The Dodge Challenger Message Board

On a stock set-up there can be a lot of free play between the camshaft and the distributor/oil pump drive gear.

The gear can also rise up under load from the engine and retard the timing. This is made even worse if the distributor is not bottomed out in the slot on the drive gear. or if the distributor shaft has a lot of end play on it.

The effect of all this is erratic spark timing. You can see this when you use a timing light. The timing mark will tend to jump around a bit. Or it may retard slightly from the expected timing curve when the engine is revved quickly.

To cure this you can fit a collar to the distributor shaft which will hold the drive gear down, thus making ignition timing more accurate and stable.

Step one is to remove endplay on the distributor shaft.

My MSD distributor has a small collar help in place with a pin. I found there was 20 thou endplay on the shaft so I fitted a home made 20 thou aluminium shim between the collar and the bearing.

The distributor still turns freely but now there is no up and down movement in the shaft.

EDIT
Don't do this. It was a mistake.

The screw on the new collar has some thread lock on it to make sure it does not come loose!

I fitted it with 4 thou clearence between the collar and the gear drive.

I found there was still some play between the distributor shaft and the slot in the gear drive.
So to get rid of this I hit the ends of the shaft with a hammer until the shaft was a snug fit in the slot.

I am not sure if 4 thou will be enough cold clearence to allow for expansion when the engine is hot or not. Ideally I would set this to zero clearance with the engine at full operating temp.

I have seen a photograph of this set up with a 10 thou feeler gauge being used. But that may be just to play safe.

Just been thinking. I usually add 2 thou to set valve lash when cold. So 4 thou should leave a couple of thou spare and maybe leave enough room for oiling the drive bush too.
It will probably be OK at that.

Posted a question on Moparts. See what they say.

I hate asking questions on there though. You normally get a hundred different answers.

So how does it run? is it smoother with the timing now?

drewcrane wrote:So how does it run? is it smoother with the timing now?

Not going to put it back together until I am sure about the clearance.
I don't know if it will make enough difference that i would feel. But it will make the timing more accurate.

Drag up a calculator and google the thermal expansion rates if you want to calculate your clearance at operating temp. Should end up pretty close.

Nice work there Dave. I would think .004 would be plenty.

dave-r wrote:On a stock set-up there can be a lot of free play between the camshaft and the distributor/oil pump drive gear.

The gear can also rise up under load from the engine and retard the timing. This is made even worse if the distributor is not bottomed out in the slot on the drive gear. or if the distributor shaft has a lot of end play on it.

Hmmmm.
Maybe I haven't understood how your engine is made, but I assume it has a "normal" camshaft drive ?
The reaction force from the gearmesh forces the intermediate shaft down.
Also, I expect little fluxeration due to the tooth play as the shaft is "tigthened" as it's transmitting torque driving the oil pump.
....I see no need for a collar , nor a need for the other modification either?

I assume the topic of the effect of a cam shaft thrust button has already been discussed.....and the importance of chain wear??.

Am I making an ass of myself ?????

No you are right. Yet the problem still exists? I know a few people now that have fitted a collar like this and their timing has turned rock steady instead of floating around a bit.

Maybe the forces of turning the oil pump drive it up?

I tell you one thing. Before I did this you could hold the rotor and when you twisted it gently you could move it side to side quite a lot. Now I have got it down to just one mm play.

If it does not improve the timing i will let you know. But several people have found it works a treat.

If it does not improve the timing i will let you know. But several people have found it works a treat.

cant wait ,is it an easy install?

You need to remove the intake and the distributor. That is it.

If I get time tonight I will put it back together and fire it up to see if there is any improvement.

Well i put the engine back together last night (took an hour) and tonight i was all set to fire her up and see if there was any effect!

But.

No voltage in the ignition system. It turns over OK but will not fire.

Too cold to think tonight. I will have to investigate another night.

well dave the old addage "it ran before",something got unplugged,and workin in the cold stops those synapses from working properly

johannes wrote:The reaction force from the gearmesh forces the intermediate shaft down.

Got an answer for you on that. It is the momentum/weight of the oil in the pump that causes lateral movement of the oil pump shaft (acel/decel).

dave-r wrote:

johannes wrote:The reaction force from the gearmesh forces the intermediate shaft down.

Got an answer for you on that. It is the momentum/weight of the oil in the pump that causes lateral movement of the oil pump shaft (acel/decel).

Hmmmm. Must admit I'm a bit sceptical to your thesis.
Do you mean sideways (out of centre) = latheral movement of the shaft ?
....if so, change the bushing.....

