I stumbled onto this mystery while checking the ignition timing. The symptoms that led me here were a series of very hard starts (at Corvair College, for example), and finally when I switched to the points system on runup and the engine quit!

But it was clear that something wasn't quite right inside my distributor. This mess appeared as though it might have been due to some collision of rotor and wear on the carbon center post in the cap, but there was no evidence of that anywhere...just the messy goop, which had the consistency of valve grinding compound. And the small shiny metallic particles sure looked like aluminum to me. Upon inspection, the points were bridged over with this goop, which explains why that "backup" circuit didn't work now.

Removal of the point plate left no doubt there was a problem, but it still wasn't clear exactly what was going on.

OK, now it's obvious...the centrifugal advance weights were being allowed to extend out too far, slowly machining a groove into the aluminum distributor housing. This also explains a lot about recent anomalies with my engine performance...certainly the hard starting and higher incidence of detonation on hot summer takeoffs after heat soaking (while refueling on a long cross-country, for example). The hard starting was intermittent, and was caused by metal chips gumming up the works and interfering with the spring return of the weights, so after shutdown the distributor was still advanced a good bit (if not completely), which left the timing too far advanced for starting. The detonation was due to the weights' increased travel rotating the rotor further than intended, advancing the ignition to the point of detonation. Fortunately the EIS (Grand Rapids Engine Information System) helps me recognize detonation almost immediately by setting off alarms whenever the CHT rises past my set point of 425F.

Given the forces on the weights when they are hurled outward, you can see that the slots would wear in the direction that they are slotted. And since those holes (now slots) are so close to the controlling contour of the cam assembly, a little slotting here makes a big difference in how far out the weights can go. The rotor is turning clockwise, so the leading edge of the flyweights are digging into the aluminum even better than if that were "rubbing" in the other direction. I don't think the contouring of the weights or the cam assembly is a factor, given that they are not a perfect match, and therefore are not the product of one wearing against the other.

You can even see how deep the groove had become by the wear marks on the outer edge of the weights. The displaced metal also contributed to "sticking" of the weights in the advanced position. I checked the "green" 1965 Corvair Chassis Shop Manual and the only mention of maintenance for these two pivot points is on page 6Y-19 to "inspect the governor weights for wear or burrs and free fit on their pins". Well, I've got burrs alright, and no problem with a "free fit" on the pins! Bob Helt's "The Classic Corvair" discusses two potential problems with Corvair distributors, including point plate pivot wear and what happens if you put the weights in upside down (it retards rather than advances), but no mention of the weights becoming slotted. So perhaps this is fairly rare stuff I see here.

David Langsather, the man behind Dale's Harmonic Balancers and rebuilt distributors (and custom violins), looked at the photos above and made the following observations:

There were two issues here. The first is the wear of the advance weight pivot pins: Both parts are hardened steel but if one part is harder than the other, the softer part will wear; in this case the pivot pin was the softer (remember that there is no lubrication at this point unless someone puts a drop on oil here once a year). This occurs over a long period of time. Eventually the advance weight will wear completely through the pin and the weight will be come a free object with disastrous results to soon follow. The wear problem described is fairly common but yours is an extreme example.

The second problem occurs when the distributor rotor is pulled off and sticks. This pulls the point cam upwards and bends the advance weight retainer plate which also is designed to keep down the point cam. When this bends enough the springs pull the point cam sideways and on to the top of the advance weights which then allow the advance weights to pivot out and contact the inside of the distributor housing with results you have documented. I have actually seen several distributors where this action cut the distributor housing completely in two, and many other examples where there was a groove machined inside the housing (as in your case).

One should carefully examine the weight retainer plate to see if it has been bent in the past. The top surface should be perfectly flat; straighten if necessary. Also be sure the rotor lifts off the point cam with no or little effort to prevent problems. Looking at this distributor, I would suspect that there is a significant amount of high frequency vibration being sent to the distributor. The thing that can do this is bad gear interface between the crankshaft distributor drive gear and the distributor drive gear. Look for uneven wear or cupping on the distributor gear and uneven teeth thickness on the brass crankshaft gear that drives the distributor.

As you know, this gear drives on only one side so if you reverse this gear on the crankshaft, the other side is unworn and like new! When this gear wears, the teeth thin but not evenly around, so the distributor ends up speeding up and slowing down each rotation of the crankshaft. A combination of these two gear wear problems can create destructive vibration to the distributor; also affecting engine performance.

