Rotax SCP2 Bosch Points
Ignition System Timing

Contents ...

Introduction ...

Equipment Required ...

Table of Timing Settings ...

Setting the Timing ...

Checking the Pole Shoe Gap ...

Introduction ...

Setting the timing of the Bosch Points Ignition system is a relatively straightforward process. If you have just replaced the points then you should set them initially and then check them again within a few hours as the cam followers wear quickly initially until they get polished in against the crankshaft. Once this initial polishing in of new points / contact breakers has happened the ignition should stay in time for many hours if all components are in good shape. If you are re-setting some existing points that have served some hours then examine the surface of the points contacts to ensure there is no significant pitting / burning. If there is consider replacing the points and testing the relevant condenser to ensure it is working correctly. Setting the timing requires access to the stator plate mounting bolts and points via the holes in the magneto flywheel. To gain access to the flywheel you will have to remove the rewind starter and the starter pulley. The magneto housing itself need not be removed. If you need to replace the points, condensers or stator plate you will need to remove the flywheel. If you are just setting the timing then the flywheel can stay put.


Equipment Required ...

You will require a screw driver, set of feeler gauges capable of measuring a points gap of 0.3mm, 0.35mm, 0.4mm, 0.45mm (0.012" through to 0.018"), an Ohmmeter, and a Dial Guage with a 14mmm spark plug adapter.


Table of Timing Settings ...

Please see below the tables of settings relevant to setting the ignition timing for the more popular Rotax aircraft engines fitted with Bosch ignition ...


Rotax 447 ...


Points Gap (mm)  0.35mm - 0.45mm 
Points Gap (inches)  0.014" - 0.018" 
Timing BTDC (degrees)  18° 
Timing BTDC (mm)  1.83mm - 2.33mm 
Timing BTDC (inches)  0.072" - 0.092" 
Pole Shoe Gap (mm)  8mm - 17mm 
Pole Shoe Gap (inches)  0.3" - 0.69" 

Rotax 462 ...


Points Gap (mm)  0.3mm - 0.4mm 
Points Gap (inches)  0.012" - 0.016" 
Timing BTDC (degrees)  18° 
Timing BTDC (mm)  1.86mm 
Timing BTDC (inches)  0.073" - 0.092" 
Pole Shoe Gap (mm)  13mm - 17mm 
Pole Shoe Gap (inches)  0.51" - 0.67" 

Rotax 503 ...


Points Gap (mm)  0.35mm - 0.45mm 
Points Gap (inches)  0.014" - 0.018" 
Timing BTDC (degrees)  18° 
Timing BTDC (mm)  1.83mm - 2.33mm 
Timing BTDC (inches)  0.072" - 0.092" 
Pole Shoe Gap (mm)  8mm - 17mm 
Pole Shoe Gap (inches)  0.3" - 0.69" 

Rotax 532 ...


Points Gap (mm)  0.35mm - 0.4mm 
Points Gap (inches)  0.012" - 0.016" 
Timing BTDC (degrees)  18° 
Timing BTDC (mm)  1.96mm 
Timing BTDC (inches)  0.077" 
Pole Shoe Gap (mm)  13mm - 17mm 
Pole Shoe Gap (inches)  0.51" - 0.67" 

Setting the Timing ...


All Rotax 2-stroke engines are intended to fire at exactly 18° BTDC (Before Top Dead Centre) so at exactly 18° BTDC we want the points to open for the piston approaching TDC. There are various different ways to go about setting components up for this to happen and this is just one of them. By way of example we are going to go through the process of doing this on a Rotax 447 engine.


There are basically 4 steps to setting the timing ...


  • 1. Centering the stator plate.
  • 2. Initial approximate setting of the points gap.
  • 3. Checking the timing with a dial guage.
  • 4. Checking / Setting the Pole Shoe Gap.

1. Centering the stator plate.

We make a start by centering the stator plate on the magneto housing ...



Rotax Bosch Points Ignition Stator Plate mounted fully clockwise

Stator Plate Fully Clockwise

Here the stator plate assembly can be seen mounted fully clockwise. Note the potential for adjustment indicated by gap highlighted by the red arrows.

