My car stalled in the pits while at the track this weekend. When I starting taking things apart to figure out the cause I was very surprised to find that the oil pump shaft was sheared at the distributor end.

Turns out that the advance tabs in my Mallory Unilite distributor broke off and jammed the mechanism causing the distributor to seize.

Anyone seen this before?

This scares me DEEPLY

Weird, the spindle actually turns the distributor, I wonder if it is simply poor quality mainland china steel in that spindle? Do you know if it was a Nissan spindle or after market?

The only other contributors I can think of are an excessive rotating mass in the distributor or some binding of the bushings in the distributor or some mechanical interference inside the distributor as the weights sling.

It is strange how the broken piece is twisted. It is as if it only made contact at the very top.

ahhha I re-read and saw the tab cause... cancel my guesses

Edited January 29, 201312 yr by Blue

Blue, I could be wrong but I think the advance tabs inside the distributor broke off and locked it up which caused the shaft to break. It was difficult to turn the distributor rotor by hand after I removed it from the car until I took the distributor apart and removed the broken advance tabs. It now turns easily.

Wow... I've not seen that happen. How old or new was that distributor? I've had the same unilite in my 72 with an L28 for..30+ years.

I'd sure contact Mallory about it - especially if it was a newer distributor.. they may have a materials or process problem to address.

FWIW,

Carl B.

I spoke to Mallory. They said that they have not heard of issues like this with the advance tabs. The distributor is out of warrantee so if I want it fixed I will have to ship it to them and pay for it to be refurbished. I think I will go a different direction.

I spoke to Mallory. They said that they have not heard of issues like this with the advance tabs. The distributor is out of warrantee so if I want it fixed I will have to ship it to them and pay for it to be refurbished. I think I will go a different direction.

The perfect time for Megajolt!!!

I considered moving to an electronic ignition but Dave Rebello talked me out of it. He convinced me to go with the magnetic pickup distributor out of a 1975-6 280Z to trigger the MSD 6AL that I already have in the car. Dave considers this to be a very reliable option for racing applications. It also has the advantage of simplicity and period correctness. I picked one up on ebay, with a mount, for $60 shipped. Electronic has the advantage of tunability and timing stability. On a street car this is important but for a track car I only really care about performance between 4000-7500 RPM at WOT. Because the ignition is fully advanced under these condtions all that really matters is the stability of the timing and spark intensity. The stability of the stock 1976 distributor is to be determined but the MSD box should deliver lots of spark.

I'm sitting here trying to imagine the scenario that caused this failure. I just can't believe BOTH advance tabs would shear off at the same time due to a manufacturing defect. One, sure. It just feels like a an instantaneous high-G event that gave the advance plate a real snap/whack to shear them both off. And because the shaft is geared and driven by the crank, I don't believe that a dizzy jamming would cause the oil pump drive tang to shear. That jam cannot get to the oil pump drive end without getting past the engagment with the crankshaft worm gear.

I've experienced an oil pump failure that sheared that drive shaft tang off, where its internal pressure relief valve failed allowing the pressure to get too high. What gave was the drive shaft tang. Now if that's what happened to you, though just for an instant, that very violent event may also have resulted in enough snap torque to shear the dizzy tabs.

Just don't want this happen to you again with a new dizzy. I'm really feeling like its the oil pump that caused the shaft tab to break off. Look at the amount of twist on that tang! No way a dizzy jam could have caused it without much more internal damage to the dizzy itself and at LEAST the gear on the shaft getting spun.

Just think this through and maybe replace the oil pump or service the relief valve and look for damage from something getting momentarily stuck in there.

Edited January 30, 201312 yr by zKars

I agree that there are some significant forces at play to twist the shaft tab and I would like to find the root cause and avoid it in the future.

I could be wrong but I don't see how the oil pump could be involved as there is no damage to the oil pump end of the shaft. Unless I am missing something, if the oil pump froze it should show damage on the oil pump end not the distributor end. I also have a low oil pressure siren that goes off if my oil pressure drops below 20psi. If I had an intermittent oil pump issue I would expect to hear the siren go off while driving but I have not.

I suspect that the advancement tabs fell off some time ago. I have been hearing an intermittent rattling near the front of the engine at idle but I was never able to find the cause. I bet the tabs were rattling around in the bottom of the distributor for the past few months until they finally jammed the distributor shaft.

Don't forget metal fatigue as a possible cause for the tabs breaking. Fatigue doesn't take much force just many many cycles. Could be that the tabs broke off one a time. Since they are advance limiters, you might have had really high advance for a while before they jammed the shaft.

BUT it is hard to see how the tabs could get in to a spot to jam the rotating shaft. Jamming the advance mechanism would just lock your timing advance in to one spot. Did you find the tabs actually wedged in somewhere? Seems like they would have done more damage if they were the cause and not an effect.

The shaft locking up from galling or some other bearing/bushing type failure might have stopped things so suddenly that inertia broke the advance tabs. Just a possibility.

Edited January 30, 201312 yr by Zed Head

Makes sense that the tabs have been off for some time. I think its a red herring. If the oil pump failed in the way I'm thinking, ie stuck relief valve, the pressure would spike HIGH, never triggering your alarm.

The thing is I can't get past these two points.

First the shaft engagement point of the crank drive gear is pretty substantial. And its between the dizzy and oil pump. How does a shaft jammed at the top, with the crank still turning, not destroy/bend/twist the shaft, gear or dizzy? Nothing appears wrong with the dizzy other than the tabs are gone. If those tabs got jammed in somewhere I'de expect some very noticable damage on something in the dizzy.

And second, if the oil pump is "fine" and is free to turn, and you lock its drive spindle, is the momentum of the pump and oil enough to twist and shear the drive tab? Doesn't feel right. If the pump is free to spin, then nothing the crank or shaft can do other than stopping dead at 8K RPM would cause that type of damage. I think.

And you say the car stalled in the pits. What caused that if the shaft can still turn the dizzy and its still in time (advance tabs being there or not at idle wouldn't matter) unless the gear on the spindle spun on the shaft during/just before the tang twisted off? This is easy to check. Compare the gear position on the busted one to a new one.

If it IS spun, it doesn't prove which end of the shaft was "locked" at the point, but without any dizzy damage, and with the force required to cause that twist and shear, I'm strongly suspecting the oil pump end is the culprit.

I'm sure you'll figure it out.

I might have this backwards, and it doesn't help that the picture of the end of the dist. drive shaft is upside down but isn't the twist backwards for the scenario where the distributor stops and the pump keeps turning? Shaft turning counterclockwise, broken end pointed up. It looks more like the pump stopped and the distributor kept going.

Maybe I'm looking at the wrong end of the shaft, or just thinking backward. I haven't actually had one out.

Is it possible that the twist is old damage that finally fatigued and broke?