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73 Javelin 8.8 Ford rear end swap

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DMack View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMack Quote  Post ReplyReply Direct Link To This Post Topic: 73 Javelin 8.8 Ford rear end swap
    Posted: Apr/16/2019 at 12:35pm
The first thing I have discovered…

There is no such thing as “a bolt in” other than another AMC rear differential. 

Tried to do a bunch of research before I picked out the differential that I was going to put into my 73’.  Many, many thanks to “Mopar_guy” for giving me some direction on this forum.

I read a great forum item that was several years old about narrowing a Ford Explorer rear end to put in a Rambler, and using two of the passenger side axles to create a narrowed rear end.  The issue for me at that point was that the narrowed rear end was “too” narrow and the pinion was still offset by 2 inches.

I read all of the information that I could find about rear differential specs for the 73-74 Javelins. Everything I read stated that the factory rear end was 60.0 inches (+/- 1/2") wide with spring perches at 43.5 inches.  My particular rear end in the car (an AMC 10) measured in at exactly 61.0 WMS to WMS. I am not exactly sure why this would be different…maybe thicker flange on after-market drums?  The measurement for the 8.8 from WMS to WMS is 61.25 inches.  I can live with it having a 1/8th inch wider track on each side. (No narrowing required).  

I was looking for a rear end that would give me a few things…low price, easy parts availability, posi traction and disc brakes.   As I searched around, I landed on a 1994 Mustang GT 8.8 28-spline trac-loc with 3:08 gears in it for $150.00.   H.P. for my set up should be somewhere in the 350-375 area, which is well within the capabilities of this rear end.  A nice feature for the 8.8 is that should the 28 spline ever fail, I could purchase a (stronger) 31 spline carrier and axles for this differential and it fits right in to this rear end. 


After cutting and grinding all of the brackets off this rear end, I ordered the 2.5 width by 2.75 perches from Chassis Unlimited for $42.00 bucks.  I also found a set of 3:55 gears on Craigslist for $25.00!  I opted to have a shop install the gears & weld on the perches.  All tolled with all new axle, pinion and carrier bearings, welding and setting up the gear set, I have about $390.00 into it. I did have to purchase a new axle.  When the shop installed the new axle bearings, they noticed some pitting on the shaft.  Add another $80.00 for a new axle.  I also bought a cast aluminum cover with a built in stud girdle for another $99.00.  New brake pads and turn the rotors, new (fabricated) hard lines, new 2-3/4” U-bolts and nuts and new rubber lines and I am in it $649.00 complete.

So…a few tips I can offer if you opt to go this route. 

After you have the perches welded on and reassemble the brakes, you will find that the support bracket for the caliper mount is too close to the perch to allow the U-bolts to fit in. You have to cut the weld on the bracket and trim it back about ½” and re-weld to give yourself the clearance to get the U-bolts that hold the differential to the leaf springs.


I also welded a plate and stud inside the perch to hold the factory Ford rubber brake lines and brackets in place.  I inserted a steel spacer over the stud to keep it at just the right height where it didn’t tip to one side or the other.  I also ground a small indentation into the perch to accept the stabilizing tab on the brake bracket.  (Pardon my crappy welding) This made for a strong and I think very clean installation.

 

Next was turning the bracket under the axle vent, so it faced in the opposite direction.  I used this bracket to hold the e-brake cable coming off the passenger side.  I also took the bracket that held the (Mustang) center rubber brake line, removed the rubber line from it, and bolted another loop (from the Mustang e-brake) in order to hold the e-brake line. (see last pic on this posting)

I managed to use the factory AMC e-brake lines on this rear end!  With just a tiny bit of modification.  I turned both lines around so the end that originally went into the bracket that is welded to the frame rail of the car, now fit into the bracket of the caliper on the new rear end.   The cable end fit into the bracket on the caliper perfectly, I just had to take a Dremel tool and cut a small groove into the housing to accept a “C” clip to hold it in place.


The other end of the cables go into the bracket welded on the car.  I had to open the holes just slightly to get the end that used to go into the brake backing plates, through the holes.

Kept the flat pinion flange on the new differential and ordered the Spicer mounting flange from Denny's Driveshafts and a set of new u-joints.  After taking a ton of measurements, this set up will not require any modification to the driveshaft other than the new flange!  Both the original driveshaft and the new flange use a 1310 u-joint that bolts right in.  The measurement of the flange to the transmission output shaft is less than a ¼’ (shorter) than the factory set up with the AMC 10 differential in place.  This was truly the only “bolt in” part of this mod.


Here is what it looks like with the wheels on...and a pic of the completed rear end.



