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How to: Project Frankentranny - A4LD / 4R55E / 5R55E Hybrid Project

Brain

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This is a current work in progress, so be patient if you think this is incomplete – I agree it is at this point.

First I’ll start off with a little bit of history of why I started this project and why it could be useful and interesting to others. I have a ’92 Eddie Bauer that I have used to tow some trailers (kiss my butt U-Haul). The problem is that I didn’t follow the owner’s manual’s directions and I towed in the overdrive shifter position. At 85K miles I had the front seal blow out and it cost a bunch of money to get a “cheap†seal replaced, but I was on the road so I didn’t have the option of doing it myself. That was a failure that set the stage for the second failure. At 125K miles, the overdrive band broke and basically chewed up everything in the front half of the case. That band was worn away faster because of my towing in overdrive, and because I had overheated it at the 85K mark when it ran for a minute or two without fluid. When that band broke, the freed band anchor plate got stuck between the overdrive drum and the case, causing the front half to run off of the centerline (cocked up) and causing parts to collide. This caused enough friction to weld things together that shouldn’t have been.

When a dealership gave me an estimate of $2500 to replace the old A4LD (referred to as the A4 for the rest of this series) with a rebuilt unit, I declined and let the truck sit until I had time and money to fix it myself. When I pulled it apart and saw what kind of damage was done and how many parts I was going to have to replace, I started to research the upgrades that were available to the A4. There are quite a few available out there, but I kept hearing about front seal problems with the A4 that I hadn’t heard of with the 4R55E (the next gen A4 used from ’95-’96, referred to as the 4R for the rest of this series), the 5R55E (the third gen A4 used from ’97-’02, referred to as the 5R for the rest of this series), or the 5R55W (the fourth gen A4 used from ’02 and up, not mentioned again). It seemed to me that hearing this problem over and over again, that somebody would have found a way to fix it. The problem is that there is no “fix†for it if you want to keep all of the A4 as it was produced without using some mix-and-matching of parts from later years. In my opinion, it was a flaw inherent in the design (but it takes some knowledge of fluid dynamics to fully understand the problem). I decided that fixing a bunch of other potential problems wouldn’t be worth it for me if I couldn’t find a way to deal with that front seal and bushing problem.

While I was researching and learning about the A4 and its offspring (which itself originated from the C-3), I found that Ford had redesigned how the lubrication for the front bushing and seal was supplied. I also found out that many of the bushings were eventually replaced with roller bearings and that the numerous thrust washers were replaced by really thin Torrington bearings. If anybody is interested in the A4, they have probably read through Glacier991’s posts about rebuilding the A4 in the most “bulletproof†fashion possible. In those posts he hits the nail on the head at the beginning and end of the series when he said that the best you could do is try to use as many parts and design ideas from the 5R as possible. His efforts inspired me to do this write-up, as well as peaked my curiosity. Hence my journey in hybridization of transmissions, Project Fankentranny, began.

First I wanted to find a solution to the front seal lubrication issue. I found that the A4 and the 4R bellhousings were the same except for a different design for the lubrication of the front bushing, an extra pump bolthole, and one elongated hole feeding the case. The 5R bellhousing was the same as the 4R, except the pocket for the starter was shaped differently (which makes sense because it was behind a different motor). So, if I wanted to have the updated way of lubricating the front bushing and seal, and keep the standard starter setup and bolt pattern, a 4R bellhousing was a must. It serves as my “adaptor plateâ€.

A4/4R bellhousing front
27133MVC-002F.JPG


5R bellhousing front (notice the different starter pocket shape)
27133MVC-018FB.jpg


A4 bellhousing back
27133MVC-003F.JPG


4R/5R bellhousing back - stock
27133MVC-001F1.JPG


4R/5R bellhousing back - stock lube pathways
27133MVC-017F.JPG

27133MVC-016F.JPG


4R bellhousing back - modified lube pathways (bigger drainback hole and cut back supply restrictions)
27133MVC-012F.JPG

27133MVC-010F.JPG


Notice that the part numbers (year reference) are stamped in the back of the bellhousings with the A4 having the number starting 90GT (starting in '90), the 4R has a number starting 95GT ('95), and the 5R starting 97GT ('97). The right one to wark as an adaptor between the 4.0L and using the new way of lubricating the front seal and bushing is the 95GT.
 


