but if at 90° to the valve stem as the tappet forces it to be it would not be able to slide over the edge of the collar… if that is the case you would want to examine the valve I guess.

I struggled to assemble the head level on the bench, as some of the shims would not stay in place to allow the tappet to be fitted with the slope at an angle. Propping up the side I was working on so the valve tops were closer to horizontal sorted that OK.
I’ve got the exhaust side reliably to 0.006" now, which took shim sizes ranging from 0.094" - 0.098". This took three adjustment cycles. I’ll now remove this cam and do the inlet side, then replace all.

I’m assuming that as with bearing clearances you set the valve clearances with everything dry, then lube on final assembly?

This sounds wrong, like you did something wrong with the #32 valve stem collets. The shims should be captured in the collars.
What sort of valve spring compressor are you using?

The valve spring pressure hold the collets in place. Make sure the collets are seated properly by giving the valve stems a sharp downward tap (ensure that there’s nothing obstructing thevalves from moving). It’s surprising how much the sring tops can rise when the collets are seated properly.

Hi Roger…be very carefull of assembling the head on the bench…as you bolt down the camshaft some valves will open and protrude below the flat surface of the head and can be easily bent…the head needs to be supported at each end to raise it slightly off the bench to avoid this …there must be something wrong in your assembly…each shim should be placed on top of the valve stem and checked to make sure it can “rock” on the valve stem…if it dosnt rock then its sitting on the collar and will cause you problems…the tappet is then dropped in place…make sure that you torque the camshaft in place…if you dont torque it then your gap readings will be incorrect…posting a photo of the top of your valve and shim position would be a good idea…Steve…best photo i could find…of a shim in place
2022-07-01 09.28.47

I use WD40 or similar, not dry

You are right to keep asking questions about this till you are 100% sure, I have seen all sorts of weird things on heads, including seats cut too deep, valve stems ends ground, undersize shims, and acquired 2 heads in a row that had no shims at all, and the job messed up by both professional and amateurs

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Hi Steve, thanks for the tips - done all of that, I’ve built loads of heads of all kinds in the past and am quite used to all the issues. The only OHC ones I’ve done were a long time ago, Stags, Lotus 907 twin cam and S54 BMW M3, all the rest have been pushrod. None of the OHC engines had this large inverted bucket tappet arrangement and were of more modern design than the XK, hence the questions.
Terry, the machining work and parts selection was all done by David Knight engine services who has been machining and building XK engines for decades. I’m pretty confident there’s no issues with their work.
I’ll take a micrometer to the valve caps that the collets sit in. The unknowns here are the caps and the collets, which are beautifully made by Rob Beere but there may be a different tolerance in the heights, which could affect the shim pad location. It’s hard to do a direct comparison because the original valve caps are the old deep kind, whilst the new are the short type - but the thickness of the top pad is what I’m really interested in. That said, in my experience the tapered collets can settle slightly soon after fitting, which would allow the valve cap to rise fractionally giving better location of the shim. So the current plan is to assemble all the valvetrain, give a few gentle nylon-drift taps on the valve stems and give it a few days bench-testing (i.e. sitting there doing nothing) and then popping the cams off again to check.
The odd thing is that the old caps and shims are even worse. That may be due to deposits and wear on the caps and collets precluding proper seating, not sure.

Am I unusual in finding setting the gaps easy? There seems to be a lot written about how tricky this is, which I haven’t found it to be. Having it sitting on the blocks on the bench makes a big difference, obviously.

No, its not hard, unless something is wrong, then things can get confusing

Its hard to visualise your “problem” without looking at it directly with my own eyes

I dont know whether you have an issue or not

Roger…setting gaps is simple…its just acurate measureing and calculating the correct shim size needed …the problems usually occur when people dont know what they are doing and measure and calculate incorrectly…im not sure how you can have measured accurat gaps if your haveing problems with your shims not staying in place…something very wrong somewhere…

Just tedious and minimal maths are involved which is too much for some :yum: