So all break downs are related to valve clearance

Agree, and its a nice phrase too, has a sort of rhyme to it. 0.25 for me all the time.

Absolutely, get the valve clearances right and your chassis won't break, your coil and points will last indefinitely, rust is but a distant memory, your hood will be perfectly water tight, your king pins will never chatter, your battery will be charged at 14.3 volts precisely and you never need change the oil again as long as you use a Purflux oil filter.

Please explain why 0.25 mm is better than Citroens recommendation of 0.20 mm
Do you know what happens changing to 0.25 mm

Love it.

The camshaft has a base circle. You measure the clearance directly opposite the point of the cam, at the lowest spot. The base circle is ground slightly oval so that as the camshaft rotates it initially lifts the valve gently before opening the valve completely.

By increasing the valve clearance very slightly it takes fractionally longer before the follower contacts the base circle and begins to open the valve.

Therefore the valve stays closed fractionally longer which allows the head of the valve to transfer heat to the cylinder head. The head of the valve can't really shed heat when it is open. Hence the tighter the valve clearance the greater the risk of a burnt out valve.

Edit

Bear in mind that we don't have leaded fuel any more, the lead acted as a lubricant on the valve seat.

If the head of the valve gets too hot it starts to weld itself to the valve seat as the valve snaps shut. When it is opened again the weld is broken and microscopic bits of valve seat get stuck to the head of the valve. That means the valve seat starts to wear. A phonomenon known as valve seat erosion. The lead used to prevent this.

Aditionally modern fuels run hotter than the stuff we got years ago.

Anyway Cloudy, beside the controversy of 0.20 and 0.25 the pictures are excellent and should be made a sticky, if that's the right term.

Viking,
out of idle curiosity, why did this cylinder head have to be removed?
http://www.citroenclassic.org.au/Galler ... 80_jpg.htm

And why was an electronic ignition unit being fitted, instead of a replacement set of advance weights?
http://www.citroenclassic.org.au/Galler ... 85_jpg.htm

ken.

Oil feeding pipe snapped.

Push rod seal leaking badly

Ignition fitted as a test (dust and moister (creek crossings)

Burning a valve or a seat is more likely to come from, worn valve guides, wrong ignition or wrong mixture than changing the valve clearance less than 0,05 mm