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The Black Mystic Art of Cam Timing!
Part One
By Peter Shearman
INTRODUCTION
Like Electric’s many people think that cam timing adjustment is a ‘Black Art’. They think that you have to be some sort of a Magician or Semi-God to be able to do it! Like most things in life experience is the best teacher and the only way to get experience is to have a go yourself.
Year ago (many!) I didn’t have a clue how to adjust Dellorto carburettors but by watching someone do it, asking questions and finally trying it for myself I learned how. If you are willing to have a go then you will always learn something. Even if you make mistakes, and we all do, you will learn from them.
I wasn’t game to try cam timing until a couple of years ago when I rebuilt the 900 engine. Before I tried it I did a lot of reading and watched it done on a belt drive at a service day before finally tackling it myself. Like Desmo shimming it isn’t very hard you just have to give yourself plenty of time to take it slowly and double check all your figures. The good thing about cam timing is that even if you completely mess it up you can always go back to the standard factory timing marks and start again but don’t try running the engine until you are sure the timing is where you want it!
This article is devided into three instalments. The first explains the role of valve timing starting with the basics and moving on to more complex explantions. The second part describes how to measure your existing valve timing and the final part gives a guide on how to change the valve timing
BASIC FOUR STROKE PRINCIPLES
For those with little knowledge of what goes on in a four stroke engine this first section will cover the simplified basics of operation including the part that valve timing plays in the four stroke cycle. If you already know all this just move on to the advanced section.
A four stroke engine crankshaft rotates twice (2 x 360° = 720°) for each cycle of operation. During this cycle the piston moves up and down the bore twice which gives us four strokes! When the piston is at the top of its stroke this is called Top Dead Center or TDC for short. When the piston is at the bottom of its stroke this called Bottom Dead Center or BDC for short. TDC and BDC are reached twice during each cycle of operation.
During these two rotations of the crankshaft the camshaft only goes through one rotation. This is achieved by driving the camshaft at an overall 2:1 reduction ratio from the cranshaft. The camshaft controls one valve cycle which covers two rotations of the crankshaft so whilst the crankshaft goes through 720° the camshaft only goes through 360°. You need to remember this relationship when it comes time to move the camshaft to change the timing.
Valve timing is usually given using two figures. The first is the number of crankshaft degrees degrees before/after TDC/BDC that the valve completely closes. This gives rise to further abbreviations of, BTDC (Before Top Dead Center), ATDC (After Top Dead Center), BBDC (Before Bottom Dead Center) and ABDC (After Bottom Dead Center).
Four stages are passed through for each four stroke cycle and these are listed below in simplified terms to explain the basic four phases involved. We will go into these phases in greater detail in the ‘Advanced’ section.
Let’s start at TDC at the end of the compression stroke.
POWER At TDC (Compression) the spark ignites the compressed mixture resulting in a burning of this mixture to create the power to drive the piston down the cyclinder. Both valves must remain closed for this power stroke.
EXHAUST At BDC the inlet valve must remain closed and the exhaust valve must be open whilst the piston in on the up stroke. This movement forces the burnt gases out past the exhaust valve to the exhaust port and the exhaust system.
INTAKE At TDC (Exhaust/Overlap) the exhaust valve must be closed and the inlet valve must be opened. The pistons downward movement causes a pressure below atmospheric in the cyclinder which allows atmospheric pressure to feed the air/fuel mixture past the inlet valve and into the engine via the inlet port.
COMPRESSION At BDC the exhaust valve must remain closed and the inlet valve must be closed. Whilst the piston is on the up stroke the air/fuel mixture is compressed dramatically ready for ignition by the spark plug. The cycles repeats with the power phase next.
If an engine was set up like this with valves opening and closing at TDC and BDC it would run at low revolutions but such basic valve timing is not good enough for an engine to develop any usable power. If you have grasped the basics and want to know more it’s time to move on to what happens in a real motor.
ADVANCED CAM TIMING THEORY
I found a diagram helpful in understanding what is happening. With reference to figure 1, I have divided the two crankshaft rotations into two circles joined at TDC this should be followed as a ‘Figure of Eight’ through the various cycles.
Starting at TDC compression follow the left hand circle clockwise until you return to TDC exhaust. Then follow the right hand circle anti-clockwise back to TDC compression. I have divided each circle into four 90° segments giving eight phases for the purpose of our advanced discussion. The actual degree settings for cam timing will be discussed later on.
