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Harleyvato1963 09-08-2008 10:11 PM

Rearend for a 1956 F100
I was hoping to get some advice on whether or not the rear diff on a 1996 Pontiac Formula will work on my 1956 F100? I took the engine an LT1-350 and the transmission a 4L60, Just trying to match everything up. :D

techinspector1 09-08-2008 11:52 PM

I think it's an excellent idea to use everything from one donor. If the track is too wide, I think I'd cut the fenders and put a splice in them.

Brian_B 09-09-2008 12:12 AM

The ford truck 9" is a bolt in from 57-73 into your 56.

What bolt pattern does your front suspension have? I would try to match that is possible.

Harleyvato1963 09-09-2008 11:01 AM

It has a Mustang II IFS from Total Cost Involved. I think it is 5x 4 1/2.

Do you think it is possible to cut the rear diff to fit?

Deuce 09-09-2008 11:34 AM


Originally Posted by Brian_B
The ford truck 9" is a bolt in from 57-73 into your 56.

The 57 to 72 F-100 rear end is a bolt in ... no cutting or having to narrow it. It is a way better rear end than the 1996 Formula rear end. The only reason I could see for using the Formula rear end is ... IF ... it is a disc brake equipped rear end.

The 9 inch Ford is a lot stronger and a lot easier to work on. I would just install the Ford rear end and then buy a center section with a gear ratio very close to what the Formula has ... if I wanted that gear ratio. A lot of the Formulas had a 3.42 gear ratio. Ford has a 3.50

The Formula rear has a lot of stuff that would have to be cut off ... new axle pads welded on ... and I believe they are too wide.

A 57 to 72 F-100 rear end is just over 61 inches wide ... measured this way

A mid 70's Firebird/Camaro rear end is about 2 inches wider than the 57-72 F-100 rear so I know the 96 is too wide.

Moser Engineering can shorten your housing ( $110 ) and shorten axles. Same price ;) But not all axles can be shortened ...

Moser has a web site ... below

Harleyvato1963 09-09-2008 12:10 PM

Excellent! Now I'm getting somewhere. My next question was the ratio, I'm trying to get past the headache of trying to match the rear diff to the transmission. The truck has the old original differential, so what you are saying is to just update the internal guts (gearing) to 3.42 or 3.5 ratio? Maybe just sand blast it powdercoat and away we go? :D

Deuce 09-09-2008 12:44 PM

The 56 F-100 does not have the same gears and interchangeability of the 57 up rear ends ... :embarrass so the original 56 F-100 rear end is not as user friendly. I would select a 9 inch Ford rear end ... from this donor list ... with the width you want ...

A rear end from a Ford passenger car would have the same wheel bolt pattern as your front end has now ( with the 5 on 4.5 pattern ) ... and use a ratio that is very close to the ratio in the Formula rear end. Do you know the ratio in the Formula rear end ??

Harleyvato1963 09-09-2008 04:06 PM

I've been looking around on the Internet for the ratio of the 96 Formula but seem to find it...can you help me out?

Deuce 09-09-2008 05:11 PM


Originally Posted by Harleyvato1963
I've been looking around on the Internet for the ratio of the 96 Formula but seem to find it...can you help me out?

Most GM rear ends have a tag on them that says what ratio they are. Also ... if you still have the donor Formula ...

This plate can tell what the ratio is ...

Harleyvato1963 09-10-2008 03:20 PM

I went to the local Pontiac dealership and they informed me that my 1996 Formula donor had a 3.23 ratio rearend. It had an option :confused: for a 3.73. My question now is, what is the difference in performance from the 3.23 to the 3.73? :confused:


onebadmerc 09-10-2008 05:48 PM

Deuce, not trying to high jack the thread here, don't the 57-73 9" rearend out of trucks have 28 splines and the newer 73 and up trucks have a 31 spline axle. This brings me to another thing, I know the two wheel drive 73-79 trucks had a wider frame and a wider rearend. I have also heard that if you want a 31 splined 9" rearend in a pre 73 truck to use a 1973-79, F100-150, 4x4, 9" rearend. I read somewhere that the 73-79 F100-150 4x4 trucks used a 67-72 frame under them, have you heard anything about this?

Harleyvato1963 09-10-2008 09:59 PM

I'm hoping to get pushed in the right direction. I'm really confused and thinking that I'll just spend the $3000 that Currie Enterprises quoted me for a ready to bolt on rearend. Unless somebody else has any ideas. :confused:

Brian_B 09-10-2008 10:09 PM

Posts #3 and #7 suggested the ford 9" which is bolt in and plentiful. You can swap the pumpkin to just about any gear ratio you want.

I have seen them go for as little as $150.

I have one for mine (traded some junk chevy parts for it)...if I ever get around to going and picking it up.

If you prefer the bolt pattern to match your front clip...a local machine shop can re-drill the shafts.

Harleyvato1963 09-11-2008 06:45 PM

When you say pumpkin you mean the internal gears? :confused:

Brian_B 09-11-2008 06:47 PM

Sorry. I meant the removable center section. Take a look on ebay for them and you will see what I mean. The last time I looked there were several center sections being sold with different gear ratios for the 9".

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