I've recently developed a regular tapping noise that seems to come from near the distributor. Around the same time, the engine is experienceing a very slight miss at part throttle. Now I'm not saying the two things are necessarily related, but having had the dist cap off today, there is quite a lot of endplay in the distributor shaft (ie up and down movement, as it sits in the block) ie a few millimetres or more. Could that looseness be messing around with the advance? Maybe not, but I still want to get rid of the noise.

When I move it, it makes a very similar noise to what I'm hearing with the engine running. Also the tapping is coming at what sounds like half engine speed, leading me to think "distributor" even more.
My question is, what is a "safe" amount of endplay for the shaft, and are the necessary bushes etc available for doing a rebuild?

And yes, I have considered whether it's valve noise, but after much pokiing around it just seems to be coming from the dist, rather than anywhere near the rocker cover.

 

Excessive dist shaft endplay will not produce a "tap".
When the engine is running, the shaft can shift up & down as it meshes with the camshaft gear (causing erattic timing), but I have never seen this shifting so violent that it produces an audible tap.

You're probably hearing a sticky lifter (or other valve train noise) underneath the manifold toward the rear of the engine.

You can get a an inexpensive automotive stethoscope at an autoparts store or wherever and try to narrow it down more. A piece of hose works also, but no where near as good as a stethoscope. I've tried & compared both.

 

:iagree:

 

Hmm, yeah could have been a coincidence that the "miss" came about when the tapping started.....
I'll get hold of a stethoscope (been using a long screwdriver "plugged" into my ear so far!) and have a look around for the source of the tapping.

Even so, can anyone tell me what is a suitable amount of endplay for the dist?? The GM manual is not a very good resource for distributor overhaul procedures.
:cheers:

 

Well, the exact same thing happened last summer on my my '66 327 car. Started making a tapping noise as described. You could tell it was coming from the distributor. It was indeed the fact that there was too much end play on the distibutor main shaft. The (outer) contact on the rotor was actually hitting the top of the cap where each contact for the cap is molded in. It even chipped it in a couple of spots. I swapped in another distributor, and no more noise. I don't remember how much end play there was, but it was sloppy. When I rebuilt and reshimmed it, the instructions said .010 for endplay.
Not sure if this is your problem, and I had never heard of this before, but it can happen!

Dick

 

Dick
Maybe I was onto something after all! Well, before I get too carried away I'll buy the shim kit and sort the endfloat out. It's way, way over ten thou at the moment, more into millimetres than thoushandths of an inch! (I'm hoping the drive gear is ok too). Then hopefully the tap and the miss will simultaneously disappear and we all get a pat on the back!
Mind you, vette ownership often teaches you not to be too optimistic! I'll keep you all posted.
:cheers:

 

I agree with Dick that the endplay can allow the rotor to hit the contacts in the cap.... but I differentiate that noise as a "click" as opposed to a "tap" as heard from a noisy valve train component. I guess it's just a matter of definition here. I have no idea what you actually heard.... a click or a tap.

In any case, if it's loose, it needs to be shimmed.

I have been rebuilding distributors for 35 years and I always shoot for .004 to .006 on the endplay. I consider .010 too loose because the shim(s) tend to "wear in" after a few hundred miles and the shaft endplay loosens up again. This is from experience. If you shim it to .010 and it loosens up to .012 or .015, then you wasted your time. The .006 gives it room to grow and still stay within the .010 limit. It needs .002 to .004 for heat expansion to prevent galling.

This is just for old GM distributors... Ford/Chrysler etc all have their own set of issues based on cam & camshaft orientation. Sometimes there's "up" pressure on the distributor and sometimes there's "down" pressure on the distributor based on design. So... the rules vary based on manufacturer etc.

Edit... I have experienced noise from a distributor when the cap was snapped on crooked.... so check that first. Even though they are keyed etc, they will still go on crooked enough to cause interference between the rotor and the cap.

And.... since you already know the enplay is excessive and you're going to pull the distributor: when you pull it, just push the shaft up and spin it.... you should hear the same "tap". This way you won't be fishing for non-existent valve train noises.




[Modified by Tom454, 9:38 AM 4/20/2004]

 

I must say it was more of a tap than a click, but hey, we're getting into semantics here!
You're right, whether its the cause of the tap or the miss or both, it still needs to be shimmed. I'm putting an order into Ecklers for some other stuff this afternoon, so will add the shim kit to my order.
And cheers for the info on endfloat, I'll aim for 4 to 6 thou..

I'm pretty sure the cap is/was straight, by the way. I'm just hoping the drive gear is in good condition when I pull it out, as the problems always seem to pile up as you dismantle things!
Cheers for all the info :)