We start at TDC compression. As discussed previously both valves are closed and the air/fuel mixture has been compressed into a small area. Because the mixture takes a finite time to become completely ignited we must start the ignition before the piston gets to TDC. I won’t get into ignition timing in depth here but suffice to say the faster the crankshaft rotates the further before TDC the mixture needs to be ignited. The net result of this is to ensure that the maximum push of the fully burning mixture peaks just as the piston starts its downward stroke regardless of piston speed.
POWER For the first 90°-100° ATDC both valves remain closed whilst the mixture burns causing pressure to rise pushing the piston down on its power stroke.
EXHAUST 1 At around 80° BBDC the exhaust valve starts to open. The main reason for opening the exhaust valve earlier than BDC is so that it will be fully opened by the time the piston reaches the start of the upward exhaust stroke. Most of the power from the burning mixture has been used at this point so there is virtually no loss of power by opening the valve early. Also we can use the small amount of combustion power left to begin the exhaust phase early even though the piston is still moving down.
EXHAUST 2 & 3 From BDC to TDC the exhaust valve is wide open and the rising
piston is forcing the burnt gasses out of the cylinder
INLET 1 At around 60° BTDC the inlet valve starts to open. As for the early opening exhaust valve we do this to give the inlet valve a chance to be fully open by the time the piston is reaching its maximum downward velocity and it gives a head start to filling the cylinder with fresh air/fuel mixture. By the time the inlet valve is fully open the exhaust gasses are moving fast out through the exhaust port and the inertia of this column of gas causes a slight depression in the cylinder which allows atmospheric pressure to feed in the fresh mixture. Exhaust systems are designed with this in mind and the term ‘Extractors’ is fairly self explanatory when you understand what is happening in the engine.
INLET 2 & 3 From TDC Exhaust/Overlap to BDC the inlet valve is wide open and the piston rapidly moving down creates a depression allowing atmospheric pressure to feed fresh mixture into the cylinder.
EXHAUST 4 The exhaust valve remains open till around 60° after TDC. This is done to purge the exhaust gasses. The rapidly moving incoming mixture not only fills the cylinder but forces the last of the exhaust gasses out through the closing exhaust valve until the returning exhaust pulse stalls the flow. The swirling air/fuel mixture at this point also provides some cooling for the hot exhaust valve.
INLET 4 From BDC to around 80° ABDC the inlet valve remains open. Although the piston is now moving upwards the inertia of the incoming air/fuel mixture is stronger and results in a mini super charging effect where the mixture is initially compressed into the cylinder.
COMPRESSION After the inlet valve closes at around 100° BTDC the rising piston continues to compress the fresh air/fuel mixture in readiness for ignition and the start of a new cycle.
When you realise that this full cycle happens around 70 times per second at high revolutions you can appreciate that cam timing is a very critical factor when designing an engine. The timing figures are dependant on the camshaft(s) which are ground specifically to suit the type of engine.
As you can imagine many factors effect the factory setting of cam timing such as, the intake and exhaust systems, fuel type used, compression ratio, maximum RPM of the engine, piston and combustion chamber shapes, torque and maximum power conciderations, fuel economy, etc.
MORE TERMS & CONSIDERATIONS
Nearly all cam timing figures rely on having a known clearance between the rockers and the valves which is greater than the normal or Running clearance. This is called a Checking clearance and is normally 1.00mm (40 thousandths of an inch). Timing at checking clearances means that you will have to temporarily change the shim or adjuster on each valve prior to measuring the timing but more on this later.
Some factory timing figures are given at Running clearance even though there is no appreciable gas flow below 0.5mm lift. This is done to provide enhanced figures which make Duration and Overlay appear much longer which is an advertising advantage when buyers think that longer must be better! If no checking clearance figures are given with the factory timing specification then you have to assume that timing is at running clearance. ie normal operating clearances are used.
The number of degrees from when the valve start to open to when it finally closes is called the Duration of the valve timing. Typical factory figures for the bevel drive 900SS are around 320° for both exhaust and inlet valves although these are running clearance figures and so are optimistic!
Another term mentioned above is Valve Overlap. This figure describes the number of degrees where both the inlet and exhaust valves are opened at the same time. The reasons behind this were discussed in the ‘Advanced’ section.
The duration and overlap can be worked out from the factory figures although as you will see later these do not always match the cam that is fitted to the bike! Lets take the bevel 900SS factory figures and do some calculations. Remember all degree figures are for the crankshaft and must be halved if applied to the camshaft.
Note, factory timing at running clearance for this model so figures are enhanced!
The exhaust opens 80° BBDC and closes 58° ATDC. The exhaust valve duration is 80° (To BDC) + 180° (BDC to TDC) + 58° (ATDC) = 318°.
The inlet opens 63° BTDC and closes 83° ABDC. The inlet valve duration is 63° (To TDC) + 180° (TDC to BDC) + 83° (ABDC) = 326°.
