HEI Ignition swap on Pontiac/GMC
By Bill Hanlon, edited by Larry Gorden
March 2006
Editors note: Although these instructions are for replacing the 57 GMC distributor with HEI ignition, they apply to any of the early year Pontiac or GMC V8 engines.
I installed an HEI distributor in my 57 GMC V8. Below I describe how I made the switch and solved minor problems.
HEI Distributor Installed in 1957 GMC Engine
The HEI puts out a much hotter spark that the standard 57 ignition. It also doesn't have points that wear (changing dwell and timing) and burn (reducing current flow and therefore amount of spark).
The GMC V8 from 55.2 through 59 is basically a detuned Pontiac V8 from the same year. Truck engine sizes were 288, 316, 347 and 336 cubic inches. Pontiac car engine sizes were 287, 316, 347, 370 and 389 cubic inches. The distributor used on 55-56 motors did not have external points adjustment. The 57-59 (and beyond) motors used distributors with external points adjustment. Pontiac used the same physical dimensions for the mounting of the distributor into the block from 1955 through 1981. They introduced the HEI distributor in 1974. The HEI cap is about an inch bigger in diameter (causes problems later, see below) and somewhat taller (no problem under a GMC truck hood).
I bought a distributor from a '76 full size Pontiac equipped with a 400 cubic inch V8. The local "pull your own" auto recycling yard charged me $15. Make sure that the one you get:
1. Has a vacuum advance (later models do not)
2. Has wire connections for only two wires. They are marked BAT and TACH on my cap. Later models have computer control and have more wires.
3. Is NOT from a 455 Super Duty equipped engine because the gear on the cam is a different size.
I cleaned it up and checked the end play. It was way more (0.085") than desired. I removed the gear from the bottom, installed 0.073" of 1/2" ID shims and reinstalled the gear. End play was now down to about 0.012". You can buy a set of shims at your local hot-rod parts shop. Mine were made by Moroso. The guys at the parts store said to leave at least 0.010" end play. While you are at the part store also pick up 3' of rubber hose that will fit on the vacuum advance of the HEI, some #12 wire (6 feet is plenty) and an inline 20 amp fuse holder with fuse. You will also need 2 yellow crimp on female spade connectors and necessary parts to make a splice in this #12 wire. Get a set of HEI spark plug wires too. If you decide to install a PCV system check below for additional parts you will need.
Before starting the changes on my truck I carefully set the timing to the specified 3 degrees BTDC and set the idle to 450 RPM. Then turn off the engine and hand crank it to TDC. Next remove the negative battery cable. Trace the #1 spark plug wire (front driver's side of the engine) and make a chalk mark on the intake manifold where it attaches to the distributor cap. Remove the distributor cap, coil, coil wire and spark plug wires. The rotor in the distributor will either be pointing roughly at the chalk mark or in the opposite direction. If it is opposite, hand crank the engine one turn and stop it on TDC again. The rotor should now be pointing roughly at the chalk mark. Wipe off the chalk mark and make a new one more accurately if need be. Later I will refer to this as MARK 1. Also make a chalk mark on the distributor case where the rotor is pointing (later referred to as MARK 2).
Remove the bolt and clamp that you loosen to adjust timing.
Remove the old distributor by slowly pulling it straight out. Notice that the rotor will turn slightly while it is being removed. Try not to turn the rotor yourself. Once the distributor is removed note the new position of the rotor relative to the case of the distributor. Make another mark (MARK 3) on the distributor case. Do your best to make MARK 4 on the intake manifold based on where MARK 3 is.
The "road draft" tube (the 50's version of crankcase ventilation) is the 1"+ black tube that plugs into the valley cover just forward of the distributor. It makes a loop just clearing the side of the stock distributor and then continues down past the back of the passenger side head. It WILL NOT FIT when the HEI is in place. I found two solutions for this problem. If your carburetor or air cleaner has a provision for a PCV valve use the second solution. It is MUCH easier than the first.
The first solution is to replace the upper piece of the road draft tube.
I removed the complete road draft tube and found that the "filter" in the middle was totally clogged with hardened carbon deposits. I tried soaking it in various solvents with little success and decided to eliminate the filter. I used 1" rigid copper tubing and fittings to make a replacement for the original road draft tube.
Start with a 1" copper 90 degree L fitting. It fits fairly well into the valley cover hole that the draft tube plugged into. I soldered a piece of 12 gauge single strand wire around the outside of the L about 1/2" from one end. This formed a "stop" to prevent the L from dropping too far into the valley cover.
On the back of the passenger's side head there are two 5/16" threaded holes. I used a 4.25" piece of 1" rigid copper tubing from the 90 degree L to a 45 degree 1" L and another 6.25" piece of rigid copper tubing to get past the threaded holes. Use one of the holes to attach a clamp to hold the tubing in place.
I put another 45 degree L fitting on the tubing and continued down past the oil filter with 1" rigid tubing until it reached about the same place the original road draft tube ended.
Once you get this tubing all soldered remove it, shoot it with black paint and re-install it. The net result is that the tube now turns underneath the distributor instead of going up the side of the distributor cap. I wonder if I could have just used the first 90 degree L to get started and used bendable 1" tubing the rest of the way?
