So after doing a compression check and oil change on the '75-'76 302 in my '66 f100, I figured that I needed to get another motor, since it would be more $$ to rebuild the old one, and would take too much time to do. I found a very nice '90 out of a Grand Marquis for $400, and I'm wondering if it will bolt up to the C4 that I assume came with the old motor, or if I'm going to need to buy a new C4 to install this motor. Were there any major changes to the 302 bolt pattern between those years that would make it unpleasant to bolt up to my old C4? I'm assuming that the flywheel might have to be taken off of the old motor, but I'd rather keep the trans that's in the truck, rather than to buy a new one.

Also, from driving this old truck I've found that whatever stock rear end that the truck was supposed to have is now long gone. It's revving way too much at 55mph to be a 3.25 rear end. Can I get a ballpark on the rear end gear by jacking up the back of the truck and spinning the tire, and then counting the number of revolutions in the driveshaft versus one revolution of the tire? Probably just answered my own question, but I never bothered to check my Falcon (too slow to matter!), and most of the other cars I've had were still stock, which this thing is most decidedly not.

 

As far as I know the 302 block is the same no matter what year. So It should bolt up to the old C4, thats basically whats in the Grand Marquis except it uses some electronics. As for the rear end I dont no what to tell you but it definitely sounds not right.

 

Okay, first things first. There's no problem with the bolt pattern, the C4 will still bolt up to that '90 motor just fine. Your problem here is going to be the flexplate. (automatic transmission flywheel)

You can't use the one from the '75 302 as it has the older 28oz. imbalance, you have to stay with an '81 and newer flexplate which has a 50oz. imbalance or you're going to have serious engine-destroying vibration problems. I don't think the '90 flexplate will work as it's probably for an A4OD, and most likely has a different offset if not a different converter mounting pattern. Compare the two very carefully. If it won't work, you'll need to find an early '80s ('81 or newer) 302 or 351W flexplate for a C4.

As to the rear end, yes, you can figure your ratio that way. To get a more accurate figure, get a helper to count the turns of the driveshaft while you turn the rear wheel 10 times instead of once. As an example, if your helper counts 37 and a third turns of the driveshaft to your 10, then you have a 3.73. If it's a 4.10, he'll be counting 41 turns to your 10.

Warning: don't try to count both of them yourself, trying to keep track will cause your head to explode!

 

Are you sure?
Because of the differential action, the turns of the tire in your example should be 20.
I would have thought that if you have just one helper, you can turn only one rear tire, with the other one one the ground. If the ratio was just 1.00, then 2 revolutions of the tire will make the propshaft turn one [1]rev.
Or 20 on the tire will make 10 one the prop.
With a 4.10 ratio, 20 on the tire will cause the propshaft to turn 10*4.10=41 turns.
If both tire are rotated the same number of turn, then you are of course right
Or?

Cor

 

Your analysis doesn't compute for me, Cor. Let me try to explain it this way:

With an open differential, if you lift both tires off the ground and turn one, the other will turn the same number of turns in the opposite direction while the driveshaft remains stationary unless there is some sort of drag on the other wheel. (Dragging brakes or something.) The trans must of course be in neutral. The torque on an open diff. will follow the path of least resistance. Set one wheel back on the ground, and the driveshaft will now turn. There is no gear reduction caused by the spider gears in the differential, they are totally independant of the gear ratio. So with an open diff, you need to check the ratio with one wheel on the ground to stop that tire from turning while you count the other one.

With a Traction-Loc or other locking-type differential, the clutches in the diff. cause the wheels to lock together until a greater force such as turning the vehicle causes the clutches to slip, allowing the wheels to turn at different speeds. So, to check the ratio with a locking-type diff, you must raise both wheels off the ground.

I think I'm right, but if you beat me over the head with proof otherwise, I might be talked into accepting it!

 

Proof? How do you do!
I think we agree that de "ratio" of a differential is the division of the number of teeth of the bevelgear(crownwheel) into the number of teeth of the pinion.
So is there is no friction on either driven wheel, both off the ground, then, with a 4.10 ratio, the propshaft must turn 4.1 times and both driven wheels will rotate one (1) turn, right?