The overlap is the period of degrees where both valves are open. For this cam the overlap is 63° (BTDC inlet opens) + 58° (ATDC exhaust closes) = 121°.
The other figures we are interested in is the point of Maximum Lift (ML) of each valve. Normally this will be half way between the opening and closing degree figures if the cam is symmetrical. So work out where the point of ML sould be we divide the duration of each valve in half and then add that figure to the opening degree figure.
The exhaust valve duration is 318° divide by 2 = 159°. The exhaust opens 80° BBDC (= 260° BTDC) plus 159° (Half exhaust duration). Therefore Exhaust Maximum Lift should be at (260°-159°) = 101° BTDC.
The inlet valve duration is 326° divide by 2 = 163°. The inlet opens 63° BTDC plus 163° (Half inlet duration). Therefore Inlet Maximum Lift should be at (163°-63°) = 100° ATDC.
It is common to refer to these cams as ‘101°/100° lobe center cams’ and as you have probably seen there is a symmetry between the ML figures either side of TDC. This is called Lobe Center Symmetry and most engines have the two ML’s at equal distance away from TDC. We will use this symmetry to decide if the cam is advanced or retarded when we calculate the real ML figures.
Figure 2 shows the relationship between the opening, closing and maximum lift of each valve for the factory specifications the 900ss Bevel Engine.
We now come to a problem not unique to Ducati which is that the cams fitted to the engines do not necessarily match the ones specified in the manual! For whatever reason most 900SS bevels were actually fitted with cams with a lobe center symmetry of ‘96°/96°’ which are not quite as good a cam as the ‘101°/100°’ items. Don’t worry about which cams are fitted to your machine as when we measure the ML it will soon become obvious what you have got! The point is don’t assume that the factory figures relate directly to what is acually fitted to your machine!
Figure 3 shows the timing diagram for the 900SS with ‘96°/96°’ cams. Note that the opening and closing figures shown on the diagram assume that these cams have the same duration as the ‘101°/100°’ cams although this may not be the case. As I have no factory figures for the ‘96°/96°’ cams the only known points are the two ML’s.
Next we will find out more about the three different ways of measuring the valve timing, how to find an accurate piston TDC, how to accurately find the point of maximum valve lift and finally how to measure it using a step by step guide with real life examples!
Part One
By Peter Shearman
INTRODUCTION
Like Electric’s many people think that cam timing adjustment is a ‘Black Art’. They think that you have to be some sort of a Magician or Semi-God to be able to do it! Like most things in life experience is the best teacher and the only way to get experience is to have a go yourself.
Year ago (many!) I didn’t have a clue how to adjust Dellorto carburettors but by watching someone do it, asking questions and finally trying it for myself I learned how. If you are willing to have a go then you will always learn something. Even if you make mistakes, and we all do, you will learn from them.
I wasn’t game to try cam timing until a couple of years ago when I rebuilt the 900 engine. Before I tried it I did a lot of reading and watched it done on a belt drive at a service day before finally tackling it myself. Like Desmo shimming it isn’t very hard you just have to give yourself plenty of time to take it slowly and double check all your figures. The good thing about cam timing is that even if you completely mess it up you can always go back to the standard factory timing marks and start again but don’t try running the engine until you are sure the timing is where you want it!
This article is devided into three instalments. The first explains the role of valve timing starting with the basics and moving on to more complex explantions. The second part describes how to measure your existing valve timing and the final part gives a guide on how to change the valve timing
BASIC FOUR STROKE PRINCIPLES
For those with little knowledge of what goes on in a four stroke engine this first section will cover the simplified basics of operation including the part that valve timing plays in the four stroke cycle. If you already know all this just move on to the advanced section.
A four stroke engine crankshaft rotates twice (2 x 360° = 720°) for each cycle of operation. During this cycle the piston moves up and down the bore twice which gives us four strokes! When the piston is at the top of its stroke this is called Top Dead Center or TDC for short. When the piston is at the bottom of its stroke this called Bottom Dead Center or BDC for short. TDC and BDC are reached twice during each cycle of operation.
During these two rotations of the crankshaft the camshaft only goes through one rotation. This is achieved by driving the camshaft at an overall 2:1 reduction ratio from the cranshaft. The camshaft controls one valve cycle which covers two rotations of the crankshaft so whilst the crankshaft goes through 720° the camshaft only goes through 360°. You need to remember this relationship when it comes time to move the camshaft to change the timing.
