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.

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