Am i at TDC? (wan't to be super sure) - Series 3

Hi everybody, finally i am finishing the engine head gasket labor, want to know the #6 TDC is properly set before attaching camshaft chain.

I did the following:

  • both #1 and #6 cyl at top, but not sure how top they are, just set them matching the block surface before head drop.
  • i rechecked with a screwdriver through spark plug holes and #1 y 6 are on top
  • distributor is pointing #6, but in my car, don’t know why the #1 (opposite) is pointing to the water pump, instead of the engine block as it should (i think).
  • then i wanted to check with the pulley mark, and it’s around 12BTDC.

here is a pic (sorry for the quality):

And here come the questions (remember head removed, camshaft removed):

  1. is that right pointing there?

  2. As far as i rode through the whole forum, the distributor position doesn’t matter as long as it points to #6 cable at #6 TDC, when doing ignition timing, right?

  3. Also, if both #6 and #1 at TDC, if i place the camshaft with the jag tool, and the little notch at 90º, that position will become the TDC for the #6 no matter what, right?

How much should i move the crankshaft, to which side or is there a reference with a screwdriver i should follow or trust the pulley mark and point it to 0º??

Want’t to be super sure i am at right TDC, so i can connect the camshaft chain, and then complete the head work so i can start the jag again.

Thanks a lot!!


You are right to verify TDC as everything else rests on that. Crank rotation is hard to detect around tdc so use the double stop method. Be careful, but if you confirm and mark same crank stop angle (say 30 degrees) either side of tdc, you can just measure and mark exactly halfway between the two prior marks for true tdc.c

Do you have any means of measuring the height of the piston such as a vernier depth guage or a dial indicator?
Either of these can be used to measure the piston when it is below the block surface, say 1/2” - 12mm on the way up and then on the way down. Make a mark on the pulley at each point, making sure you only turn the crank one way. The mid point between the two marks is your true TDC. If this dosen’t line up to the current position of the pointer then you need to move the pointer to match.

As mentioned, the double stop method is the easiest and most accurate.

Pszemia …

The absolute most accurate way to insure the #1 piston is at TDC is by using a mechanical depth gauge

Either use a magnetic holder or simply cut a hole in a small piece of wood that the dial fits tightly into.
Have a helper manually rotate the engine while you check that the dial has reached its highest point before it decreases again. Since you don’t have the timing chain connected yet there is no problem rotating the engine back and forth when you get close. Remember there is a direct mechanical connection between the crank shaft and pistons.

When you are sure that the #1 piston is at TDC mark where the indicator is pointing on your picture (damper).

Now use the Jaguar tool (or make one yourself) to ensure the two camshafts are at their TDC position in the head and bolt it back on. Connect the timing chains being sure you take out any slack without rotating the engine.


If there is a discrepancy between TDC measured by piston height and TDC pointer, Ariel - recheck…:slight_smile:

Piston TDC is always correct, but difficult to measure properly - and both Peter and Robin presents ways and means. Using a vernier depth gauge is most precise - but still requires some fiddling.

When the engine is set to piston TDC; then you ponder a bit. There are two reasons for pointer misalignment; pointer may be bent out of position - or the vibration damper, which carries the scale, may be adrift. This may happen when the damper rubber deteriorate - requiring damper replacement…

There is no surefire way of detecting an iffy damper - but it’s not a frequent occurrence.

A crude way is to hold the damper steady while turning the engine back and forth - deteriorated rubber may show up as excess engine motion before damper moves. Or removing the belt and try moving the damper, of course…

Loose damper is really a side issue - included only for the record and your ‘super sure’…:slight_smile:

As an aside; the rotor pointing roughly to #6 is to verify that you are on the correct TDC mark - set to TDC the rotor will alternately point to either #6 or #1 when the engine is turning. When the head is put back; #6 is automatically the cylinder ready to fire - so the rotor must point to that. Tnen, indeed, the ignition timing is set using the pointer and scale on the damper. Which is an important point of the TDC mark precision - as Peter says; everything rests on the TDC…

xj6 85 Sov Europe (UK/NZ)

If you have a suitable depth gauge, the best way to determine that you are precisely at TDC on #1 and #6 is to measure the height of #2, #3, #4, and #5. All four should be exactly the same.

This method is more accurate because those four pistons are in the midst of a stroke so even a hair off would make a big difference. #1 and #6 at at the top of a stroke where the piston is nearly stationary, so a little this way or that barely registers.


Well, some time happened and i have been doing a lot of work trying to get the correct valve clearance, so far I have count 4 complete assembly, measure each one, take note, TDC and star again.

My last measure is the following (in mm):
Cylinder | intake | exhaust
1 | 0,40 | 0,35
2 | 1,05 | 0,35
3 | 1,10 | less than 0,05
4 | 0,35 | 0,35
5 | 0,60 | 0,35
6 | 0,40 | 0,35

And the shims:
1 | 2,25 | 2
2 | 1,90 | 2
3 | 2 | 1,9
4 | 2 | 1,9
5 | 2,25 | 2,15
6 | 2,05 | 2

I am way off in some and almost nailed on others. The ones that are around 1 are the most wired for me. I am not an expert but that seems to be too much clearance for those valves, am i right?