Skip to step M.
After much messing with copper tubing I decided to take the easy way out and install a PCV valve. Of course this method depends on having a source of engine vacuum designed for PCV valves, which was not standard on 57 GMC V8s. I had previously changed the carburetor on my truck to an Edelbrock 1404 which does have a 3/8" PCV port designed in. A second option would be to use a later model (or after market chromed) air cleaner that has provisions for a PCV valve.
Go to your local FLAPS (Friendly Local Auto Parts Store) and look at the "HELP" products on a pegboard. These are in red blister packs and include things like window crank handles, throttle return springs and shims for front end alignments. HELP part number 42054 is a PCV valve grommet for Pontiac V8s. They package says from 63 to 75, but it fits nicely into the valley cover hole. Also buy a Fram FV112DP PCV valve for the same application. Other numbers from the back of the Fram box are AC CV622, Purolator PV-679M and GM 25040143. While you are there also buy 2' of 1/2" ID black rubber hose suitable for PCV systems. Get the same length of 3/8" ID hose.
The PCV valve originally plugged directly into the grommet underneath the distributor and will have the same clearance problems with the larger diameter HEI distributor that the road draft tube has. To get around this I used a 6" long piece of 1/2" diameter copper tubing. Make a 90 degree turn near one end of the tubing. Then put a slight flair on that end to help lock it into the groove in the grommet.
Snap the grommet into the valley cover and snap the 1/2" copper tubing into the grommet. Install the 1/2" rubber hose onto the copper tubing. Connect the 3/8" rubber hose to the PCV vacuum source. Find an appropriate place to install the PCV valve, cut the hoses to length and slip them onto the valve.
Remove the cap from the HEI distributor. Lower the HEI part of the way into it's hole on the engine, but not so far that the gear starts to engage. The vacuum advance should be pointing roughly at the gas pedal. Turn the rotor so it points at MARK 4.
Lower the HEI until the gear starts to mesh. Continue lowering and notice that the rotor should turn slightly and line up with MARK 2 when the HEI is all the way seated. Make sure that there is no gap between the flange on the HEI and the engine block. If there is, the HEI is not all the way in. What is probably stopping it is the shaft for the oil pump. This shaft engages the bottom of the distributor shaft. Try wiggling the rotor a little and see if you can get it to line up. If not, remove the HEI, eye-ball the tab in the bottom of the HEI's shaft and then use a long screwdriver to reach down into the block and turn the oil pump to line it up. Repeat step N until you get the HEI to drop all the way in. DONT DROP THE SCREWDRIVER INTO THE HOLE!!! I did, but was able to retrieve it. You may not be that lucky.
Install the hold down clamp leaving it loose enough that you can move the distributor to set the timing. Line up the nearest plug wire connector on the HEI cap to MARK 2.
Put on new spark plug wires. Your old ones wont fit the HEI. Use the firing order found on the intake manifold in front of the carburetor. Start with cylinder # 1 (at MARK 2) and proceed counter clockwise around the distributor. When you get all 8 wires installed go back and check them again.
Hook up the vacuum advance. I used rubber hose all the way to my after market carb. If your vacuum advance line is still a metal line, I would recommend completely removing it and replacing it with rubber instead of cutting the metal tubing. You may decide to restore the truck to stock some day and wish that you had not cut the tubing.
Hook up the BAT electric connection to the HEI. The easiest way to do this is:
1. Attach a spade connector to one end of the inline fuse holder.
2. Attach the other end of the fuse holder to the 6' of #12 wire.
3. Attach the spade connector from step 1 to one of the two unused IGN connectors on the ignition switch. The IGN connector is hot with the key in either the start or run position.
4. Feed the far end of the #12 wire through the firewall and cut to length to reach the BAT connection on the HEI.
5. Attach a spade connector to the wire and slip it on to the BAT connector on the HEI.
6. Install the negative battery cable.
It is now time to crank it up and reset the timing. The timing may be off quite a bit. This may result in backfires through the carburetor. Make sure the air cleaner is installed (less chance of burning off your eyebrows) and have a fire extinguisher nearby.
Disconnect the vacuum advance line at one end and block the vacuum back into the carburetor. Connect a timing light. Try to start the engine. If the engine is hard to turn over the timing is too far advanced. Rotate the distributor clockwise to retard the timing. Once the engine is running set the timing back to where it was in step A. Tighten the clamp bolt.
Reconnect the vacuum advance line and let the engine warm up. Check the idle speed. Once mine was warmed up it was idling about 50 RPM faster than with the stock distributor. I attribute this to the better spark provided by the HEI. Reset the idle to desired speed.
I left my stock wiring (with the ends taped up) in place for a week just in case I had to go back to the stock distributor. After that I removed the ballast resistor and it's associated wiring from the firewall as well as the wiring for the coil.
Another "improvement" that came from installing the PCV system is that the slight oil film that I had before on my rocker arm covers near the breathers is gone. This is because the engine is now ventilating the crankcase and burning blow-by gases instead of venting them out through the breathers.
The TACH terminal on the HEI drives a Sun Super Tach II just fine. Someday I'll write up how to install the tach in the dash and reface it for 4000 RPM to look near stock.