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Rotax Bosch Points Ignition Stator Plate mounted fully clockwise

Stator Plate Fully Counter Clockwise

Here the stator plate assembly can be seen mounted fully counter clockwise. Note the potential for adjustment indicated by gap highlighted by the red arrows. We want to start off with the stator plate in the centre of it's adjustment range and adjust it from there if needs be ...

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Rotax Bosch Points Ignition Stator Plate Assembly Centred

Stator Plate Centered

Slacken the two stator plate mounting bolts and move the stator late to a postion where there is a reasonably equal gap either side of the mounting bolts. Here the stator plate assembly can be seen mounted in the centre of it's adjustment range.

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Tighten the two stator plate mounting / adjustment allen bolts so that it reliably stays centred with as equal amount of adjustment either side as you can e.g. aim to set the gaps of available adjustment A and B to be about the same.


2. Initial approximate setting of the points gap.

Consulting the table of timing settings for the Rotax 447 above we can see that the outside limits for adjustment of the points gap should be 0.35mm - 0.45mm. Set the points gap on BOTH sets of points to the centre of this range e.g. 0.4mm.


So now you your stator plate should be centered and your points should be set to something close to the middle of their range of acceptable settings for your particular engine. Note that the points gap can change as you tighten them so ensure that you check them once you have adjusted and re-tightened them. Your timing shouldn't be a long way out now but being a two stroke engine timing is very important and we may need to fine tune it to ensure that it is as close to 18° BTDC as possible to avoid any premature or retarded timing problems.


3. Checking the timing with a dial guage.

If the flywheel isn't already mounted on the crankshaft put it on now taking care to ensure that the woodruff key is in good condition and the corresponding groove on the magneto / flywheel is clean, clear of debris and in good condition. Having locked the crank with the Rotax 2-stroke crank locking pin, replace the magneto washer and nut and tighten the magneto nut to the correct torque. Remove the crank locking pin. Now rotate the magneto flywheel in the normal direction of rotation which is clockwise as you face the flywheel until mag piston comes up to TDC (top dead centre). You can check this by looking through spark plug hole. Insert your dial guage mounted in it's 14mm spark plug adapter into mag side spark plug thread and tighten finger tight. Find TDC exactly using dial gauge and zero the dial guage.

SkyDrive CCBCT - Cyclone Conrod Bearing Clearance Tester

Dial Gauge

Here is a suitable dial guage mounted in it's 14mm spark plug adapter ready for insertion into the spark plug hole of the magneto side cylinder. This one is the dial guage that comes with the SkyDrive CCBCT (Cyclone Conrod Bearing Clearance Tester). Ignore the tubing and syringe, they serve no purpose here but come with the bearing tester and are only shown here as it is more convenient to leave them in place than remove them!

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Cyclone Conrod Bearing Clearance Tester in a Rotax 447 mag cylinder

And here is the dial guage plus adapter again mounted in a Rotax 447 with MAG cylinder at TDC.

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Rotax Aircraft Engines Bosch Points Ignition Stator Plate Assembly

Rotax 447 with MAG cylinder at TDC

And here is the magneto flywheel of our Rotax 447 with MAG cylinder at TDC. Note that arrow A points to the ignition point alignment marker on the magneto housing. Arrow B points to the alignment marker on the flywheel. These two markers should line up exactly with each other at the exact time the points open and the ignition is supposed to fire e.g. at exactly 18 ° BTDC.

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So having set both stator plate assembly and points gap in their center of their respective adjustment ranges we need to verify exactly when the points are opening for each cylinder. Before we start it is helpful to know ...


MAG Cylinder Timing

MAG Cylinder Timing is controlled by the upper points set.

The lower ignition coil creates the spark for the MAG cylinder.

The SOLID BLUE wire connects the lower ignition coil to the MAG cylinder points.


PTO Cylinder Timing

PTO Cylinder Timing is controlled by the lower points set.

The upper ignition coil creates the spark for the PTO cylinder.

The BLUE WITH RED STRIPE wire connects the upper ignition coil to the PTO cylinder points.


