Show some Love to your Marelli Ignition Distributor

Marelli S112AX S112BX ignition distibutor used in Porsche air-cooled motors

Today we are going to rebuild a Marelli Ignition Distributor.

The Marelli distributor is used in Porsche 911T carbureted engines from 1969 to 1971.

There are two Marelli part numbers, S112AX and S112BX. The AX is used in non-CDI cars and can be recognized by the condenser. The BX is used in CDI-powered cars and has no condenser.

Two times speed Marelli Ignition Distributor rebuild video

Taking a Marelli Ignition Distributor Apart

We start the disassembly by removing the cap. The cap hold-down clips need to be worked off with a flat blade screwdriver. The clips can be sent to plating and get a black oxide finish.

The rotor is held in place by two screws, these can be tight so use a quality screwdriver that fits the slots.

The Marelli has a primary and a secondary distributor spring that is not the same. Take a note, or photo of your setup when you take off the rotor. Pop the springs off of the posts and work off the weights.

If you have excessive corrosion in your weights or sliding plate, your advance will not be functioning correctly. We have always cleaned and reused these parts. One case where the distributor was in a chemical environment after we rebuilt it came up.

To fix this we used ceramic coating paint on the plate and weights. Do not paint the shaft or cam lobes. This fixed the problem and protected the parts if they were to go back into a toxic environment.

Combating corrosion on a Marelli Distributor
Combating corrosion on a Marelli Distributor

Removing the points

The points in distributors are a wearing item. Points on a non-CDI ignition will wear 10 times faster than a distributor that runs a CDI. This is because on a Kettering or standard ignition car, there is 3 to 4 amps going across the points. On a cdi ignition they are just a trigger signal and have only about .2 amps going across the points.

To remove the points, rotate the shaft till you can access the screws holding the points in place. Remove the points through-block on the side to fully remove the points.

The Marelli Ignition points part number is, 911-602-960-00.

Preparation for cleaning

To remove the distributor drive gear, we drill out the pin. Set up your distributor square to the drill and carefully drill out the pin. The Marelli distributor has a different size pin than the Bosch distributors. The drive pin is 5mm. We use a 4mm drill to remove it. Retain the shims, but you will need a new drive pin.

When you can remove the shaft, you can take the rest of the distributor apart. We wire the parts together for cleaning and polishing. To protect the housing, we plug each end with a rubber cap.

Reassembling the Marelli Ignition Distributor

With all your parts clean and dry, you can start to reassemble the distributor. We use three different lubricants during re-assembly.

High pressure, high temp, heavy grease

Transmission fluid

Bosch Distributor grease, part number 5 700 002 005 for the small tube

Bosch has started making their special grease again, under the new Bosch Classics Line! It is made for the points cam and it has the consistency to stay put. We have not found any other grease that is adequate. It is good to note that some other point sets come with a small blister pack of points cam grease. You can buy the points just for the grease pack!

It is very hard to purchase from Bosch, and maybe the re-released lube has not infiltrated the market fully as yet. The grease was unavailable for a very long time. I found a company with stock.

https://www.vwnos.com/5-700-002-005-2

You do not need much of this grease and a small tube will last you and your friends a long time!

Oiling the Felt

When you re-install the felt this is the part that takes the transmission fluid. We like transmission fluid, but you could also use engine oil to lubricate this felt. In a Marelli they have two pieces of felt; one that presses into the grove in the housing and the other that sits on top under the holding shim. Oil them both.

Press the shim back into the groove with a socket that fits. The bearings are made of a powdered material that is porus, even though it looks solid and the oil will infuse the bearing.

Setting the Distributor Shaft End Play

We use the high temp high-pressure grease when re-installing the shaft in the housing. Some grease at the top under the plate and in the shim that goes under the plate. A small amount of grease is also placed in the top and bottom of the housing.

With the shaft back in the housing replace the shims that came off. Use a 5mm pin to temporarily lockup the drive gear. Use your feeler gauge to check the play. The play should be .15 to .2mm. Adjust the shims till you get the correct play. Different metals heat and expand at different rates, you must have the correct play, or you can damage your engine.

We use a heavy rolled 5mm roll pin to re-pin the drive gear. When the solid pin was added at the factory they had a machine just to press that rivet in place. You can risk damage to the shaft or gear by pounding a solid pin till it peens over enough to hold.

Setting the Points

Kurt has been doing distributors a long time and you may have missed him setting the points in the video. The points have a set gap when they are most open on the cam. For a Marelli ignition distributor, the point gap is between 14-16 thousandths.

Points gap

14 -16 Thousandths

or 0.3556 – 0.4064 mm

Leave the points held down loosely, put the cam on the highest point. Slide the points plate until the gap is open. Put in your feeler gauge and tighten down the points. Spin the cam and check again. The feeler gauge should have some drag but not move the point’s arm.

You can set this up with a feeler gauge, but the final check is the dwell reading in the distributor machine. The dwell should be 40 degrees. You can see that Kurt sets the final points position on the run test. 

Marelli Distributor Points Bounce

The Marelli distributor is a good ignition distributor. They got a bad name because the parts supply ran out. These days there are aftermarket parts, and Porsche genuine parts available for them. One note is the rotor should always be checked as 5000 Ohms resistance. If it does not have this, you will not be running with optimal resistance.

Partsklassik has made a resistor for the coil plug to get 5000 ohms in the right place it you have zero resistance in your rotor.

You may have seen in the video that the Sun Machine lit up extra as the distributor was put through its paces. We have found that points have changed the metal spring over time and are more prone to “Points Bounce”

Waveform showing points bounce
Waveform showing points bounce

To combat this we added a shim to the points on install and check the amount of bounce on the Sun machine. The result of bounce is more firing signals. This is not as bad in a non-CDI car but can lead to problems in a CDI car as it will fire multiple times. You can work the points a bit tension-wise to lessen this, but it may be impossible for you to see without a distributor machine or oscilloscope.

Marelli Ignition Distributor Timing

The carbureted 1969 to 1971 engine can have a Marelli or a Bosch. The Marelli has a great timing curve and has more total advance than the Bosch.

The Marelli Distributor Advance Curve

Servicing your Marelli Ignition Distributor

If you are going to remove your Marelli from the engine, make sure that you have your car on top dead center, cylinder one, before removing your ignition distributor.

If you are not going to do a full-on take apart of your Marelli Ignition Distributor, than you can still service it.

Check your Rotor and cap for wear, the brass can wear out, and the center wiper contact is made of carbon and wears out too. Do the resistance check on your rotor. Check your points, replace or re-gap as needed. Lube your points cam with Bosch grease. Oil the top felt.

You can replace your points with an electronic set. We generally only do this if the points cam has worn out. Electronic points can also have over-firing conditions, and they cost a lot more than regular points!

When reinstalling your ignition distributor, never force it in, never pull it down with the hold-down clamp. Fully seat your solid core ignition wires in the cap in the correct places.

Show a little love to your Marelli!

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