Modified ignition system in 1970 impala

Greetings all, it’s my first post here. I’ve had my 1970 impala for going on 10 years now. It has a 400 small block engine. At some point before my ownership the distributor cap and related parts were upgraded to circa 1975 vintage.

This car has been incredibly reliable about starting even after sitting for long periods, with one exception. The distributor cap gets little burn marks on the metal contacts inside and I’ve had to change them out often. I never had this trouble with a '76 impala I owned.

My first question is, does anyone know if there is something I can do to make the distributor caps last longer?

My second question is that when I go to the parts store to buy new plugs and wires, should I get 1970 or 1975 models? At autozone, their computer listed different models for 70/75.

My third question is, what gap should the spark plugs be set to have.

I humbly thank anyone who has any expert advice or insight into this matter.



In 1975, GM came out with HEI. This replaced the old points ignition and was/is a VAST improvement. Few ignitions can equal it. HEI stands for High Energy Ignition. Gap your plugs at .045. Plug wires must be for a 1975 or later HEI system.

Minor burn marks on the cap contacts are normal over time. If the marks appear to be excessive then that is usually caused by faulty plug wires or spark plugs that are misfiring due to age or the gap being too wide.

You should get the HEI (75) wires as these are likely to be 8MM and the 1970 wires are probably 7MM. The latter would not fit the cap correctly.

I agree with Caddyman about this era of HEI ignition. It’s simple, pretty much bullet-proof, and I’ve done an HEI conversion to every older GM car I’ve ever owned.

If your system was upgraded to HEI, you should check to be sure it was done correctly. Many times people just slapped it in drove. But there’s one thing that must be done during the conversion for it to work properly. HEI must have the full 12 volts on it and many times they are installed without replacing the resistor wire that fed the original distributor. That voltage drop will cause problems.

thanks fella’s for all the great information, I believe the modifications were done by competent people, all sorts of work had been done when I bought it, I guess my last question is do I need plugs specific to 70 or 75? thanks

I get the impression from your reply that you trust they did it right and are not going to verify it. It only takes 10 seconds to measure the voltage at the HEI connector and put this question to bed. You know what they say about ASSUME? :wink: My boss also has a poster in his office that is appropriate: In GOD we trust, all others bring data. If I misread your intentions, I apologize.

The plugs should be appropriate to the engine and ignition system set up. You state there has been a lot of work done to the car. I’d suggest you gather as much of that information as possible if you haven’t already. To select plugs and appropriate gaps on modified engines, the more information you have the better like; ignition type/voltage, compression ratio, cam specs, mechanical timing setup etc. Of course, you could just throw in a set of OEM plugs set to .045" (HEI equipped) and check them periodically to see how they are performing.

The 1970 engine probably takes a RJ12Y Champion, a short plug with a gasket. Starting in 1971, Chevy changed to a RBL13Y which is a longer plug 5/8 hex, tapered seat (no gasket). In 1975, with HEI, it was RBL12Y6, a wide gap (.060) plug. In 1976, an RBL15Y4, (.045 gap)

So, do the plugs in your engine have a 13/16 hex and a gasket, or a 5/8 hex and a tapered seat?

I will double check, but I’m pretty sure it’s the larger size and with a washer that squashes when you install them. A former mechanic had installed Bosch plugs that he determined were good for this particular car. (I’ll pull one out and get the model #)

The car is running well, and I’m planning to put on new wires/plugs just for good measure. I’m just wanting to do what’s best for this situation and hoping to make the distributor cap last longer.

The distributor could be installed so that as the spark adcances the rotor is in the gap ahead of the cap contact when the coil fires at higher RPMs. This causes a great deal of arcing and erodes the cap and rotor contacts while blistering the cap and rotor plastic.

It’s not that I wouldn’t like to verify the HEI conversion was done correctly. I just don’t know where I would measure this voltage. I do have a basic meter so if I knew where to measure, it would be nice to know. But just so you know, the car has always run very well for me except for these distributor caps failing.

There is a cylindrical part connected to the control module and there is a set of wires leading from that which plug into the distributor cap. Are those the wires I should measure for voltage? Do I have to be cranking the car to measure?

Regarding the plugs, the engine has been rebuilt, and I do have paperwork from that. I remember that the cylinder walls have been bored “30 over.” But I don’t know more than that for sure. It still has a two barrel carburator so, it hasn’t been totally enhanced. I’m not sure if any other performance upgrades were done during the rebuild. I have some paper work from when it was done. I will look at it. Otherwise, I don’t know how figure out the things you listed (ignition type/voltage, compression ratio, cam specs, mechanical timing setup etc. ) I did not realize it was so complicated to figure out which spark plugs would be best!

Anything’s possible, but the plastic in the cap/rotor have never looked bad, only the contacts are pitted with dark soot around the erosion. Also the cap wiggles if it’s not aligned, so it seems slightly “fool proof.” haha

I just recently got the car out after leaving it set for 2 whole years. It wouldn’t start and I went thrugh this cap/rotor business again. (This time the contacts in the cap had a white oxidization around each pitted area, I assume it was the burnt areas changing from sitting so long.) The only other thing besides the cap/rotor issue was once I had to replace the control module to get it to start. So I tried taking that off when the new cap/rotor didn’t get it to start. I deced maybe there was just a dirty connection so I jiggled the connections and just put it back together without actually replacing the control module. The point of this story is that now I have to wonder if the times replacing the cap/rotor could have jiggled a loose connection and replacing them wasn’t necessary. However, every time this happens the contacts do look burnt with a pit in each one.

Not sure if any of that is useful to reaching a conclusion…

How long do the caps actually last before needing replacement?

Have you checked the distributor for wear? Lateral looseness on the shaft that drives the rotor? If your ignition timing wanders or jumps around, that can be a sign of distributor wear also.

At less than $100 for the distributor, plus another $100 for cap, rotor and wires, throw the kitchen at it and fire it up. If you are determined to make it an “as needed repair” you should consider ohming the pickup coil. It is testable and can be replaced with basic tools. Also, when the cap was changed could the dog legged grounding bar have been left out?

trixonian here near the great lakes taking a spare rotor was a must on a service calls on these cars. The HEI would always burn through the centre and it was not always visable. If you live in a moist area carry a spare

Definitely 13/16 hex and a gasket (metal washer)

The model is Bosch “super” R6 962
(There’s also WR9FC stamped into the metal part of the plug)

I’ve seen a few in which the rotors were burned through but that’s not because of a bad design. It’s caused by going a long time with with an engine performance problem; usually an ignition miss due to plugs and/or wires.

I’ve had the car since '97 and put just under 24,000 miles. I’ve put on 4 or 5 distributors/rotors and changed the control module 1 time. So the distributor/rotors seem to average 4 or 5 thousand miles. It’s not the end of the world because this is just a fun car, but it seems like they should last longer than that.

I’ve been getting distributor/rotors for about $30 for the pair from Autozone. I’m sure it’s not OEM equipment but they have brass contacts. I’ll have to research how to ohm the coil.

So far I have not forgotten to keep transferring over the dog legged grounding bar as I go through these caps. So far… :slight_smile:

You mention that you have had to change the distributor cap often, How often is often? You don’t mention any performance issues such as hard starting or misfiring. Are you changing the cap just because you don’t like the look of it? If that’s the case ,buy a new cap and keep it in the car along with a screwdriver to change it with but wait for a performance issue to change it.I had a 72 Impala and never got more than a year out of a cap. It was two cars in one, my first Chevy and my last.