Hi Scotti
I have a antique 78 dodge Aspen with a slant 6. I inherited this car when my dad died. I used it until 2011 on a regular basis. It has 78K original miles. Now it's a weekend toy. I babied the car which is probably why it is still on the road. One problem that it always had is the distributor caps over time develop cracks and in humid or rainy weather it backfires and runs ruff. In the old days you could by a slant 6 distributor cap at a Channel home center. I would buy them 5 at a time and keep them in the trunk. Both me and my wife became adept at swapping them out in grocery store parking lots. Now these caps are getting harder to get and more expensive. Typically they last about a year. Is there something that can be done to extend the life of the existing caps like a spray sealant? The car is only 43 years old.
By the way, a 43 year old car is considered "Vintage", not antique.
I stand corrected.
I'll take a wild stab.
heat makes plastic more brittle. Maybe covering the cap with insulation would help it last a little longer.
You can't prevent UV damage from the arcs though.
There is something mechanical causing the caps to break. I understand cheap junk, but this goes beyond that. I still think maybe some tabs aren't lining up when he installs the cap and is cracking it when it's installed.
yep or maybe overtightened
My 2 cents? For around $150 you can get a electronic ignition conversion kit, which will give you better performance and fewer headaches.
https://www.google.com/search?client=firefox-b-1-d&q=electronic+ignition+conversion+for+slant+six
I had a slant six Dart (70) that I drove for 200k miles, and I think I only replaced the distributor cap once. Plugs and points and condensers, more often, but the distributor caps should last you years, not weeks.
A 1978 model has factory electronic ignition. Chrysler started with that in 1973 or 1974 as I recall.
There must be a reason the caps keep breaking. If there is wear and play in the distributor shaft, the rotor could be hitting and breaking the cap. Check the distributor shaft for looseness and wear and consider a new distributor. Also, check to see that it is seated properly before tightening it down.
Good point, but given the poor quality of many new replacement parts these days, if the shaft is loose it might be better to replace the worn bushing if feasible in that distributor, or send the whole thing out to be rebuilt.
This is not from miss alignment or rotor interference. Hairline cracks form and in humid weather these cracks allow sparks to jump to the wrong cylinder or be rerouted to ground. I found this out but running the car at night in total darkness. You can see these little bolts of lightning across the body of the distributor.
Unfortunately it's very hard to get quality ignition parts these days for older vehicles. Aluminum contacts are typically used instead of brass in caps, and quality control is poor, rotor-to-cap gaps are all over the place. This all makes for unpredictable ignition performance. An exception might be performance-oriented ignition parts (MSD, Accel) but I don't think they make caps for the slant-six. All I can suggest is source the best quality cap and rotor you can find and favor brass over cheap aluminum contacts. As far as wet weather problems, WD-40 can help, the old trick is to spray it on cap and wires, also inside the cap if needed, to drive moisture away.
Here is a discussion of the problem in a slant-six forum from a few years back:
https://www.slantsix.org/forum/viewtopic.php?f=19&t=60552
try hemmings motor news for nos parts
Hard to find? Rock Auto is showing 8 distributor caps on its website for your vehicle.
Good ones are hard to find. At least he has electronic ignition, most of the point sets available today are garbage.
Car has electronic ignition. Tried the Rock Auto and the JCwhitney. Made in China garbage. One was delivered already broken. Another had a misslocated center pin which did not line up with the rotor.
Unfortunately the only decent stuff out there seems to be for the performance crowd. If you had a small-block V8 you'd have your choice but I've checked in places like Jegs and Summit Racing and I cannot find anything for six-cylinder Chrysler distributors.
The slant-six was used in marine applications through 1991, I wonder if marine-grade cap and rotor would be better made?