Once you have the engine back together an important job is to adjust the valve clearance (sometimes called “valve lash” or “tappet gap”). The clearance you are adjusting is the gap between the “tappet” and the tip of the valve. Tappet is an Engineering term meaning:
“a moving part in a machine which transmits motion in a straight line between a cam and another part”
In this case the c90 the tappet is a grub screw that goes through one end of the rocker arm and contacts the top of the valve tip below, transferring the motion of the cam shaft to the valve.
The idea is that you must adjust the clearance between the end of the tappet and the tip of the valve so that – when the engine is running at operating temperature – the tappets are just about making contact with the valve tips when the valves are shut. If the gap is too big then, when the tappet pushes down on the valve, it would “hammer” the top, potentially damaging the parts. If the gap is too tight then the valves can be held open when they should be shut and this can potentially burn the valves, preventing them from making a good seal with the valve seat. The adjustment to set the correct gap must be done when the engine is cold, for reasons that will become clear later.
valve gaps too large
I am a bit hazy on the explanation of the “gap too big” problem, but it seems to come down to the difference between pushing something and striking it. The cams on the camshaft are shaped like a ramp and are designed so that the momentum of the tappet increases gradually until the valve is fully opened but if there is a gap between the tappet and the tip of the valve then this delays when the tappet contacts the valve and the extra momentum gained causes it to strike the valve, a bit like a hammer.
As can be seen in diagram of a cam profile below, the cam is designed so that the face of the rocker arm initially contacts the shallower part of the cam. This is intended to smooth the acceleration of the part, allowing any play in the cam and valve mechanism to be gradually taken up and reducing the impact. When the valve clearance is too large this part of the ramp is bypassed and instead the follower collides with the harsher part of the ramp and is accelerated more aggressively:
The other effect of valve gaps being too large is on the power the engine generates . This is because the gap has an effect on valve timing: increasing clearance between tappet and valve effectively reduces the time the valve is open, and this interferes with the draw of fuel/air mixture into the engine.
… and gaps too small
What happens when valve gaps are set too tight? In extreme cases the tappets can be over tightened to the extent that the valves are kept open all the time and never properly seal the cylinder.
Problems can still occur when the gap is sufficient to allow the valves to close properly but tighter than specified by the manufacturer. This is because the contact between the valve and valve seat is what allows heat from the valves to transfer to the air-cooled cylinder head:
A too narrow gap causes the valves to open sooner and close later, meaning they stay open longer than normal and this reduces the time for heat dissipation. If the valves get too hot they can ‘burn’.
This tends to happen on the exhaust valve which, unlike the intake, is not cooled by air drawn from the carburettor, and therefore gets correspondingly hotter.
If overheated valves are burned this can damage the seating surfaces and – since hot gasses cab now leak past the damaged areas – this makes the problem worse. Here are some burned exhaust valves showing the damage that can occur:
Valve gaps at operating temperature
Since the metal components in an engine expand with heat, the valve gap at operating temperature is different from the valve gap when the engine is cold. The amount the valve gap grows or shrinks is determined by the materials of the components involved in operating the valves:
The rocker arms are housed in the aluminium head which, as it gets hot, tends to move the rocker arm away from the valve, but at the same time the valve gets longer as it heats up, offsetting the effect.
The other variable is heat: we know aluminium expands more rapidly than steel, which is why we heat aluminium components to insert or remove steel parts like bearings, but the steel valves get very hot compared to the cylinder head which is cooled by the air that flows around it.
No doubt someone with an engineering background could calculate what will happen to the valve gaps in a C90 when the engine reaches operating temperature but luckily we don’t have to as someone on the c90 forum has measured it and confirmed that the valve gap in this engine increases as the engine gets hotter. Note this measurement was done on a OHC C90 engine, I have read somewhere that the valve gap on the older pushrod engines decreases as the engine warms up , but have not been able to confirm it.
So now we know why everyone says it is important to set the gap when the engine is cold: if you set it when the engine is hot the gap will be insufficient when the engine cools down.
How much clearance is needed?
Because the consequence of valve gaps being too tight tend to be more serious than those caused by gaps that are too loose, it is sometimes said that a ‘noisy tappet is a happy tappet’ (the noise – from the hammering parts – is a sign the tappet is not too tight).
But, as we have seen, there is a trade off between engine wear (and therefore maintenance schedules), heat dissipation and performance, so the preference must be to get the adjustment just right. But what is ‘just right’? We will look at this in the next post.