Technical What the hell happened?

Drove the roadster last week and ran perfectly all the way home and into the garage, no issues at all...no odd sounds, stumbling, NOTHING to rouse any suspicions of any kind.

Decided to pull the hood off this morning to some cleaning of the engine department, and found the dizzy cap busted wide open. It was violent enough that the broken section of cap slammed up against the firewall hard enough to make some major league scratches in the paint. The broken piece was laying on top of the transmission bell housing.

The rotor hold down screws were both real loose and both indicating tits were broke off and stuck in the weight assembly. I can see no damage inside the cap and all 8 brass tits are flawless in shape and condition as well as the rotor as viewed from the top side. There is a burn spot inside the cap which I circled, but thats the only issue I can see.

Now I had a similar thing happen 30 years ago to a 318 that instantly started running real shitty and before I could get it pulled off the road it threw a rod out the side of the block...found that the distributor had some slop in it allowing the rotor to move up and down and had caught one of the distributor tits which broke the plastic that held the rotor pointer allowing it to spin about 45 degrees retarded, but the distributor cap was damaged at one of the tits and it was painfully obvious what had happened.

But, back to my current issue, I can see no damage that may have caused this on this engine. I replaced the cap and rotor and it fired right up as usual and runs flawlessly. This engine has less than 5000 miles and the distributor was brand new. There is no noticeable up and down movement in the dizzy shaft...

I am a little perplexed as to what happened here. Ran perfectly before and after so this had to happen right as I turned the engine off last time....any ideas?

68 4 bolt main 350 sbc.

 

Bad engine mount?

 

Damfino. Subscribed.

 

See if there is a gas smell in the oil. A friend had gas in the oil and it ended up blowing all the sheet metal off the engine. It looks like an internal explosion to me.

 

Was the cap still in place with one clamp? Wondering if it was running like that.

 

How close is it to the firewall as mentioned earlier? Really looks interference related..
An clearly it would still run in that condition

 

its close to the firewall, but clears by about 1/2"....no gas in oil...the clamp was still attached but you could move the cap around very easily, which makes me think it had to happen just as I shut it off last time, but heard and felt nothing...weird

 

I had a rotor on one of them come loose and destroy the cap...but I was at the top end of the dragstrip at the time. The screws were loose, one had come out all the way, the other was still engaged a few threads.

I think I just didn't tighten it up all the way.

You have a good mystery there. I like it.

 

The leading edge of the rotor looks like may have been contacting the poles, possibly from the loose screws, and also perhaps hard enough to break the cap.

 

I wonder if the burn spot is the cause, coincidental, or damage.

Where's the other piece?

 

was there more than one piece broken off? my guess is a small piece broke, from age, defect or mishandling, and got between the rotor and cap and popped the big section off. I bet it was running like that for a little while. put it back on and video it running.

 

Blowby my experience in a situation like your's was that when the rotor contacted the contacts in the cap it broke one of the plastic holders that hold brass contact in place. Your's does not seem to indicate that, it could really just be a defective cap that was cracked in that are all along. It would run fine with a crack there as opposed to between contacts. It is certainly a mystery.

Pat

 

Ron, from the pictures I can see the marks at the inner cap points were the rotor hit them. This happens a lot on later model gm products especially vans. the screws come loose and wipe out the cap, rotor and some times take the top plate right of the distributor. Why it ran till it was parked is a unknown....

 

I'm having a hard time believing the paint scratches were caused by a sudden impact.

You might try contacting the manufacturer.

 

Are there any witness marks on the edge of the rotor?
There appears to be a few witness marks on the inside of the cap at the same level.

Does that rusty orange mark, that is inside the cap and on the edge of the broken section, also show up on the edge of the broken piece?

When installing a cap rotor they should be of the same brand/type. Tolerances can get sloppy or tight if mix/matching brands.
Had an OT HEI MSD rotor that wouldn't work with an HEI ACCEL cap, or vice versa. Rotor was physically smacking the inside towers. Could hear it on cranking. Was being super lazy and didn't want to change the cap(wire/coil/ground strap/blah blah blah).

Does that cap have any vent holes?
Does your engine have an active PCV system or just a vent?

Jags were known to blow up their caps from fuel fumes.
There have been a few stories related to Chevys blowing their caps at track events. Saw one, it didn't have any kind of PCV or active ventilation system, just a pair of vents on a valve cover. Cap was blown to pieces, rotor was more conical than disk shape. Bit hard to tell what had happened.

Being the cap still had about 225° of foot coverage, the weight of the plug wires, and the single clamp were probably heavy enough to keep the partial cap attached. You may have been driving it like that since the time before. Fresh air is a great insulator, to a point.

 

I've seen something similar this happen when one of the cap latches wasn't latched properly. If not properly latched the spring tension on the opposite latch can tip other the cap off the distributer and....wack goes the rotor.

 

well...i suspect the loose rotor was in fact the problem and if the cap was cracked, it wouldn't take much...I only found 1 piece that had broke...the piece with the access window is gone...it did damage the firewall, i think the metal window actually dug into the firewall. that section of the firewall has a small layer of bodyfiller and it dug thru the paint, bodyfiller and down to the metal. ive touched up the firewall, replaced everything and seems all is well....takin it out tonite, so may find out...hopefully not

 

hey....you may have something here

 

Maybe that's impact damage, not a burn mark.

 

I have had the same thing happen as Doublepumper said. That makes the most sense. Did you happen to
rev it up, like a lot of guys do, just as you turned off the key? Anyway, take it for a spin, bring alone extra parts and a screwdriver and see what happens. Let us know!

 

What I learned is to make sure the screws are tight.

Sent from my Trimline

 

Anyone know why someone would do that?

 

I had something similar happen, but I tracked it down to the distributor hitting the firewall under load. Problem went away when I swapped to a smaller diameter distributor. I was actually able to buy my truck cheaper that way because the previous owner couldn’t figure out the lead had been knocked off the rotor.

 

I'm going with this. Maybe a transmission mount issue, too?

 

I was thinking like others tranny and/or engine mounts might be worth checking too.

 

I'm with the group that thinks it's an "interference with the firewall" crowd. It's also the most likely reason there was damage evident on the firewall.

Loose screws may just be a coincidence, most of us have been accused of having at least one loose.

 

THIS
Ben

 

Evidence of firing up instead of out( to the side ) excessive distributor shaft end play ??

 

Put a new cap on and try again. Lippy