Normal amount of play in distributor shaft...?

[agfours]

I'm working through troubleshooting a noise at higher rpms, (diesel truck sound at +3k RPMs) and trying to figure out if this dizzy is having an issue in timing advance.

In a 3.2 dizzy, with Motronic, what is (or is there?) a normal amount of play in the distributor shaft in the up and down movement? (vertical play)

My dizzy shaft can be pulled up by hand about 1/8 to 3/16" - i'm thinking that if this can be raised, the force between the heilical gears at high speed might be causing the shaft to raise and allowing the position of the rotor to fall back slightly in its rotation/timing, to the point where the mechanical advance / motronic is not allowing full advance...? Make sense or not?

The mechanical advance weights seem to be operating ok - I can turn the shaft to activate the weights with a few ounces of by-hand-generated torque.

[Steve@Rennsport]

Hi Paul,

While 3/16" of axial play isn't good for the cap & rotor at all, the timing isn't affected on a Motronic engine. All timing functions are done in the ECU; the "advance mechanism" inside the distributor simply provide rotor phasing to ensure the rotor is properly lined up with the proper contact of the cap at the moment of firing.

Your distributor should be reshimmed as needed using the proper Bosch shims made for this express purpose.

You can see if the noise you hear is coming from the distributor by using a long screwdriver as a stethoscope and listening to it while holding the engine at 3K RPM.

[agfours]

Thanks Steve. I appreciate the response and will try to localize the sound as mentioned.

[Eagledriver]

There are two sources of up and down slop on your distributor. The lower shaft that has the helical gear on it and the upper shaft that rotates from the centrifugal weights and springs. You are probably just feeling the upper shaft move. It's held down by the rotor cap anyway so not to worry. Like Steve says, the distributor doesn't affect the timing anyway.

-Andy

[Tidybuoy]

Is the noise louder when the car is cold?

I recently read a post where this was diagnosed as a loose or broken head stud. The person who started the post showed pictures of the top of the engine where you could see a lot of oil/dirt accumulated at the base of the cylinders.

[agfours]

Just dropped it by my trusted local Porsche mechanic... good news is that it is not a broken head stud or anything serious. He believes it is pre-ignition, likely due to the chip advancing timing too far coupled with the probability that it it running lean. Will be replacing with stock chip to troubleshoot further.

[Ferrino]

Hi - I'm seeking clarification on the "vertical distributor shaft play" that is commonly discussed here. When I grab the tip of the shaft (near the felt), I can move the shaft up and down perhaps 1/8" (also a Motronic 3.2 dizzy). Someone mentioned that this is different from play at the lower end of the shaft (where the gear lies). So is 1/8" of play at the upper shaft normal, or just "nothing to worry about"? Thanks!

[Steve@Rennsport]

Originally Posted by Ferrino

Hi - I'm seeking clarification on the "vertical distributor shaft play" that is commonly discussed here. When I grab the tip of the shaft (near the felt), I can move the shaft up and down perhaps 1/8" (also a Motronic 3.2 dizzy). Someone mentioned that this is different from play at the lower end of the shaft (where the gear lies). So is 1/8" of play at the upper shaft normal, or just "nothing to worry about"? Thanks!

Hi,

The shaft is a one-piece unit so any play can be felt at either end.

Bosch uses special shims at the bottom, just above the drive gear to set the end-play and these can exhibit wear over time. The proper way to set this is by removing the drive gear and installing shims of the correct thickness so that you have about 2.0-2.1mm of movement. The problem is that Bosch doesn't sell those shims so one either needs to make them or salvage some from another distributor to get the end-play you need.

I would just keep a sharp eye on the distributor cap for wear, When or if you start seeing excessive wear on the contacts, that will tell you its time to rebuild it.

[Ferrino]

Thanks Steve. I am going to check the old cap tonight - how would the contacts on the cap be affected by axial play of the shaft (and rotor)?

[john walker's workshop]

************ sells the shim kits and bushings. one thing that happens when the upper shims go away is the rotating plate wants to machine the housing below it. if that happens, you need to use a lathe and take off just enough to smooth that area so the thick oilite bronze bushing in the kit will last a long time. then shim the gear end and you're done, except if it has radial play and needs new bushings.

[MConn]

I got the vertical slop out of my Motronic distributors by installing a new bronze washer under the advance mechanism. On one distributor, the washer was completely gone, and the washer was paper-thin on another one. Heres a picture of the old one and the new one.