If you mean torsional vibration caused by the spring effect of the steel shaft, and/or the oil itself,.....remember oil is the perfect damper, and even if the shaft is thin,...it's also short, thus relatively stiff towards twisting.

If you mean vertical movement of the shaft, you've lost me...

Chain slack and missing cam shaft thrust button are my main candidates...
Please keep in mind, that the rotating ignition mark is on a damper, thus the outer part (containing the ingnition mark) will to some degree "twist back and forth" relativ the crank....... I have NO numeric values for this movement/twisting,...so they migth be neglectable.......

Hey,....I just want to understand your way of thinking,...not causing troubble....

The pump drive rises and falls vertically. This alters timing. You have to stop the vertical movement. As I already explained this is caused by the inertia of the oil in the pump system.
This is a known and has been discussed on many forums.

There can also be some rotational play due to the end of the distributor being a loose fit in the slot on the top of the oil pump drive. You have to remove that too and make sure that the distributor engages fully in that slot to prevent damage to the two parts.

Since starting this thread I have found that that racers have been doing this modification for some time without telling the rest of us.

Cam thrust button? Not needed on a normal big block unless you are using a roller camshaft is it?

RESULTS

First of all I found that shimming the distributor does not work. It had the effect of locking out the timing in the distributor. So I had to remove the shim I fitted there.

After starting again I found the timing is very stable now.

At anything above idle the timing light shows the timing mark completely rock steady. It made adjusting the timing much easier.

At idle the timing mark still moved around a tiny bit in the light but much less than it used to before. Again it was easier to read the timing mark because it was much more steady.

So a good result.

Glad to hear it! Thanks for posting this Dave! I am going to do the same with my engine project. I think you may have found out how much it does raise/expand to the bottom of the shaft collar you installed. So without the shim there, how much clearance from the top of the intermediate shaft to the bottom of the shaft collar do you reckon is there?

There is 20 thou up/down play on my distributor. Maybe if I had pushed the shaft up (instead of pulling it down by inserting a shim) and pulled the collar further down it would be better? But it is OK now anyway as it is. I don't think improving it any further would be an advantage. You can read the timing clearly now and it hardly flickers at all at idle. Rock steady over 1500rpm.

To recap.
The idea is to place the collar on the distributor shaft in such a way as to prevent any vertical movement in the oil pump drive which has the effect of retarding the timing or making it erratic.

But you must also make sure the end of the distributor is a good fit in the slot on the top of the drive gear. Make sure it is well engaged (it need not go all the way to the bottom of the slot if you have the collar) and a snug fit so there is no rotational play.

Sorry this has not been a very scientific test. But rest assured (on my engine at least) it had an effect.

I am going to use a Milodon Bronze I.S. with standard width oil pump. http://www.mcmaster.com/#shaft-collars/=5cejzf I am looking at the two piece clamp on---My only concern is the soft bronze material possibly 'riding' up on the lower dizzy shaft and the collar acting like a lathe on the soft metal The .020 clearance must be the 'ticket',, scientific or not it is a good test of what works and what doesnt. Thanks

I would get the clearence down less than that Eddie. If I could be bothered to take the intake manifold off again I would get it down to less than 10 thou.

dave-r wrote:I would get the clearence down less than that Eddie. If I could be bothered to take the intake manifold off again I would get it down to less than 10 thou.

Sounds good Dave! I'm still waiting for my Bronze I.S. / roller lifters/ Cam Button/ RollMaster timing set. I'l post pics as soon as I get setup and the parts in.

Dave, what's the shaft diameter on the MSD (I assume 8546), and what collar did you use? Did you machine a recess for the screw or did it bite enough into the shaft to hold (not that it might go anywhere if it gets loose)

fbernard wrote:Dave, what's the shaft diameter on the MSD (I assume 8546), and what collar did you use? Did you machine a recess for the screw or did it bite enough into the shaft to hold (not that it might go anywhere if it gets loose)

Don't know.

I was given the collar by a friend and he had a few made for him by his local machine shop. Another friend of mine is thinking of making some too.
It was a tight fit in the distributor shaft and I had to give the inside of the collar a rub with some rolled up abrasive paper until it was loose enough to move up and down the shaft easily with one hand.

The end of the grub screw was concave (rather than convex) so it is a better fit on the curve of the shaft surface.

I put a high strength thread/stud lock liquid on the screw before fitting it.

My mate that can make them says he can use grub screws with a serated end which really lock in place. But a good dab of thread locking compound is always the safe bet.

I'd rather use a pin, like MSD does with the upper collar (the split-tube-like pin, I don't know how you call these in English). I'll look into this. I have an aluminum bar somewhere I bought when I needed to make spacers, there should be some left.