So far, it is working out well, but we will see once I have all of this back on the car… The factory mounting plates for the U-bolts will work, I just need to open the holes up 1/16" for the bolts to fit through. (more than enough material remains around the hole).

Just a side note.  I sent this last pic to Mopar_guy and he suggested I check the rear cover dimensions to make sure it wouldn't hit the gas tank when installed!  Never even thought about that until he mentioned it.  Good news, after measuring the clearance it has about an inch of clearance behind the differential.

Will keep updating as I go.

Thanks again to this Forum.  A ton of really helpful information in here.



Edited by DMack - Apr/16/2019 at 2:57pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Red Devil Quote  Post ReplyReply Direct Link To This Post Posted: Apr/16/2019 at 1:27pm
Originally posted by DMack DMack wrote:

I read all of the information that I could find about rear
differential specs for the 73-74 Javelins. Everything I read stated that the factory rear end was 60.0 inches (+/- 1/2") wide with spring perches at 43.5 inches. My particular rear end in the car (an AMC 10) measured in at exactly 61.0 WMS to WMS. I am not exactly sure why this would be different…maybe thicker flange on after-market drums


Problem is most sources quote tread width. Tread width is centre of tire to centre of tire and does = 60". But the wheel flange is offset 1/2" from the rim centre, so the axle measurement from WMS-WMS does = 61".

I've posted this before about the later Javelins, but you'll see lots more posts stating 60", so need to check if people are talking about tread width or wheel mounting surface width. Most technical specs are tread width. Unless you know the wheel specs, you need to measure to get the WMS width.

Hope this helps,RD
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Apr/16/2019 at 1:52pm
One piece of advice, soak your clutch plates in gear oil before final assembly.
Was passenger in a one the other day that I shortened but did not do final assembly.
One wheel hit some sand on the road and under power the differential hunted one side to the other until he took his foot off the gas pedal. Didn't loose it but could have.
A sign of dry clutch plates on a new install, he did put in the additive.
Told him to do a few figure 8's over the next few months before he hits the road. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Apr/16/2019 at 2:06pm
Originally posted by Red Devil Red Devil wrote:

Originally posted by DMack DMack wrote:

I read all of the information that I could find about rear
differential specs for the 73-74 Javelins. Everything I read stated that the factory rear end was 60.0 inches (+/- 1/2") wide with spring perches at 43.5 inches. My particular rear end in the car (an AMC 10) measured in at exactly 61.0 WMS to WMS. I am not exactly sure why this would be different…maybe thicker flange on after-market drums


Problem is most sources quote tread width. Tread width is centre of tire to centre of tire and does = 60". But the wheel flange is offset 1/2" from the rim centre, so the axle measurement from WMS-WMS does = 61".

I've posted this before about the later Javelins, but you'll see lots more posts stating 60", so need to check if people are talking about tread width or wheel mounting surface width. Most technical specs are tread width. Unless you know the wheel specs, you need to measure to get the WMS width.

Hope this helps,RD


Once i saw this thread, i knew you would reply.

As for a reply to the OP.

Yep, there is so many factors in what fits. The only bolt in is a 68 to 70 B body. Slight issues with shock plates... you must use th plates and U bolts from the rear you obtain. Though, the brakes are huge! So be aware when braking.

EVERY THING ELSE FROM OTHER MAKES IS NOT A PERFECT FIT. Even though you can bolt in a 71 TO 73 Mustang 9" the spring pad width is not the same distance, and is off 1/2".

Some probably will shake their heads on my 65" wide 2WD truck rear end. Yes it is 2.5" wider on each side than stock. But I chose to go with modern rims, which eat up the extra width.

I made my decision based on cost and availability at the time, plus I also made it a point to have something big enough to make changes with, as I figured there will cutting to make it fit. I even started cutting a weld on the tubes once I laid it all out, then it hit me... all it needs is proper pad placement. So there you go, I almost cut a perfectly good axle tube to fit for stock width, as that was the numbers running in my head, at the time. Ha!

One consideration... the 31 spline Explorer axles work just fine in 9" rears. From what i gather some 8.8 rears are 28 spline as well as some 9" though i never came across a F150 with 28 spline axles. Though i wouldnt say there is none out there.

71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMack Quote  Post ReplyReply Direct Link To This Post Posted: Apr/16/2019 at 2:38pm
Thanks everyone for your comments and replies.  I don't know why my pics didn't post.  tried it through Google photos.  I'll try again in this reply to post a pic of the finished product.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mopar_guy Quote  Post ReplyReply Direct Link To This Post Posted: Apr/16/2019 at 2:57pm
Glad to hear it will fit with that cool cover Dave. Thumbs Up  I'm glad I could help. Smile

"Hemilina" My 1973, 5.7 Hemi powered Javelin
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