Is there good literature on how Bosch Distributors work?

[jwakil]

I'm tackling one part of the car which I've always been afraid of, ignition and timing. Is there a good tech article or book which explains the inner workings of the Bosch distributors? Haven't found anything which explains clearly and adequately.

[James Brown]

Problem is that Bosch hates people doing just that, they want a certified service center to perform that. Even parts are hard to come by.

[Bob Kontak]

I think you will be way ahead if you send it off to somebody like Jerry Woods or similar for a rebuild. These places can rebuild them and change your mechanical timing curves to match your engine/application if need be.

[HarryD]

Hi,

Here are three web pages to get you started. Use a search engine like google and look for others.

How Does My Ignition System Work - Coil, Distributor and Cap, Rotor, Spark Plugs, Wires

HowStuffWorks "How Automobile Ignition Systems Work"

Ignition system - Wikipedia, the free encyclopedia

[911pcars]

With the exception of perhaps vacuum retard in emission-equipped vehicles and the method of signaling the ignition system (points or reluctor), the Bosch distributor works like most any other distributor (pre-84). The references listed by HarryD should work.

Sherwood

[Bob Kontak]

jwakil - Are you looking for info like HarryD posted or a specific exploded view of yours or similar distributor with technical info? You used the word "tackling" so I assumed you were going to roll up your sleeves and tear it down.

[mpetry]

Couple years ago I had the opportunity to spend an afternoon with Don Marx in his shop in Oregon as he ran thru some distributor curves. Don's no longer with us, however he worked with Gary Emory at Part Obsolete and his specialty was better than new carbs and distributors. All the Emory racecars - 4 and 6 cyl - have Don's stuff in them. His mods included a revised advance curve that was somewhat steeper than the original Bosch curve, to get more torque in the 2500-3500 rpm range, using pump gas.

The Bosch distributor shaft has a cam (see pics below) and little arms that act on that cam, rotating the advance plate as said arms are pushed away from the shaft by centrifugal force as engine RPM increases. By revising the shape of that cam, and carefully selecting the springs, you can control how much advance is applied at a given RPM. The Sun distributor machine reads advance in degrees as the rpm is varied.

It was an interesting day and I learned a lot from Don. He was a bit gruff but if you sat there and listened, the knowledge just came pouring out. Thanks Don, you are missed!

[Bill Verburg]

Originally Posted by jwakil

I'm tackling one part of the car which I've always been afraid of, ignition and timing. Is there a good tech article or book which explains the inner workings of the Bosch distributors? Haven't found anything which explains clearly and adequately.

distributors, points and timing are second nature to those of us of an age. The first car I owned w/o points was a '78 IH Scout.

to change points you need
timing light
feeler gauge set(a dwell tach is nice to have too)
a small flat blade screwdriver set, 2 will be used at a time
replacement point set
replacement condenser, not absolutely necessary but might as well swap it out
a small dollop of lube for the cam

here's a blow up of a typical point type dist, this is a 4 but a 6 is the same except for the am and the cap


Hopefully you have a Bosch, much easier to work on


I like to remove the dist from the car and put it in a bench vise, it makes it much easier to work on

to remove the distributor note(photograph or write down) the orientation of a cylinder terminal on the cap, most chose #1 as it is the cylinder just beneath the dist(rear of car, L),

remove the cap, note(photograph or write down) the orientation of the rotor(some will bump the engine to TDC on compression for #1 but that is unnecessary as long as you do not rotate the engine w/ the dist. out) the rotor only goes one way as it is keyed. but remember to know the direction that the rotor points before removing the dist and do not rotate the engine w/ the dist out)

remove the dist, place in a bench vice padded by a shop cloth, remove the rotor

the points are held in be a single screw at position P w/ an arrow in the diagram above, unplug the wires and remove the point set. This is a good time to check for the free movement of the advance weights, they pivot at one end and are restrained from swing out by the small springs, be sure they move freely, lightly lube as necessary

if the dist is in a bench vise it's easy to clean the cam and lightly lube it by rotating the dist from the bottom

replace the new points in the same place as the old do not fasten all the way just lightly held down is fine, reconnect wiring, rotate the dist cam so that the point rubbing block is on a high point, use the second screwdriver to pry the points to the desired gap(usually 0.015 to 0.017in) I'd rather be high than low. tighten down the screw holding the point set in place, rotate the cam from below and note the opening and closing of the points, remeasure to be sure.

