This is the third time I have seen exhaust valve issues on Panhard engines. The first one was intermittent loose, the second one dropped out, and now this one in Brian’s engine which was damaged by valve seat recession.

Many years ago, I looked at a loose valve seat and wondered how if these things that were put in using cryogenics they could come loose, because it is rare in modern engines. I used a spreadsheet calculation to plot the relative sizes of the machined seat recess and the insert outside diameter as the temperature rose. Ultimately as the aluminium head gets hotter the dissimilar materials will expand, and loose their interference fit. It happened at around 320ºC, and the percussive action of the valve would almost certainly be moving the seat by then.

I have been told the expected temperature of the piston will be around 280ºC, but longer advance values from say, a tired distributor will mean the average head temperatures will almost certainly be higher. If you throw in a weak mixture to one cylinder being driven by a good one down the motorway on a hot day for example, and you’ll could easily meet this high temperature condition, but the valve runs much hotter at certain parts of the combustion cycle anyway.

When combustion is initiated the exhaust valve head is heated on the face closest to the piston, and when it opens, the hot combustion products wrap around the valve head, heating the back of the valve head, the seat insert, and the stem. Whereas the inlet valve gets a cooling effect from the fresh charge, the exhaust relies on loosing this heat through the seat insert primarily, which can be as high as 75%. Typically the stem to guide takes the other 25%. These values can vary, depending on what you read, but the primary heat path is the valve seat insert to cylinder head interface. If this has poor conductivity, then the valve seat will not lose its heat, and run hot.



The other side of the coin is other air cooled engines and do they drop valve seats? I don’t know of a problem with Citröen 2CVs, but it was an occasional occurrence with 412 VWs, so perhaps it is the way of things to be. However, cooler oil, non weak mixtures, better ignition control and within specification valve guide clearances should all help reduce the temperatures that the valve seat is exposed to, but is there anything else that could be done.

A quick look into the history books, might reveal a few answers, and they do with exotic materials being used, valve gear exposed to airflow, excessive finning to lose heat, rich mixtures to create a cooling effect, and even reducing head diameter to reduce the surface area exposed to the combustion process. Anything else?

Here’s a Panhard valve seat insert, and below that is the same Panhard exhaust seat insert & a modern equivalent from a Harley Davidson…spot the difference? The clue is in the sides.





Notice the carbon deposits from the combustion products caking up the sides of the seat.



Panhard valve seat inserts, have peripheral grooves in them, nearly every other seat has smooth sides to maximise the contact area, so perhaps this explains the issue more than anything else. Panhard valve seats should be replaced with smooth bore equivalents, and possible have slightly more interference fit.

Brian’s engine will have this fitted to his affected cylinder.