Plug gap for 1978 302 in '67 Mustang?

danube7

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Jul 4, 2019
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Hi all,

This is my first post to this Mustang related forum. I'm going to try a few others as well, as I work to get my car back out on the road.

Here's the story. I have a 1967 Mustang C-code convertible. In the 1980's the engine died and it was replaced with a 1978 Mercury 302. It has a replacement "2150"-like 2bbl carburetor. Since I can never get the original engine back, my goal is to make the current one "better" with stock parts. Years ago the distributor failed so I now have a HiPo dual point centrifugal advance distributor. Sitting on the shelf ready to be put in (in winter) are an Autolite 4100 carb and a 289 HiPo 4bbl intake manifold. But that is for later.

Now, I need to replace my spark plugs. Originals were 18mm BF42 (BF32 in the HiPo). But the 1978 vintage 302 has 14mm ASF52 plugs. But what gap???? My 1967 shop manual clearly has 0.032-0.036" for the gap, but I cannot find the gap for the 1978 vintage engine anywhere. I looked through the 1978 shop manual Engine volume and it is not listed. Does anyone know where I can find the right gap for my engine? (And I can't tell you what engine I actually have, unless I can figure it out from the block casting numbers somehow).

Some guidance would be very much appreciated. Thanks in advance!

Dan
 
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2Blue2

I partied with that dude!, um girl, um whatever...
Mar 5, 2019
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Hey Dan Welcome aboard.
My Mustang 2 manual says 1978 takes an ARF52 spark plug with a .044gap.
But it does come down to ignition in use not block application, 78 was electronic ignition = bigger plug gap
points ignition = smaller plug gap ( .032-.036). You got points then use the 32-36 gap

A suggestion, I bought one of these small cap electronic ignitions with coil and wires for less than 150$ on flea bay.
Installed in a hour now I have rock solid timing and advance and reliability! No points to Re-adjust too.
There are other approaches that work well too. Pertonix, MSD etc.
Post a pic of your car, I love those early verts.

s-l64.jpg
 
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danube7

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Jul 4, 2019
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Virginia
Thanks 2Blue2. I guess 0.032-0.036 it is. I measured 2 of them at about 0.04, so they are worn, and firing voltage on cylinder 6 is high, so I bet it's fouled or has a big gap.

Thanks, but I'm keeping my HiPo distributor. I like the solidity of the mechanical parts. Treated right, they really are reliable. I can't *see* if electronic points are working, but I can see that my points are still good. Also I am stubborn and OCD, so I want to try to end up with a "K-code" car using original stock parts. It's a project.

I will eventually post pictures, but my car needs to be detailed, so it is a bit embarrassing at the moment. When I get a good shot up on a hilltop and I am sure I can get the car home from there, I'll take some pics!

Dan
 
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rbohm

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your best bet is to dump the dual point distributor and go with a duraspark system. teh dual point was great in its day, but has long since been surpassed by much better ignitions. even the nascar boys are running electronic ignition, and have been for decades. but if you like the dual point system, more power to you. ultimately you need to do a little experimentation on point gap setting to find out what your engine wants. i have run gaps as small as .020, and as wide as .060. in the end i have settled on .040 for most of my vehicles.
 

danube7

Member
Jul 4, 2019
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Virginia
Hi folks, and thanks for the replies.

I have a set of NOS ASF52 plugs, all re-gapped to 0.036", ready to go into the engine. But then I measured them since they are resistor plugs. They all read 12.5 KOHms except one, which reads 20KOhms. Is this too high? This seems like a pretty high value considering the plug wires are about 1 KOhm and the Ford shop manual specs those at <5000Ohms per foot (so they can be up to 10K Ohms for the longer ones). Should I use the 20 KOhm plug or get another one?

But the ASF52 plugs that came OUT of the engine, and are very old, are all between 4 and 8 KOhms (except one which is very low at 250 Ohms). But it's the same plug. Does the internal resistance get lower with use, or did my NOS parts get higher by sitting on the shelf for 40 years?

Interestingly, I can find out just about anything about spark plugs on the interwebs. Plug thread diameter, length, gap, tip type, temperature,... everything except core resistance of resistance type plugs. Is there some reference manual for Motorcraft spark plugs that lists the specified core resistance of the plugs across model years? What *should* it be for a points-based ignition system from the 1960's? The problem as I see it is I have too many different parts from different years. I have a 1967 ignition system, feeding a 1978 engine, with aftermarket plug wires from the 1980's. I'd really like to know what the specs of the system *should* be for a stock 1967 points based ignition.

Thanks in advance for the help.

Dan

p.s. My apologies if this post ends up appearing twice, but I tried to read my post and it looks like it got cut, so I tried again.
 

2Blue2

I partied with that dude!, um girl, um whatever...
Mar 5, 2019
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did my NOS parts get higher by sitting on the shelf for 40 years?


Plugs can become more resistant over time.

500 to 5000 ohms ( with 9 volt Ohm meter) is the rule of thumb cut off points for spark plugs to making engine run rough.
New plugs shouldn't show more than 4000ohms
Aviation mechanics has a pressure box to test spark plugs in a pressurized environment. But plugs are cheap - when in doubt throw it out and get a new one.

This qualifies as another vote for a HEI type ignition with its hotter spark over points. I want an arc welder going off in there if I can.

The problem as I see it is I have too many different parts from different years.
This isn't a problem.

aftermarket plug wires from the 1980's.

a normal ignition tune up gets new points, cap, wires, and plugs.

Just Get some new plugs and wires ect! And don't get too hung up on NOS parts that are considered expendable (plugs wires belts hoses ect.)
 

rbohm

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Apr 12, 2002
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op at this point i think you are over obsessing your parts selection, just put the plugs in and run them.