We will check the timing of the MAG cylinder first.


setting the MAG Cylinder Timing on a Rotax 447

MAG Cylinder Timing ...

To verify with your Ohmmeter when the points for this cylinder are opening connect one probe to earth e.g. the magneto housing. Connect the other probe to the SOLID BLUE wire coming from the MAG points going to the lower ignition coil primary winding. This is accessible by pulling back the rubber boot (arrow A) from the double lead wire that connects to the ignition coil. Or you can connect your probe to the connector for the dampening box (arrow B) having disconnected the dampening box first. Either connection will do. To electrically isolate both ignition coils you will need to temporarily remove the earthing connections (arrow C) and (arrow D).

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Rotax Bosch Points Ignition System Circuit Diagram

Now your Ohmmeter should be measuring the resistance between the two contacts of the points for the MAG cylinder. If the points are closed this resistance should be zero (for short circuit) and when they are open (referring to the circuit diagram) you can see that the resistance measured should be that of the primary winding of the ignition coil e.g circa 2.0 - 2.5 ohms if your MAG ignition coil is in good health.

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Now rotate the flywheel counter clockwise approximately 20 - 30° so that the alignment mark on the flywheel is to the left of the alignment mark on the magneto housing. The points at this point should be closed indicated by your test meter as a measurement of zero ohms. Very slowly rotate the flywheel clockwise until the resistance changes to 2.0 - 2.5 Ohms. This indicates the exact moment your points have opened and a spark would be generated if your engine was turning over at cranking speed. If your ignition timing is set correctly then the alignment mark on the magneto housing and the alignment mark on the flywheel should line up opposite each other like this ...

Rotax 447 Bosch timing is set correctly

When your timing is set correctly ...

The alignment lines should look like this. The line on the flywheel is lined up with the arrow on the magneto housing.

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Bosch Rotax 447 timing is set early

If your timing is set early ...

The alignment lines might look like this. Note the line on the flywheel is counter clockwise from the arrow on the magneto housing. If the resistance on your Ohmmeter changes when the flywheel arrow is to the left of the magneto arrow then your spark plug will fire too early e.g. your engine will be pre-igniting.

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Bosch Rotax 447 timing is set too late

If your timing is set late ...

The alignment lines might look like this. Note the line on the flywheel is clockwise past the arrow on the magneto housing. If the resistance on your Ohmmeter changes when the flywheel arrow is to the right of the magneto arrow then your spark plug will fire too late e.g. your engine will be retarted.

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If the Ohmmeter indicates that your ignition timing is either set too early or too late then slacken off the stator plate mounting allen bolts and rotate the stator plate until the Ohmmeter see a resistance change at the exact point the alignment marks on magneto housing and flywheel line up. If your ignition is too early rotate the stator plate clockwise. If you ignition is too late rotate the stator plate counter clockwise. Test the timing for both cylinder again. If the points gaps are exactly the same for both sets of points then the timing for both cylinders should be the same. If they differ then you can position the stator plate in the best position for one cylinder and adjust the points gap on the points for the other cylinder.


If the timing is too early on a particular cylinder then decrease the points gap toward the lower limit of the adjustability range and then check the timing again.


If the timing is too late on a particular cylinder then increase the points gap toward the upper limit of the adjustability range and then check the timing again.


Checking the Pole Shoe Gap ...

Once you are satisfied that the timing is correct for both cylinders then you must check the Pole shoe Gap.


Rotax Aircraft Engines Bosch Points Ignition Stator Plate Assembly

What is the Pole Shoe Gap? ...

When the alignment marks on the magneto housing and flywheel are aligned and the ignition timing is correct the last check that must be done is to check the distance between the pole shoe and the magneto magnet as per this diagram. Consult the timing tables per engine (above) to check what the acceptable range of values are for your particular engine. The pole shoe ranges seem to vary depending on which manual you consult but you can use the results above for guidance.

If the pole shoe gap is out of tolerance / range the flywheel may have turned on the crankshaft. In this case remove the flywheel and inspect the taper, slot and woodruff key. If they are damaged repair / replace as necessary.

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Once the timing for both cylinders has been set, you have finished setting your timing and you can reconnect all wires and re-assemble the engine.


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