The engine in this thread is a 300TDi, but the method is the same for any engine that uses a notched head gasket.
In order to determine which thickness head gasket to use, you need to measure how far the piston protrudes above the block (also known as the deck).
Some people just use the thickest gasket, and there's no real harm in doing that - the fact that you may have had the head skimmed will make no difference to the thickness of the head gasket you use - however, if you use a thin gasket in place of a thicker one there's a chance that the piston will hit the face of the head
This is a no-notch gasket (the thickest available) - showing where you should read the old one.
3-thicknessess are available from Land Rover, but a 4th (no-notch) is available from after market suppliers.
1-notch being the thinnest, and no-notch being the thickest. Each notch is worth .1mm (10th of a mm)
The term 'notch' isn't entirely accurate - there are normally holes, but 'V'-shaped cut-outs are sometimes found. The engine in this thread is a rebuilt 300TDi, so it has been rebored and new pistons have been fitted - requiring measurement for the correct gasket.
The block already has the crank fitted and is resting on two wooden blocks so that the crank is clear of the bench and can be turned.
You will need a dial guage and mag mount (or some other way of securing the guage to the top of the block)
The measurement scale and what gasket to use is as follows (courtesy of Porny. Part Numbers courtesy of Ralph):-
Protrusion 0.50 to 0.60 mm - select gasket with 1 identification hole - part No ERR5261
Protrusion 0.61 to 0.70 mm - select gasket with 2 identification holes - part No ERR5262
Protrusion 0.71 to 0.80 mm - select gasket with 3 identification holes - part No ERR5263
Protrusion 0.81 to 0.90 mm - select gasket with 0 identification holes - part No ERR7154
Always use the highest reading to determine what thickness gasket to use.
Two ways of doing it - the first is the recommended way to do it and the second is not.
Make sure the top of the block is spotlessly clean, turn the crank until the piston is close, but below TDC, and then secure the dial guage so that the pointer is over the piston.
Slowly turn the crankshaft, and as the piston continues to rise, the needle of the dial guage will move clockwise. Keep an eye on the guage while you turn the crank and eventually the needle will hesitate and then start to fall - you need to stop turning the crank at the point where the needle hesitates. The reading is unimportant - you're only using the dial guage to determine that the piston is as far up as it will go.
It's vital that the crankshaft isn't moved at all as the setting will be disturbed.
Place an enginneering ruler or similar accurate straight edge across the two pistons that are now up.
The gap you can now see is how far the pistons are protruding from the block - using a feeler guage - measure the gap either side of each piston and make a note of the gap (they will most likely be different to each other).
Do the same with the other two pistons - a 30cm (12-inch) ruler is sufficient to do the measuring.
You should now have 8-readings, and the highest one is the one that will determine which thickness gasket you have to use.
I did the following sketch:-
In this instance the highest point is the rear of No1 piston (.084mm), so I will have to use a no-notch gasket)
2nd method that isn't recommended, but is an alternative:-
Position the pointer on the block - close to the edge of the bore but outside of the fire ring marks (this area might be pitted and not indicative of the true level of the face of the block.
Zero the guage.
Rotate the crank until the piston is close to the top of the bore and carefully twist the dial guage until the pointer moves over to the edge of the piston. The pointer will obviously drop, but ignore the reading. As you turn the crankshaft the needle will start to move clockwise and once it reaches the zero mark once again, keep an eye on it until it reaches the maximum height - in the same way as using the feeler guage method.
This picture is the top of the piston level with the top of the block - the reading from this point upwards is what you need to know.
The reading on the back of No1 piston - .084mm
Take a reading from the front and rear of all 4-pistons and make a note of each one, and again refer to the highest reading to determine which gasket to use.
It's important to note that moving a dial guage once set is not recommended as doing so may well alter the zero setting and therefore give you a false reading. You could minimise this by taking 2 or 3 reading from the same position and take an average if any of them are different.