Before I transplanted the 4.3L Vortec, I ran a 4.2L straight six.  The ignition was less then exciting and the engine didn't run great using the stock ignition.  Based on information I read on a couple of mailing lists, I installed a GM HEI distributor.  It was definitely one of the best modifications I've ever made to my jeep.  While I no longer have that motor or distributor, it is worth considering if running a 4.2L engine.

A GM HEI ignition (distributor) from 1978 - 1982 GM 250 I-6 motor retrofitted to the AMC 258 greatly improves problems with missing at idle and cold-hearted attitude of the engine.

Readers with 1974 - 1978 258's having the Prestolite ignition may directly bolt on an HEI distributor and only need to swap the distributor drive gear off your old distributor on to the new HEI.

Those of us (myself included) with the newer Motorcraft ignitions need to find the proper distributor drive gear (The Motorcraft distributor gear will not work). The proper drive gear to swap on to the HEI shaft can come from 1974 - 1978 258's with the Prestolite ignition, or from 1978 or newer AMC V-8s with the Motorcraft distributors. OR you may buy the gear from a dealer *if* they have it in stock... The OEM part number for the correct gear from the Prestolite ignition is: 320 8615.

The part number listed above is a good part number, but will cost you $55US. I found the correct drive gear in a 79 '304 in a Wagoneer, I had to buy the whole distributor to get it, $20. I have heard mention of the gear needing to be modified, a collar ground off. I did not have to do this, the 304 gear worked perfectly. The gear does have a long collar on it, but it does not interfere with anything internally.


1) Regap plugs to the .050 - .055 range
2) Use Ford 300 I-6 plug wires. It has a center mounted Distributor like the 258 *and* has the proper ends for both the HEI distributor and the spark plug itself.
3) A new (rebuilt) HEI distributor will cost $125 plus, so try to find one at a junk yard. I-6s are harder and harder to find, but there are still some out there. GMC or Chevy trucks are good donors. I found my distributor in a 78 Chevy pickup, cost $40

Those of us with the newer computer controlled carb will have some other modifications to make. I'm not sure of them all yet, but it has to do with that huge electrical plug protruding from the back of the carb. Someone that did this said if you can jam the plunger/solenoid inside that housing to stay half open all the time that you can leave it alone. Otherwise you have to swap an older carb without the computer control, thus eliminating the whole computer and its associated spaghetti-like wiring harness. While desirable, this is not always the most legal thing to do in some states.

Other resources:

GM HEI Distributor Upgrade for 258 I6 Jeepsby Larry Maggio
GM HEI ignition Swap for a 258 by dperk
HEI Ignition Upgrade for the AMC 258 by Keith Hardt
Cape Conversions - Source for HEI distrbutors.



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HEI Installed

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HEI Closeup

Copyright 2000 - 2006 Jim O'Brien