Why you should drive your SM

From Citroen SM Wiki

Drive your SM!

© 2004, Rolf Knijff

The other day we were attending a wedding party in Brussels. To our amazement one of the guests turned up in his SM. Using it, more or less, like a 'daily' car as the owner explained.

The web master of citroensm.net, Clemens Maas, uses his car to visit club-meetings all over Europe. He doesn't turn his hand to do 700 km a day, going abroad from the Netherlands to Sweden or France. Drive it!

Is that special? Well, in a way, yes. How many SMs you know of are really used on a regular basis? How many SMs are languishing in garages without much use? Of course a lot those garage-queens are not being driven because they broke down and awaiting repair or are in the midst of a restoration. But if your SM is OK and can be driven, drive it! It's better for your car! This is why.

Wearing down your car by not using it.

If you want to learn about car engines, no better place to start than internal combustion airplane engines. This is what we found in Kas Thomas book 'The Major Overhaul. A guide for pilots' . Kas is an engine nut. He dug up this information in a Lycoming Service Instruction, valid for all Textron Lycoming aircraft engines. The subject of this particular instruction is 'Recommended Time Between Overhaul Periods'. This is what it says:

Because of engine deterioration in the form of corrosion (rust), drying out and hardening of composition materials such as gaskets, seals, flexible hoses, fuel pump diaphragms and abnormal wear during starting on soft metal bearing surfaces due to the loss of a protective oil film during extended periods of inactivity, all engines that do not accumulate the recommended operating hours between overhaul in a twelve year period must be overhauled during the twelfth year

What this long phrases, and following text, says is: use your engine at least once a month

If you don't it will have to be overhauled before it's 'due date', expressed in hours of use! Textron says that an engine preferably should not be out of service for more than 30 days. That seems not very long! It seems fair to say that the same advice applies to SM-engines. But lack of use will 'damage' more components of the car than just it's engine. Hydraulic systems, hoses, etcetera, won't like lack of use either.

After the major overhaul

What can happen if you don't use your car for a (very) long time? We heard this story some years ago. A friend garage-owner had a customers engine overhauled at the Maserati works in Modena. It cost a lot of money. For some reason or other however the engine was not as it should be. It was leaking oil. Our friend, the garage-owner, who installed the engine was very careful not to touch the engine and void the guarantee. The resulting 'discussion' on costs between Maserati, the garage and the owner took quite some time. The SM not being used for a couple of years without repair, because the owner was not sure what other costs he had to face. When the problem was solved, more or less, and the engine started up again all was wrong. Especially with the valves! Sticking valves, no compression. A (partially) new overhaul was necessary. What the story proves, apart from the need to be careful in the organization of major engine work, is that SM engines don't like to be idle!

Breaking Exhaust Valves

SMs exhaust valve are hollow. They are filled with sodium, which when shaken by the up-and-down movement of the valve in a running engine, gets rid of engine heat quicker. That is of course when adjusted properly, with sound valve guides and seats! A number of specialists however seem to think that the hollow valve is a mixed blessing. If engine are not used on a regular base, the valve will corrode and eventually may break. Causing a lot of nuisance. Going back to aircraft engines, Kas Thomas informs us that Lycoming engines also have sodium-filled exhaust valve. Thomas remarks however that to do as designed, thus transfer heat, the sodium-filled valves need a close contact with the valves guides! The tighter tolerance, Thomas suggests, the more rapid the wear, which when not attended to leads to 'haphazard seating and inadequate transfer of heat'. And thus: engine failure.

Running hot?

Sodium filled valves were developed for high power aircraft engines by Sam Heron (an engine designer of legendary fame). Consulting another authority, Kevin Cameron writing on motorcycle engines, we learn that sodium-filled valves are so efficient in transferring combustion heat to valve guides, that engines with sodium-filled valves sometimes need extra cooling. In (radial aircraft) engines, Cameron informs us, rocker boxed were filled with oil to prevent valve problems! Thus: sodium-filled valves my be a mixed blessing when surrounding design isn't 'synchronized', or the engine isn't maintained properly. There remains one mystery. Alfa Romeo used sodium-filled valves on all it's classical engine with no problems! A number of questions concerning the Maserati design remain.

  1. Is breakage of exhausts valves happening on worn and overhauled engines?
  2. More precise. To what extent does play in valve guides and the resulting problems of seating geometry (heat transfer problems) play a role?
  3. Is breaking a quality problem? What quality steel is used? Why don't Alfa Romeo valves break.

Solid replacement

To prevent breaking exhaust valves Dutch SM specialist Han Wandel (see links section) informs us he asks his customers when an engine is overhauled how the car will be used. If not used regularly Han will install solid steel exhaust valves. Finally, some people think they can circumvent the problem of lack of use, by 'warming up' their engine in the garage. By having it run for a quarter of an hour. It is an approach also used by owners who don't want to use their car in the winter. Well, this is not a good practice, we think. Engines only warm-up when doing work. Warming your engine in the garage will concentrate heat in the cylinder head. The rest of the engine remains cold, more or less. Thus condensation inside the engine is encouraged, like in the exhaust system. That may lead to breaking valves. How to care your engine when you don't use your SM, is a following story.


  • Thomas, K,(1994), The major overhaul, TBO Advisor books, Old Greenwich.
  • Cameron, K.,(1998), Sportbike Performance Handbook, MBI, Publishing, Osceola.