Now, what does the differential do? It provides rotation to the wheel with the least resistance.
If one driven wheel is on the ground, the least resistance is the wheel off the ground.
If you now rotate the propshaft 4.1 turns, the speed of this wheel will be twice as much, hence it rotates 2 revolutions. It is simply the way a standard differential works.

I do agree with your observation, that the spidergears in the differential have nothing to do with the differential ratio, but they never-the-less cause a doubling in speed and number of turns of one driven wheel, if the other of the driven wheels is held steady.
If you do not agree, I would say, try it for yourself which will provide the proof!
Succes,
Cor

 

Very well then, I have a homework project to undertake...!

 

...and I found, you were right! I must have been having a brain fart! Now that I reread your earlier post above, it makes perfect sense. I stand before you, (well, sit, really) humbled.

I can admit when I'm wrong, and I certainly was in this case.

So, if you are checking a posi or locking-type rear, lift both wheels off the ground and count the total number of turns of the wheel. If it's an open diff, lift only one wheel and turn it 20 turns to equal 10 turns of the ring gear.

 

Well TigerDan, good to see we agree; it does not matter who is right, but what is right, and considering the problem that Kartracer originally posted, he whould have to apply the right procedure, in order to work out his diff. ratio, hence my reaction.

See you,
Cor

 

Well, we jacked the truck up a few days and counted a bit over 16 driveshafts revolutions with 10 turns, so had just assumed we had to double it. I knew it was no 1.625 axle!!

Also connected a tach and went out driving and wrote down rpms and mph in each gear at different points. The stuck all the data in a program we have called gearcalc and it matched up on the button with it being a 3.25 rear axle.

 

Hi charlie, and welcome to FTE, where I specialize in giving the finest misinformation!
Please take a moment to check out our guidelines, they contain important information you'll need when posting here:

https://www.ford-trucks.com/guidelines.html

 

Got a question on Kartracer using a 90' engine, which might be roller cam. Shouldn't he swap the distributor gear to a hardened gear to mesh with the cam?

 

Hi, TigerDan. BTW, IF you haven't figured it out, I happen to know Kartracer...........I'm his dad.

Trying to get all the details on these different versions of the 302 is kind of mind boggling!
The one we acquired came out of a 1990 Merc Gran Marquis. It is NOT a 302 HO, more of a 302 LO!!! As best as I can determine, which may not be all that accurate, this non-HO 302 is basically the same as the older versions of the engine. Being a non-HO, it is supposedly not a roller cam engine......?? If it is not a rollercam engien, our older distributor should be ok, then...right?

Also, on ID'ing the flex plate, I can tell you thhis, I wish the marked these things with more info! I count 164 teeth on it. The weight is marked EOSP-AA and is a bar about 4in x 1in x 1/2in. Total weight is 5lb 14oz. So how do I tell if this is a 28oz or 50 oz unit?? BTW, we intend to use the C4 in the old truck. Haven't got that engine pulled yet, so hard to compare with the old flexplate at this point.

 

When an engine's been around as long as the 302, from its roots in the 221 back in 1962 all the way up to the last 5.0, yup, theres gonna be a few changes along the way!

I assume that you're going to pull the intake manifold anyway so that you can put an earlier carb manifold on it? At that point it'll be easy to see if you have a roller cam or not. Also, you can pull the dist. and look at the driven gear. For a roller it should be bronze, for a non-roller it should be cast.

Ah, 302 balance...for some reason, this subject has come up a lot lately. I wish I could tell you more about the difference in physical size of the weights between the 2 different imbalances on a flexplate, but I just don't mess with automatics much. I went back through past posts and found this thread from a few months back. Take a look at it and see if there's anything there that helps you:

https://www.ford-trucks.com/forums/s....php?&t=398046

But, if you're positive that the 302 you're replacing is really pre-1981, then you can be sure it's 28oz. imbalance.

 

Well we're assuming that the old 302 is out of the same vehicle that the steering column came out of...that and the location of the dipstick leads us to believe that the motor came out of a '75 or '76 Ford. It honestly could have come out of just about anything though.

Yeah, we are going to put a 4bbl on it, amongst other things.