Valve timing is usually given using two figures. The first is the number of crankshaft degrees degrees before/after TDC/BDC that the valve completely closes. This gives rise to further abbreviations of, BTDC (Before Top Dead Center), ATDC (After Top Dead Center), BBDC (Before Bottom Dead Center) and ABDC (After Bottom Dead Center).
Four stages are passed through for each four stroke cycle and these are listed below in simplified terms to explain the basic four phases involved. We will go into these phases in greater detail in the ‘Advanced’ section.
Let’s start at TDC at the end of the compression stroke.
POWER At TDC (Compression) the spark ignites the compressed mixture resulting in a burning of this mixture to create the power to drive the piston down the cyclinder. Both valves must remain closed for this power stroke.
EXHAUST At BDC the inlet valve must remain closed and the exhaust valve must be open whilst the piston in on the up stroke. This movement forces the burnt gases out past the exhaust valve to the exhaust port and the exhaust system.
INTAKE At TDC (Exhaust/Overlap) the exhaust valve must be closed and the inlet valve must be opened. The pistons downward movement causes a pressure below atmospheric in the cyclinder which allows atmospheric pressure to feed the air/fuel mixture past the inlet valve and into the engine via the inlet port.
COMPRESSION At BDC the exhaust valve must remain closed and the inlet valve must be closed. Whilst the piston is on the up stroke the air/fuel mixture is compressed dramatically ready for ignition by the spark plug. The cycles repeats with the power phase next.
If an engine was set up like this with valves opening and closing at TDC and BDC it would run at low revolutions but such basic valve timing is not good enough for an engine to develop any usable power. If you have grasped the basics and want to know more it’s time to move on to what happens in a real motor.
ADVANCED CAM TIMING THEORY
I found a diagram helpful in understanding what is happening. With reference to figure 1, I have divided the two crankshaft rotations into two circles joined at TDC this should be followed as a ‘Figure of Eight’ through the various cycles.

Starting at TDC compression follow the left hand circle clockwise until you return to TDC exhaust. Then follow the right hand circle anti-clockwise back to TDC compression. I have divided each circle into four 90° segments giving eight phases for the purpose of our advanced discussion. The actual degree settings for cam timing will be discussed later on.
We start at TDC compression. As discussed previously both valves are closed and the air/fuel mixture has been compressed into a small area. Because the mixture takes a finite time to become completely ignited we must start the ignition before the piston gets to TDC. I won’t get into ignition timing in depth here but suffice to say the faster the crankshaft rotates the further before TDC the mixture needs to be ignited. The net result of this is to ensure that the maximum push of the fully burning mixture peaks just as the piston starts its downward stroke regardless of piston speed.
POWER For the first 90°-100° ATDC both valves remain closed whilst the mixture burns causing pressure to rise pushing the piston down on its power stroke.
EXHAUST 1 At around 80° BBDC the exhaust valve starts to open. The main reason for opening the exhaust valve earlier than BDC is so that it will be fully opened by the time the piston reaches the start of the upward exhaust stroke. Most of the power from the burning mixture has been used at this point so there is virtually no loss of power by opening the valve early. Also we can use the small amount of combustion power left to begin the exhaust phase early even though the piston is still moving down.
EXHAUST 2 & 3 From BDC to TDC the exhaust valve is wide open and the rising
piston is forcing the burnt gasses out of the cylinder
INLET 1 At around 60° BTDC the inlet valve starts to open. As for the early opening exhaust valve we do this to give the inlet valve a chance to be fully open by the time the piston is reaching its maximum downward velocity and it gives a head start to filling the cylinder with fresh air/fuel mixture. By the time the inlet valve is fully open the exhaust gasses are moving fast out through the exhaust port and the inertia of this column of gas causes a slight depression in the cylinder which allows atmospheric pressure to feed in the fresh mixture. Exhaust systems are designed with this in mind and the term ‘Extractors’ is fairly self explanatory when you understand what is happening in the engine.
INLET 2 & 3 From TDC Exhaust/Overlap to BDC the inlet valve is wide open and the piston rapidly moving down creates a depression allowing atmospheric pressure to feed fresh mixture into the cylinder.
EXHAUST 4 The exhaust valve remains open till around 60° after TDC. This is done to purge the exhaust gasses. The rapidly moving incoming mixture not only fills the cylinder but forces the last of the exhaust gasses out through the closing exhaust valve until the returning exhaust pulse stalls the flow. The swirling air/fuel mixture at this point also provides some cooling for the hot exhaust valve.
INLET 4 From BDC to around 80° ABDC the inlet valve remains open. Although the piston is now moving upwards the inertia of the incoming air/fuel mixture is stronger and results in a mini super charging effect where the mixture is initially compressed into the cylinder.