I found a replacement here -

washer

I'm not sure if the original was bronze, but the replacement parts fit , and the distributor spins free without vertical slop. I went ahead and got new new advance springs from the same place.

Mike

[DRACO A5OG]

Try hi-temp stainless steel shims from McMaster Carr.

I am sorry to write but if you have axial play, it means the internal bearings are shot and there is no replacement available unless you build one. ask me how I know.

Best best is to find a known good one and test it. if you spin it and there is some resistance you found a winner. keep it lubed and it will last you.

If spins too freely, go to the next one.

Adding the shims will help but it will eventually wobble again go sloppy.

my 2 cents,

Jim

[Ferrino]

Jim - wouldn't radial play indicate shot bearings/bushings and axial play indicate worn/disintegrated shims? Mine just has axial play - you can lift the shaft up and down relative to the body, but you can't "wiggle' it side to side.

[DRACO A5OG]

I stand corrected, yes, per your diagram it would be radial. Sorry about that in that case you are good to go, just replace the shims with SS shims and it should last you awhile.

[MConn]

Ferrino,
You have to remove the gear to replace the bronze washer. I took a dremel tool and ground off the peened end of the pin holding the gear, then knocked it out with a punch. The gear came off pretty easy after that with a small gear puller. My gear and shaft had marks on them for alignment - make sure you put the gear back on in the same position it came off or you timing may be off. The bronze washer goes between the advance mechanism plate and the upper housing. Once i installed the new bushing, the original shim washers at the gear fit pretty good to eliminate the vertical slop, but since the shim kit comes with extra shims, mixed and matched them to get the feel I wanted. The kit comes with a new drive pin for the gear, it was hollow like a roll pin instead of solid like the original, so I flared the ends after installing it.








There is a great thread somewhere here about this.

Mike

[Ferrino]

Thanks very much Mike!

I removed the dizzy last night and it's clear that all the play felt at the rotor is coming from the small upper shaft/cam that it presses onto, and not the main shaft (there is negligible movement between the gear and the dizzy body).

If you peek into the small hole where the felt lives, you can see the upper shaft moving relative to the main shaft. Do I even need to worry about this, given the dizzy cap will hold the rotor down relative to the main shaft? Or can I insert an additional washer underneath that tiny "C-clip" that holds the upper shaft onto the main shaft?

I actually overestimated the total vertical play and it comes to around 1mm/0.04".

[MConn]

0.040" sounds about like the thickness of the bronze washer. Once that is worn away, the aluminum housing is next.

Originally Posted by john walker's workshop

...one thing that happens when the upper shims go away is the rotating plate wants to machine the housing below it. if that happens, you need to use a lathe and take off just enough to smooth that area...

I'm not sure about the stuff under the felt, I just put a few drops on the felt - I think that's the only lubrication for the bronze washer.

I bought the bushing for the housing (radial) but didn't need to replace it, but I did clean the Archimedes grove on the shaft. Another proof that the 911 oil system is AWESOME - 3000 engine RPM x .5 distributor RPMs = 1500 @ 1 mile/minute x 100,000 miles = 150 million distributor revolutions

Mike

[Ferrino]

Originally Posted by MConn

0.040" sounds about like the thickness of the bronze washer. Once that is worn away, the aluminum housing is next.

Thanks, but it sounds like my washers are intact if almost all of the play is in the upper shaft and not the main shaft? To me it seems that the destructive play is when the main shaft develops play and can smash against the dizzy cap. In short, I think I am (as usual) worrying about nothing as the dizzy cap will hold the rotor in place. Actually, my condition seems to match what Eagledriver discussed:

There are two sources of up and down slop on your distributor. The lower shaft that has the helical gear on it and the upper shaft that rotates from the centrifugal weights and springs. You are probably just feeling the upper shaft move. It's held down by the rotor cap anyway so not to worry.

[Ferrino]

OK, did some more measuring. Of the 1mm of vertical play I see at the rotor, only 0.15mm/0.006" is from the main shaft and the rest is from the upper shaft/cam. There seems to be a lot of debate on what the spec is for the lash on the main shaft, but I'm guessing 0.006" is OK?

[Ferrino]

I spoke to a dizzy rebuilder and they clarified that 6-thou is fine for main-shaft lash and that the play I am seeing at the rotor-shaft is from the circlip used to secure it to the main-shaft. The other style of fastening is with a screw, which is a better system. I am just going to disassemble the weights, clean and re-lube.