here's a pic of a 911 point set, 1 piece , note the rubbing block and adjustment slot



replace the distributor in the car w/ the rotor pointing in the same direction as when it was removed

replace the cap, it is keyed and can only go in one position, be sure it is the same position as before.

double check you work,

engine should start fine, double check your work w/ a dwell tach, w/ this you want to be on the low side of spec which is usually 40*+/-3*(ie, i'd rather be at 37 than 43)

check timing w/ a timing light, for most models it will be near TDC or a little after w/ the dist vac connected

I'm sure I forgot something but it's really not a big deal

[mpetry]

Great post and diagram Bill !

Dribble a little oil down the center of the distributor shaft ?

[jwakil]

Thanks for all the great info. This will certainly get me started. I'm not that afraid of replacing the points myself now, especially after reading Bill's and HarryD's info.

[Neel]

Bill, don't be tellin' on us 'old' guys.. Great post, BTW!! Other than points, condenser, cap and rotor, 'messing' with your distributor should be left to the pros.. A distributor machine, springs, shims, bushings, reamers etc are necessary to start and then the 'unknowns' begin. FI, carbs, street, track~you get the idea.. I built many a distributor for hot rods, race cars, street cars, boats etc.. The wrong setup will lead you to a very expensive pile of metal very quickly.. (Please don't ask how I know!!)

[RWebb]

2 add'l tidbits:

1. that lube is a special high temperature grease - Bosch makes it (I bought a tube in 1971 and have only used about half of it...)

2. Warren says to tear the thing down & re lube all the internals every decade or so; you will want to saturate the felt over & over until it cannot take any more 30 wt. engine oil every year


if anyone has a question about the advance, send it in to get that changed on a Sun machine to one of the usual suspects

[RWebb]

oh - check the cap and its button periodically and replace every few years

[Neel]

+1 on the Sun machine .. I started out working on a very old~ then~ POS allen machine. Progressed to a Sun 500 and then to a Sun 504 like the one picture in an earlier post...

[DRACO A5OG]

The inner plastic like bearing where the shaft spins is impossible to replace, a Bosch Factory part only. when that goes the Distributor is shot and must be replaced.

To check, remove from motor and spine the rotor, if it resonates noise i.e.vibrates, it is toast.

Dropping oil underneath the felt pad is a must, I try to do it every oil change.

[jwakil]

I'm just planning on replacing cap (has a big crack on one side), points, and rotor, and maybe clean what I can without taking everything apart. Not planning on doing anything else.

[DRACO A5OG]

Just make sure you drop some engine oil down underneath the felt cap which is under the rotor.

If you have a DVM, check the ohms on the rotor.

[HarryD]

Originally Posted by jwakil

I'm just planning on replacing cap (has a big crack on one side), points, and rotor, and maybe clean what I can without taking everything apart. Not planning on doing anything else.

Just go slow and you will be fine.

The screw that hold the points in place is very small. I like to use a pair of tweezers to get it positioned and then carefully start it with a small screwdriver.

When you switch you cap, I like to do it one wire at a time so I do not put the wires in the wrong place. As Bill already noted, there is a key in the distributor cap so you can verify the orientation of the cap. Use some dielectirc grease in the boots to help keep water out of the joint between the wires and the cap itself. You can get it at any autoparts store.

Be sure you get some distributor grease (local auto parts store or here: Pelican Parts - Product Information: 5-700-002-005). It is specifically made for distributors and as Randy already said, a tube will last you a lifetime.

Do not forget to pull the little felt plug out of the center of the distributor shaft and add some light oil.

[RWebb]

oh yeh - everybody should carry a business card with them - slide it in to clean the points in an emergency

- amazing how this topic is starting to trigger all those old memory traces in my brain...

[Bob Kontak]

Let's say you found the exact replacement parts and flawlessly rebuilt the distributor. Should it be put on a machine to be sure you are in the operating range like folks that put on new carbs/turbos get their car dynoed?

Edit - Like Neel says - after I worked on my old SC distributor, I have a very nice piece of metal in the basement I can practice on.