308 GT4 1979 - Aligning distributor Magneti Marelli SM805A with Camshaft | FerrariChat

308 GT4 1979 - Aligning distributor Magneti Marelli SM805A with Camshaft

Discussion in '308/328' started by B3Cabrio, Jun 29, 2021.

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  1. B3Cabrio

    B3Cabrio Rookie

    May 10, 2021
    9
    FRANCE
    Full Name:
    Emmanuel RdC
    Hello everyone,

    I am calling for some help of the community. I am new here as I just bought my Dino 308 GT4 (from Oct 1979) about two weeks ago. Chassis # is 14992.
    I had to dismantle the ignition distributor (Magneti Marelli SM805A) because the outer bearing was totally destroyed.

    [​IMG]
    I have the 308 GT4 workshop manual but there is no information related with electronic ignition.
    So, I have not been able to identify how to set/align the distributor axis with the camshaft. Also, being French sorry if my english is not perfect.

    It is only once I succeeded to open it I discovered that I had the electronic ignition, meaning without contacts.

    [​IMG]

    [​IMG]

    As I have hard time to find maintenance information about the magnetic pulse electronic ignition with distributor Magneti Marelli SM805A, as tutorials, DIY etc

    So I have several questions for you. I hope that someone will be able to answer or at least to point me to the right place to find the information (forum, documentation, etc)
    I have found that post ( https://www.ferrarichat.com/forum/threads/rebuilding-the-308-electronic-ignition-sm805a-distributor-marelli-aei200-ignition.435679/ ) but not sure to fully
    understand all the discussions.

    1/ What is the method to make the correct alignment between the distributor shaft and the camshat ?

    [​IMG]

    When I removed the distributor (single one) from the cylinder head, I realized that it is simply a 22 groove shaft without any location notch.
    So the distributor shaft can be introduced in the camshaft in any of the 22 positions. I was hoping that it could be 20 grooves because the 4 tips Distributor Reluctor Ring passing in front the impulse unit would have created a uniform 18degrees increment. But as there are 22 grooves, it makes a unique solution (out of 22, with 16.36° increment).
    And as there is no contact, I cannot easily check the opening time with a simple bulb light. How to measure the signal of magnetic pulse (mainly as static) ?

    I am making a little tool to correctly apply the 0.3mm distance between the sensors and and the tips of the
    Distributor Reluctor Ring

    2/ Where are the marks (on flywheel, pulley, etc) to make the alignments with the stroboscopic lamp ?

    Thank you in advance for your answers. Emmanuel
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    NoGoSlow likes this.
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,799
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #3 Steve Magnusson, Jun 29, 2021
    Last edited: Jun 29, 2021
    To add a little more clarity to the text in those referenced posts, set the crankshaft to the AF6 mark near PM1-4 (or using the PM1-4 mark is usually close enough) on the flywheel when both cyl #1 valves are fully closed (i.e., the end of the compression stroke for cyl #1 when cyl #1 is ready to fire). Then when you install the distributor to the cam, pick the spline position that most closely aligns the distributor rotor end serving cyl #1 with the "red notch" on the distributor housing when the dist housing mounting holes are centered in their slots:

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    It would not hurt to ensure that the red notch on the distributor housing corresponds to the cyl #1 contact inside the distributor cap (that is the usual Ferrari convention).

    On the edge of the flywheel, but it is useful to make a larger corresponding paint or nail polish mark(s) on the clutch pressure plate housing for better visibility. Here is an example of a carbed US 308 that has marks at 7 deg BTDC, PM1-4, and 3 deg ATDC:

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    And a more readable example of the PM1-4 mark with the pointer set exactly to the PM1-4 mark (from some other model/version 308):

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    Your OM indicates that you should have similar marks at 6 deg BTDC and PM1-4 (but don't know if there are others).
     
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  3. B3Cabrio

    B3Cabrio Rookie

    May 10, 2021
    9
    FRANCE
    Full Name:
    Emmanuel RdC
    Hello Steve,

    Thank you so much for your detailed explanations.
    The red little notch in distributor housing that must be aligned with cylinder #1 at PM was the key information that I was looking for.
    That's so easy when you have the information.

    Regarding the marks on the flywheel, I had found it on my own last night (not that difficult).
    [​IMG]
    PS: your engine is Sooo clean !

    Now, I would like to share the technique that I used to make sure that I have exactly (and even for both sides) 0.3mm between the magnets and the tips of the rotor.
    I have made a centering tool of 30,6mm diameter that goes into the bearing. The rotor is 30mm so, I just added 2 x 0,3mm to get 33,6mm. I just have to put the magnets in contact of the centering tool then tight the bolts.
    [​IMG]
    [​IMG]

    Thanks again. I should be able to start the engine tonight without any difficulty !
     

    Attached Files:

  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,799
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    Thanks for the timing mark photo -- I'll add that to my collection.
     
  5. Dr Tommy Cosgrove

    Dr Tommy Cosgrove Three Time F1 World Champ
    Owner Rossa Subscribed

    May 4, 2001
    36,431
    Birmingham, AL
    Full Name:
    Tommy
    Your English is pretty damn good
     
  6. B3Cabrio

    B3Cabrio Rookie

    May 10, 2021
    9
    FRANCE
    Full Name:
    Emmanuel RdC
    Ooops, I realized that I made a typo:

    Now, I would like to share the technique that I used to make sure that I have exactly (and even for both sides) 0.3mm between the magnets and the tips of the rotor.
    I have made a centering tool of 33,6mm diameter that goes into the bearing. The rotor is 33mm so, I just added 2 x 0,3mm to get 33,6mm. I just have to put the magnets in contact of the centering tool then tight the bolts.

    Here are the dimensions that you would need

    [​IMG]
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  7. B3Cabrio

    B3Cabrio Rookie

    May 10, 2021
    9
    FRANCE
    Full Name:
    Emmanuel RdC
    Haha, Thanks. I lived in USA, Michigan for almost 7 years (1995-2001) and I practice my "FrEnglish" :p every day for my job
     

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