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Brain

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This requires the use of a separator plate from either a 4R or 5R (they are the same).

A4 separator plate
27133MVC-012F1.JPG


4R/5R separator plate
27133MVC-011F.JPG
 




Brain

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These different kinds of separator plates work together with the front of the pump and the back channels in the bellhousings to make the different fluid pathways. Here is a look at the different types of channels on the front of the pumps.

A4 front pump channels
27133MVC-014F.JPG


4R/5R front pump channels
27133MVC-015F.JPG


I modified the drainback hole on this 5R one (enlaged according to a tech bulletin)
27133MVC-013FB.jpg


I find it very curious that Ford would publish a tech bulletin about enlarging the drainback holes and not incorporate it into every subsequent design and build, meaning it comes from the factory needing modifications. :rolleyes:

The pump produces pressure by spinning two gears together in an eccentric fashion. Where the volume is increasing, suction is created, and where the volume is collapsing, pressure is created. There is a difference between the A4 gears and the 4R/5R gears, as the A4 must have fluid pass by the inside of the small gear to lubricate the front bushing and seal, where the 4R and 5R have an o-ring on the inside of the small gear and route fluid around the outside of the gears to lube the front bushing and seal. I also find it interesting that the original factory gears had more teeth and the replacements had fewer of them. The A4 rebuild shown in Glacier991's post has this, and the 5R gears that I have show this also. I will be using the last set of gears shown (4R/5R replacement gears) in Frankentranny.

Original A4 pump gears
27133MV-002F.JPG


Original 4R/5R pump gears (with o-ring)
27133MV-003F.JPG


Replacement 4R/5R pump gears (with o-ring)
27133MV-004F.JPG
 




Brain

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Upgrading to the new way of lubricating the front bushing and seal require the use of either a 4R or 5R pump, but they are NOT the same. The A4 and 4R pump works with the “old†overdrive drum (stamped lightweight unit riding on a bushing), but the 5R pump can only be used with the “new†5R overdrive drum (solid heavyweight unit riding on bearings).

The fifth gear of the 5R is achieved by overdriving first gear (for an extra gear between first and second), which means that the overdrive band is applied in second and fifth gears in the 5R (4th gear from the A4 became 5th in the 5R, 3rd became 4th, 2nd became 3rd, and a “new†2nd was born). What this means is that the 5R has the same lowest and highest gear ratio as the 5R, so it doesn’t change the low end pulling power or the fuel efficiency and speed of the high gear ratio…they are the same. This also means that there are no new gear components to try to fit in the case, and that a complete “guts†swap may be possible. This would allow the use of the newer components without needing to swap the computer/controller packages or rely on specialty controller packages like this one ($425):
http://www.becontrols.com/products/tcs.htm

Ford redesigned the overdrive drum because the band is applied much more often and needed to be more durable. The specifics on the drum come later, but first come the bearings that support the drum. After I saw these bearings I knew I wanted them in my tranny, even if I had to do some machining and finagling to get it to work out right.

Bushing support of the A4/4R pumps for the “old†overdrive drum
2713319.JPG



Bearing support of the 5R pump for the “new†overdrive drum
2713320.JPG
 




Brain

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Between the pump and the overdrive drum there was originally a metal thrust washer, and this was eventually replaced with a plastic (polymer) thrust washer. I’m not sure if the 4R had metal or plastic. It is referred to as the #1 selective washer because you can get them in different thickness so that there isn’t too much/too little play (slop). This is one of only two thrust washers in the whole 5R transmission (the other one is metal and at the other end of the case). It is also one of only two selective washers or selective bearings in any of the A4/4R/5R transmissions. The plastic one for the 5R on the right has a little bit bigger hole in the center so that it slips easily over the roller bearing on the pump, but the metal one can fit over it too, it is just a tight fit.

A4 and 5R #1 selective thrust washers
27133MVC-001F.JPG
 




Brain

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Now we get to the overdrive drums. The original A4 and the 4R had a stamped lightweight unit riding on bushings. The one used in the 5R is solid, heavy, and needs the support of the roller bearings on the pump snout. There is a bearing race (steel) on the inside instead of a bronze bushing (which rode against the steel race on the A4 and 4R pumps, as shown earlier).

