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GM HEI dizzy swap in 4.2l how to

#1 User is offline   1BDYJ 

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Posted 18 May 2005 - 01:44 PM

I purchased a used distributor from a General Motors 250ci straight six cylinder engine for $50 from a junk yard. This distributor has a much larger diameter cap for less chance of crossfire (like the Ford set up).

Unlike the Ford conversion with the GM dizzy you replace the entire unit and discard the Motorcraft ignition system that gave Jeeps so much trouble in the past. The GM unit has the ignition coil in the top of the cap and the module inside the dizzy. It is just a 1 wire hook up to the side of the cap for keyed up power. The whole conversion only takes about an hour with hand tools and you will need a chop saw or something to cut the dizzy gear. I'll try to describe in detail and any questions please feel free to call me, I'm sorry I didn't get any pics but my camera was broken.

Before you start make sure that #1 cylinder is at top dead center, the rotor points at the #1 spot on the distributor cap.

1-disconnect the battery

2-remove spark plug wires and sell them (or throw the away)

3-remove the distributor, and the drive gear from the bottom of the Jeep dizzy, you do this by removing the roll pin that goes through the gear, you tap it out with a small punch or finish nail if you don't have a punch, and save the pin for re-use.

4- remove the gear from the GM dizzy (the same way as the Jeep dizzy)and discard---you won't be using it.

5-cut a 1/4" off of the bottom of the Jeep drive gear, try to keep the cut as straight as possible.

6-with a small grinder or cut-off wheel, bevel the edges of the gear very slightly for easier installation and less wear on the camshaft gear. This only has to be slight and only on the very edges.

7-purchae a small roll of metal tape (used for air conditioning vents found in your house, any good hardware store will sell this.

8-put the GM dizzy in a vise upside down, (I left the cap on as to prevent the shaft from falling out), take metal tape and make exactly one wrap on the distributor shaft where the gear mounts. If you test fit the gear you'll see why you have to install the wrap of tape. this will allow the gear to fit tightly over the shaft. trim the tape so no tape sticks out of the bottom of the gear, install the gear and the roll pin. you will have punch a hole through the tape where the pin goes through, you can do that with the punch or nail.

9-install the dizzy like you would any other dizzy, find a keyed power source (one of the wires from the
factory ignition system should be keyed power) and hook it to the terminal on the side of the dizzy cap
marked positive and hook up the vacuum advance.

10-purchase a good quality set of spark plugs and ignition wires (wires I used are for a 96 Jeep Cherokee with a 4.0 litre engine (the terminals will slide right onto the GM cap terminals and the wires are the same length as the factory 4.2l wires) set gap on plugs and install wires a nd your ready to set base timing.

11- remove entire Ford/Jeep factory system and tape up any loose wires and secure

Remember to set base timing with the vacuum hose for the advance disconnected!
Good luck

I can tell you from experience that the GM dizzy holds up to water intrusion alot better than the factory dizzy! My Jeep starts on the very first crank and seems to have a little better throttle response.
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#2 User is offline   Jim B 

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  Posted 19 May 2005 - 07:15 AM

:behind: nice write :unsure:

One question, what model vehicle did you install this in :unsure:
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#3 User is offline   1BDYJ 

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Posted 19 May 2005 - 08:41 AM

1984 CJ7 258/SM420/D300
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#4 User is offline   hiway 

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Posted 20 May 2005 - 06:38 AM

So the metal tape is used as a shim between(under) the shaft and the gear? I assume it is a real tight fit. Sounds cool but seems a little scary. Hope ya get your camera fixed....I'd like to see some pictures.
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#5 User is offline   1BDYJ 

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Posted 20 May 2005 - 09:44 AM

Hiway....yep thats right, just used as a shim to keep the gear from wobbling. It's not a press fit if thats what you mean....the gear will slide over the shaft with a little push.
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#6 User is offline   hiway 

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Posted 20 May 2005 - 09:52 AM

I'm not in the know on jeep motors....is the distributer the only thing that is turned by the gear? A lot of small blocks also have the oil pumps turned by the gear/other end of the dist. 60psi cold startups and high reving could make it hard on that roll pin if everything dosen't fit real snug. Am I making any sense or is it my "meds" kicking in... :sleep:
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#7 User is offline   1BDYJ 

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Posted 20 May 2005 - 09:58 AM

the fit is as tight as the factory fit for the gear. Yes the shaft does turn the pump and it does it the same on the GM as the Jeep. There is no load on the roll pin unless something in the dizzy breaks and jambs the shaft from spining.
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#8 User is offline   hiway 

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Posted 20 May 2005 - 10:56 AM

If the shaft turns the pump and the gear turns the shaft..... :unsure: Ok, if you like it then I'll quit pestering ya. :sneak:
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#9 User is offline   JeepinIan 

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Posted 20 May 2005 - 01:28 PM

hiway, on May 20 2005, 10:57 AM, said:

If the shaft turns the pump and the gear turns the shaft..... :nerd: Ok, if you like it then I'll quit pestering ya. :sneak:

IIRC, most engines run the oil pump off the dist. The older Jeep V-8's w/ the pump on the front of the engine are an oddball.
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#10 User is offline   hiway 

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Posted 20 May 2005 - 05:38 PM

What I was getting at is that if there is ANY slop in this set up, the roll pin may fail. Now you not only have to look for pin parts, but "plummers tape" in the pan. I've delt with billet roller cammed motors in the past that used brass dist. gears. They had to have the oil temp up before you rev'd them up or they'd chew up the gear. That strapping used as a shim is softer than the shaft or gear and would seem to produce more clearence over time. More clearence, cold morning, broke pin, removing the pan. Maybe there is a gear available with a smaller bore. Maybe I'm being to ticky. :tdown:
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#11 User is offline   cj-5 

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Posted 07 June 2005 - 11:31 PM

What you did is all good. I have done this mod myself with NO problems.
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#12 User is offline   1BDYJ 

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Posted 19 July 2005 - 03:48 PM

just an update.....had some trouble with my drive gear shim and the gear wobbled....I ordered a v8 amc gear up to 1993 and it fit perfectly and solved my problem.......shoulda done it that way from the start!!!
Well at least I'm man enuf to admit it......hehehehehehehehe
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