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Ford Bronco Parts - Classic Vintage Early Bronco Parts

Improving the C4 automatic

Tech article by BUCKETOBOLTS and filed under - Automatic, Transmission

Tech article by Aaron Modlin (BUCKETOBOLTS)

I snapped a few PICs as I’ve been working on my C4 rebuild and thought I’d share how I modified my transmission. Most early Bronco and truck application transmissions are already well equipped. The input shafts are usually hardened. They have the “H” servo and five forward clutches. They are also all the pan fill case design, which is referred to as the stronger casting. The Bronco version also had a deeper sump pan.

I’ll start with the cooler circuit. I drilled the aluminum ball plug out of the stator to remove the 2 check balls and spring. Then tapped the hole 1/8″ pipe and plugged. This allows full volume to the cooler. Then I reamed the 5/32” cooler return at the rear of the case to 3/16″ to get rid of the restriction there.

I then drilled the #9 thrust bearing lube circuit. The 1/8″ hole intersects the cooler return I just reamed. This allows for a pressurized lube for the bearing which is prone to shearing the ears off and spinning in the case. This circuit may already be drilled on later model transmissions.

I bought Alto Red Eagle clutches and Kolene coated steels. I’ve had very good service from these in my Mustangs. My transmission already had 5 disks in the forward pack, so I just set the clutch pack clearance at .025″. The high/reverse on the other hand only had 4. I added one friction and one steel by machining the pressure plate

to allow for the five total disks. I set this clearance at .025″ also. I relieved the high/reverse drum by drilling 5/16” holes around the band area. This allows the clutches to displace the oil faster and help release the band quicker for a good 2-3 shift.

My kit also came with a Kevlar intermediate band. These bands will last much longer and stand up to more abuse than stock replacement bands. The intermediate servo is used to apply the band. Most early Bronco C4’s have the “H” servo. It has a large 2.960” surface area on the apply side. This servo does well and doesn’t usually need any type of modification with a stock valve body. A manual valve body will have specific directions on how to modify the servo to work with the valve body.

To firm up the shifts I did a few simple tricks to the valve body per the BURLY4x4.com website. First I disabled the intermediate servo accumulator valve by switching the position of the valve and spring. This valve acts as a cushion for the 1-2 shift. I then drilled the two holes in the spacer plate to 3/16”. Whe

n I reassembled the body, I only used two check balls. One in each oblong passage in each half of the body.

I replaced my factory deep sump pan with a BTE racing cast aluminum pan. This pan holds approximately two more quarts of fluid. It also came with a matching deep pickup and a drain plug in the bottom of the pan. Because of its cast design it adds extra rigidity and strength to the case.

Tech article by Aaron Modlin (BUCKETOBOLTS)

6 Responses to “Improving the C4 automatic”

  1. mecone Says:

    Do you have any sources to build my C4 automatic trans so its more reliable to tow with?
    My truck has a 1-1/4 inch body lift and a 3 inch suspension lift with 35 inch mud terrian BFG tires. Double rancho 9000 shocks on all fours and 4 wheel disc brakes.
    Big question. Do you think its safe to tow a 18 foot Carson Titan toy hauler with two quad racers in side?
    Any answer greatly appreciated.
    Thanks
    Martin

  2. lynn w gray Says:

    great item, thanks must use info, lynn

  3. bigun-1 Says:

    I have found that the little C4 is a lot stronger that it would appear. If built properly it can withstand a lot.
    I built one for my drag car a few year’s back using mainly original part’s. It never gave me a problem. If your interested here’s a look at the car.
    http://www.fordexp.freehomepage.com/photo3.html
    The intermediate servo list here is the best, but they are getting very hard to locate. I have found that if you remove the seal from the apply side of of the piston (small end) and then equally position groove’s around this area (3 worked fine for me) It is a must that a “Hard Band” be used when doing this.

  4. brocobilly Says:

    I just found the site with this article and that’s some impressive work you’ve done Aaron! That’s a great resource you’ve given to the community, thanks a lot.

  5. PSIPIG Says:

    DOES ANYONE SELL A REASONABLY PRICED C-4 SPECIFICALLY GOOD FOR FOUR WHEELING AS OPPOSED TO THE DRAG RACING C-4 YOU SEE IN THE PERFORMANCE CATALOGS. I THINK RE-BUILDING A C-4 LIKE AARON DID WOULD BE FUN BUT IT IS PROBABLY MORE THAN I CAN HANDLE. I ALSO FIGURED WHAT WAS GOOD FOR DRAG RACING MAY NOT ALWAYS BE GOOD FOR A STRONG RELIABLE C-4 FOR FOUR WHEELING.
    ’73 WAGON, BUILT WINDSOR, C-4, DANA 20, 4 WHEEL DISC, BUILT 9″, STOCK DANA 44 UP FRONT.

  6. Jack Says:

    Man alive I have the same set up and have been searching for the right solution for months I can figure out if I should change over to the Aod from my c-4 as its starti g to act up and if I rebuild it the costs for me as a lamemen starts to get closer to just getting a Aod set up. But I must say its been confusing at times keeping up with what I need to do! I Rock climb and then I go into muddy mountains then dunes so I’m in a lot if different settings . Be nice to drive her down the freeway to the trail head 1 hour away then run the trail and drive home ( don’t we all?) but the c- 4 needs another gear up and then when in Moab the flat or slight downhills in the rocks it goes to fast!! I would like to not put 5 sticks down there( doubler) but if anyone can just point me I. The direction of someone who understands this and will take my money and set me up with EVERYthing I need I can get it put in! Help!!!!!! Pretty please with a bored out 351 w on top!!!

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