A4/4R overdrive drum inner bushing (beat up really bad)
27133MVC-005F.JPG


5R overdrive drum inner bearing race
2713332.JPG


I must tell you now all that my pictures of the old drum show how bad the drum can be. This is where my tranny failed so I don't have a A4 drum in good condition, but it works to illustrate what changes were made and why I think one is better than the other.
 




Brain

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A4/4R overdrive drum - stamped construction (those are parts of what used to be springs)
27133MVC-004F.JPG



5R overdrive drum – solid contruction
2713318.JPG
 




Brain

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The A4/4R used three friction plates in the overdrive drum where the 5R used only two. You might think that this provides less holding power, but the 5R drum uses a bigger piston to put force these plates. Where the A4/4R pistons have a large step in for the outer piston seal, the 5R doesn’t have as large a step. This larger outside diameter is where the increase in piston size comes from, as the inner piston diameter and seal is the same size.

A4/4R on the left, 5R on the right -> overdrive piston top
27133VC-003F.JPG

There are 20 posts to put springs on on both pistons, but the A4 and 4R only use 15 springs (5 empty), where the 5R version has all 20 spring posts filled (to supply more force to return the piston).

overdrive drum piston bottoms
27133VC-005F.JPG


pistons side by side, side view
27133VC-004F.JPG

The top surface is the same diameter, but the step in accounts for the smaller diameter outer seal in the A4/4R. The 5R version is also slightly shorter.
 




Brain

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The numbers work out that the stock holding force is about equal between the two (three plates and smaller piston size vs. two plates and larger piston size). I calculated a value showing that they are within 4% of each other (the A4/4R being slightly higher). Now, there are a couple of reasons I don’t like the two-plate setup of the 5R. It’s true that most people drive around in the overdrive position and don’t tow stuff in the drive position very often, so fuel economy is increased (not much) by not having the viscous drag (fluid friction) of an extra plate in a clutch pack that isn’t engaged. My thoughts are that I’m going to be towing things with this vehicle (not U-Haul though – kiss my butt), and that three plates will provide 50% more life if this is the first clutch that wears out. The other issue is that although Ford uses the same number of teeth on the friction plates (same as the A4 and the 4R, as well as the C-3 for that matter), by reducing the number of plates and keeping the force that those plates are holding the same, they are increasing the localized stressing the splines of the planet carrier by 50% over the three plate design. This wouldn’t be an issue if Ford had thought about this and used a stronger/harder material for the splines on the planet carrier where the friction plates transmit their force, but they used the same material, and I’m seeing evidence of yielding in those locations.

Damaged 5R overdrive planet carrier friction splines
27133MVC-001FB.jpg


It looks to me like they used a laser to weld on the senor spokes, so they didn't want to use a harder material that would be more difficult to weld.
 




Brain

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I don’t like that at all, so I’m working on a modification for getting three friction plates to fit in that clutch pack. I’ll update this when I get it to work.

(Update #1)
I have two 5R pistons, so I can show what I did to it side-by-side. I put the piston on a lathe and cut off a bit off the top (back) of the piston. The goal was to remove 0.062", but I ended up removing 0.064" (which is fine). I would have taken more off of the piston so that the pressure plate didn't have to be modified, but I was concerned about not having spline engagement of the planet carrier to the lowest (furtherst forward) friction plate lugs, as it pretty close to the front of the carrier splines the way it is.

My machine work put a finer space between cuts (because I don't have to do one every 2.5 seconds ;) ), so the surface has thinner bands as compared to the stock piston. Here are some pictures with the stock OD piston on the left, and the modified piston on the right:
27133A-001F.JPG


27133A-002F.JPG


I'll post another update when I get the pressure plate completed (the carbide bit that I needed should arrive in a couple of days).
 




Brain

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The latest version (but not the early version) of the 5R overdrive drum also has a little different design where the adaptor plate sits. Although it uses the same adaptor plate, it has rounded inside corners where the plate fits to the drum, where the A4 has sharp inside corners. These rounded inside corners serve two purposes: it lets fluid pass through, and it reduces the localized stress at those corners. Sharp inside corners concentrate the localized stresses and allow cracks to form and propagate (you don’t have to fully understand this, just know it’s a good thing).