So apparently pulling a stuck rotor off is all it takes to "uncage" the weights, which can lead to the "self-machining" distributor housing! The cure sounds easy enough...just pull the point plate and cage out, bend the weight plate back, and slide the weights back into their captured position and then reinstall the cage and plate. But given the photo above that shows everything located in its proper place, yet the weights are still machining the housing, so uncaged isn't the problem. And my pins were worn somewhat, but the pin holes in the weights were really worn into a slot.

In an effort to understand the cause of the worn slot and the method of how it happened, I took a stock distributor apart to take a look. It is shown with the springs removed and the weights extended outwards to their maximum extent. As you can see, the weights don't start out slotted, and the weights don't extend anywhere near the housing, so mine wore that way over a period of years, as I have about 500 hours on my distributor. But why the rapid wear? Closer examination of the weights in my distributor shows displaced metal around those slots, as if the metal was soft and somewhat "plastic". But how do you find out how soft? A Rockwell hardness tester would give a pretty good indication, and fortunately I have access to one where I work.

The results were amazing. I tested three OEM GM weights (on the left, above) and got 30, 36, and 68 "Rockwell C" readings on them. Then I tested one of my weights, and the reading was completely off the scale by a large margin...certainly down on the B scale, which is in the aluminum and brass territory of hardness. You'd have to see the machine work to know what I mean, but it was considerably weaker than the GM weights. You can see from the photo above that the diamond cone barely dented the GM weight, but the black weight on the right has at least four times the displaced metal in the dent, and even shows displacement in the form of raised metal around the indentation, similar to a volcano. In fact, the samples exhibited some visible "creep" after the cone was dropped on the sample, slowly penetrating deeper over a period of ten seconds or so. Clearly, this is a problem. All three of the "dark" weights yielded the same result.

It turns out that the darker weights (which resemble gun-bluing) are aftermarket weights that come with the aftermarket "high performance" distributor advance spring kits sold for small block Chevys. William has been using these weights in some of his distributors, since they are "free" with the spring kits. They are distinguished by their darker almost black appearance, as opposed to the more nickel-like appearance of the GM weights. William provided these weights for the Rockwell test, in the interest of getting to the bottom of the problem. Fortunately, he didn't install many of these weights, he's settled on stock weights and says there are not that many of the darker aftermarket weights out in the field. We may only be talking 5-10 sets of them in all the distributors he's rebuilt for CorvAircraft. Given that this took a long time to manifest itself, perhaps it'll be sufficient to just keep an eye out for "valve grinding compound" inside your distributor cap, and try a few twists of the rotor to make sure its operation is smooth whenever you're in there. Any sign of metal chips should be your clue to investigate further. Next time you get a chance, take a look inside your distributor cap and see if your weights are a much darker shade of metal than the surround points plate and other steel in that area, although it may be difficult to see them under the point plate. If they are not, you probably have no concerns. If they are, keep an eye on them for slotting. Another indicator is the igntion timing as checked by a timing light for full advance. If the distributor is not advancing smoothly throughout the RPM rang, this would be another clue.

OK, now that I've thrown this out there, let me say this doesn't need to become another round in the internet debate over distributors vs crank-fired ignition, or anything else. They both have their downsides. I recently flew down to visit William and Grace Ellen, and while there we discussed the distributor issue, checked some others, and took a good look at the weight situation. I've also mentioned this to Joe Horton, and his dark weights show some slotting also, but didn't get to the point of gouging out the distributor housing. William has volunteered to swap out his weights for him ASAP. Something else William is willing to do is inspect and repair any distributor that has been FLYING for 100 hours or more. This keeps him from spending the next two months checking distributors...he has other parts to make and orders to fill, and SNF is just around the corner. Spreading out the inspections and repairs minimizes the impact, while remedying the problem. Once the 100 hour distributors are checked, the rest could be checked. If you're not close to flying though, I'd wait until it's more convenient for him though.

The soft weight problem is slow to manifest itself, and is limited to some of the black weights sold as "high performance" parts with the advance spring kit. Because the advance is limited by the hole in the cam plate, the distributor advance doesn't grow, even in the extreme case like mine, so ignition timing really isn't affected. If you find a problem with slotting, remove the distributor and send it to William. Any attempt to fix it and reinstall the point plate, especially if it is a "late" model plate with the Crane ignition module mounted to it, because the alignment between point plate and distributor housing is best calibrated on a distributor advance machine.

One or two folks will be quick to say that William somehow doesn't know what he's doing, but I wouldn't call it a mistake to put brand new "high performance" parts into his distributors. Let's cut him some slack on this, and consider how we'd like to handle it if we were in his shoes. He's planning a sort of "service advisory" on these distributors, and plans to post something in the next 10 days or so on the subject...

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