Also the one which is pretty closed, how should i correct it? The first measure on that (3 exh) was around 0,20 but now closed. That shim was 2,30. Maybe use on around 2,40?

One more question, I arrive to this last numbers exchanging the mixed shims, maybe i am doing something wrong or should i go buy new ones for the ones which are off. And that would be all?

The shims seems to be special size, is there another one that fit? Maybe from another car brand?

All advices accepted!


0.35mm is the correct clearance so I assume you are doing everything correctly. You can mix&match the shims. 3 exhaust needs to be corrected. 2, 3, 5 are too wide…


And 5 inlets…

Since you are working in metric instead of the usual imperial, Pszemia; clearance is 0,305 to 0,356 mm both inlet and exhaust. The simpler imperial is 0,012 to 0,014" - sort of easier to measure with standard feeler gauges. You don’t say why you are working in metric…?

Nominally; the shims are marked with their appropriate thickness - though it is strongly advised to measure each shim prior to fitting. Shims are commonly used for valve clearance with overhead cams - but the ones used for Jaguar are usually imperial, and marked accordingly

However, if the shim diameter is appropriate - either metric or imperial will work in either case; the method is the same in either case. First measuring actual clearance on each valve - then measure/check the actual shim. It’s important to note and keep careful track on these numbers.
Call actual clearance A, desired clearance B, actual shim C and desired shim D.

A-B gives the desired change - C-D gives the required shim change. Which requires keeping track of whether you need a thicker shim (to reduce valve clearance) or a thinner shim to increase clearance.

For this reason; it is better to juxtapose actual clearance with actual shim for each valve. Like #3 valves in your table; 1,10/2,00 (inlet) and 0,00/1,9 (exhaust). Using 0,35 (for desired clearance) that gives desired inlet shim 1,10 - 0,35 = 0,75 mm thicker; or 2,75 actual shim.

Likewise exhaust; 0,00 (actual clearance) + 0,35 (desired clearance) versus 1,9 (actual shim) to 2,25 (desired shim). And so forth - given that you seem to be working in increments of 0,05 mm…

You can then check all the ‘desired’ shims against your ‘actual’ shims - and shift them accordingly. However, you will likely require some extra shims - and have some spare ones. And honestly; it is a common experience, when you recheck clearances, that it’s not necessarily all correct…:slight_smile:


xj6 85 Sov Europe (UK/NZ)

Maybe it’s me but I think this should be 0.70?


The V12 cams have “quieting ramps”, gradual closing up of the valve clearances on either side of the actual cam profile. If a mech isn’t aware of that, he can really mess up adjusting the valves. Before taking a clearance measurement, you need to make sure the cam lobe is completely around on the other side from the tappet. I presume the sixes have similar cam profiles.

It’s possible that whoever set these valves last time didn’t know that, and set a couple of valves based on measurements that were actually on a ramp. That’d make the clearance too large. I dunno if that’d explain how they got THAT large, but it’s something to consider. Whatever, make sure you do it right from now on.

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clearance at the heel of the cam lobe. that is what one measures and adjusts. differing ways for differing critters.

Never did them on this engine. But, it seems that the dial guage, a micrometer and a feeler gauges are essential.
Dial indicator to assure valve off the ramp and on the heel.

Feeler to measrure lobe to follower clearance.

Mic to measure the shims…


Correct, and that correct way is to adjust with the base circle directly pointed at the tappet.

True, TDC is not required but the lobe must point away from the valve. Not exact science! Also no dial indicator needed and with plugs out the engine can be turned by hand.

TDC is important for cam timing, but it will run if a few degrees out without one even noticing.

There’s a lot in the archives about the benefit of advancing the cams a bit (some say 4 crank degrees IIRC) in 4.2 saloons. The argument is more low end torque without sacrificing much at the high end. I’ve not tried it, but last time I set the timing I aimed at 1-2 deg BTDC, having measured TDC as best I could. That way, a error in one direction would get me closer to spec, while in the other it would get me to where some argue it should be.

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I’m sorry but none of my Jaguar shop manuals provide valve clearances in metric, and I find it a little difficult to translate all of these mm into inches without using a calculator. Why did you take these measurements in metric instead of inches?

I have done valve clearance adjustments on 4.2L XK engines dozens of times, and once on a 5.3L V12. If you provided the clearance measurements in inches, the way that Jaguar does, I would be more than happy to help with advice. I find these measurements in mm difficult to look at and understand. And yes, I do live in the USA. :wink:


I did! I advanced the intake cam by almost the width of the notch and now it idles very well but lost some top end. Also, there was some part throttle pinging and I had to retard it a little.- before, the intake cam was retarded and it never idled well, but it was much quicker. I‘d definitely recommend advancing the intake cam by, say, half a notch! No harm in doing so.
I set TDC with a screwdriver and it was perfect 0° on the timing scale.

An easy conversion is .004” = .01mm So .012” =.03mm

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