COMPRESSION After the inlet valve closes at around 100° BTDC the rising piston continues to compress the fresh air/fuel mixture in readiness for ignition and the start of a new cycle.
When you realise that this full cycle happens around 70 times per second at high revolutions you can appreciate that cam timing is a very critical factor when designing an engine. The timing figures are dependant on the camshaft(s) which are ground specifically to suit the type of engine.
As you can imagine many factors effect the factory setting of cam timing such as, the intake and exhaust systems, fuel type used, compression ratio, maximum RPM of the engine, piston and combustion chamber shapes, torque and maximum power conciderations, fuel economy, etc.
MORE TERMS & CONSIDERATIONS
Nearly all cam timing figures rely on having a known clearance between the rockers and the valves which is greater than the normal or Running clearance. This is called a Checking clearance and is normally 1.00mm (40 thousandths of an inch). Timing at checking clearances means that you will have to temporarily change the shim or adjuster on each valve prior to measuring the timing but more on this later.
Some factory timing figures are given at Running clearance even though there is no appreciable gas flow below 0.5mm lift. This is done to provide enhanced figures which make Duration and Overlay appear much longer which is an advertising advantage when buyers think that longer must be better! If no checking clearance figures are given with the factory timing specification then you have to assume that timing is at running clearance. ie normal operating clearances are used.
The number of degrees from when the valve start to open to when it finally closes is called the Duration of the valve timing. Typical factory figures for the bevel drive 900SS are around 320° for both exhaust and inlet valves although these are running clearance figures and so are optimistic!
Another term mentioned above is Valve Overlap. This figure describes the number of degrees where both the inlet and exhaust valves are opened at the same time. The reasons behind this were discussed in the ‘Advanced’ section.
The duration and overlap can be worked out from the factory figures although as you will see later these do not always match the cam that is fitted to the bike! Lets take the bevel 900SS factory figures and do some calculations. Remember all degree figures are for the crankshaft and must be halved if applied to the camshaft.
Note, factory timing at running clearance for this model so figures are enhanced!
The exhaust opens 80° BBDC and closes 58° ATDC. The exhaust valve duration is 80° (To BDC) + 180° (BDC to TDC) + 58° (ATDC) = 318°.
The inlet opens 63° BTDC and closes 83° ABDC. The inlet valve duration is 63° (To TDC) + 180° (TDC to BDC) + 83° (ABDC) = 326°.
The overlap is the period of degrees where both valves are open. For this cam the overlap is 63° (BTDC inlet opens) + 58° (ATDC exhaust closes) = 121°.
The other figures we are interested in is the point of Maximum Lift (ML) of each valve. Normally this will be half way between the opening and closing degree figures if the cam is symmetrical. So work out where the point of ML sould be we divide the duration of each valve in half and then add that figure to the opening degree figure.
The exhaust valve duration is 318° divide by 2 = 159°. The exhaust opens 80° BBDC (= 260° BTDC) plus 159° (Half exhaust duration). Therefore Exhaust Maximum Lift should be at (260°-159°) = 101° BTDC.
The inlet valve duration is 326° divide by 2 = 163°. The inlet opens 63° BTDC plus 163° (Half inlet duration). Therefore Inlet Maximum Lift should be at (163°-63°) = 100° ATDC.
It is common to refer to these cams as ‘101°/100° lobe center cams’ and as you have probably seen there is a symmetry between the ML figures either side of TDC. This is called Lobe Center Symmetry and most engines have the two ML’s at equal distance away from TDC. We will use this symmetry to decide if the cam is advanced or retarded when we calculate the real ML figures.
Figure 2 shows the relationship between the opening, closing and maximum lift of each valve for the factory specifications the 900ss Bevel Engine.

We now come to a problem not unique to Ducati which is that the cams fitted to the engines do not necessarily match the ones specified in the manual! For whatever reason most 900SS bevels were actually fitted with cams with a lobe center symmetry of ‘96°/96°’ which are not quite as good a cam as the ‘101°/100°’ items. Don’t worry about which cams are fitted to your machine as when we measure the ML it will soon become obvious what you have got! The point is don’t assume that the factory figures relate directly to what is acually fitted to your machine!
Figure 3 shows the timing diagram for the 900SS with ‘96°/96°’ cams. Note that the opening and closing figures shown on the diagram assume that these cams have the same duration as the ‘101°/100°’ cams although this may not be the case. As I have no factory figures for the ‘96°/96°’ cams the only known points are the two ML’s.

Next we will find out more about the three different ways of measuring the valve timing, how to find an accurate piston TDC, how to accurately find the point of maximum valve lift and finally how to measure it using a step by step guide with real life examples!