A4/4R/early 5R overdrive drum adaptor plate teeth
27133MVC-006F.JPG


Latest 5R overdrive drum adaptor plate teeth
2713334.JPG
 




Glacier991

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Believe me guys, he's ON to something here. This is a TON of work - what he's doing here, and I for one will be anxiously awaiting this thread's completion (though I can guess it will be an ongoing saga). Congrats Brian for getting this started, and in advance... thanks!
 




Brain

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The adaptor plate and sun gear are the same. The Ford part seems to have a little sharper edges and corners than the aftermarket unit, which means score one for Ford on this little $1.50 part, because it won't become disengaged as easily.

27133MVC-014F1.JPG


27133MVC-015F1.JPG
 




Brain

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The previously shown overdrive planet carrier is a little different in that it has outer spokes that allow a sensor (proximity sensor located on the center support) to send pulses (as each spoke passes) back to the computer to allow it to determine how fast the input shaft is turning. This is the input speed sensor as the input shaft is splined directly onto the planet carrier. Between this sensor reading and the output shaft speed sensor (shown later in the post), the computer can determine if bands or clutches are slipping and launch an error code (like “2nd gear ratio incorrectâ€￾ or something like that).

A4 overdrive planet carrier (front)
27133MVC-002F1.JPG


A4 overdrive planet carrier (side/back)
27133MVC-016F1.JPG


4R/5R overdrive planet carrier (front)
27133MVC-003F1.JPG


4R/5R overdrive planet carrier (back/side)
27133MVC-020F.JPG


Here's a closeup of the sensor ring wth spokes (it serves no other purpose that to give signals to a sensor I'm not going to be hooking up in this project)
27133MVC-021F.JPG


That plastic ring in the front encloses the adaptor plate and sun gear
27133MVC-008F.JPG


(Update #1)
I saw somthing last night that made me cringe, and then it made me think, and then cringe some more. I decided that since the OD planet carrier from the 5R that I got from Jefe wasn't going back into any tranny because of the deformations on the friction plate splines (on the front caused by having only two friction plates), that I would pull out the plastic/metal part that was pressed into the front of the unit. I wasn't even sure why this part was pressed in there, and I figured that if I was careful getting it out, I might be able to press it back into my replacement part (from the A4, which doesn't have the sensor ring). The construction was as much metal as plastic, and it was pressed in with such geat force that it was cupped after installation (meaning it had to be pried out from the outside diameter, not the inside of the ring). What I found underneath was a big fat o-ring. Here's some pictures of those parts (I didn't bugger it up too bad, and then flattened it with some pliers).
27133A-003F.JPG


27133A-004F.JPG


and the other side with the o-ring off
27133A-006F.JPG


I figured that the o-ring and plate were for fluid control, and they might be, but something about the looks of that part bothered me. It looked like a bearing. So I looked at what it would be bearing against, and found the likely suspect...the spring retaining ring. The top ridge of the retaining ring fits exactly inside that piece of plastic, and to confirm that was the case, I looked at the OD drum and where the spring retainer would be located. It was a dead-bang match.
27133A-005FB.jpg


So then I start thinking, "WHY would you want to have this part bearing against the reatining ring? The retaining ring isn't designed to be a bearing..." So I stuck the retaining ring inside the front of the carrier unit and saw how close the clearances between the retaining ring and the inside of the OD planet carrier actually were. It didn't inspire any confidence.
27133A-007FB.jpg


Now I had it figured out...they were using that plastic part to keep the retainer ring from contacting the inside of the planet carrier. Did it work? NOPE!! :fire: Here's is a close up of the contact wear that occured in Jefe's 5R (the updated non-contact design :rolleyes: ):
27133A-008FB.jpg


Those grooves that were worn into the metal tell me that contact was definitely made, and shouldn't have been. I think that this issue is the genisis of the spring retainer problems that have plagued the A4 thoughout it's history. Here is a closeup picture of the first type of retaining ring ("old") and snap ring (that holds down the retaining ring) that was used (from my A4):
27133A-021F.JPG


Notice that the snap ring has points outward (typical for outside snap rings, or you can't get them on and off). If the spring reatainer starts to spin, even if the tips of the springs that are contacting it can slip against it (which they would more likely roll because they are springs) and not cause any damage there, the spinning reatiner could have one of its raised tabs catch a point and open the snap ring so the whole assembly self-dissassembles. What did Ford do about this? Correct the interference, right? Wrong!!! :fire: They just changed the design of the retaining ring so that there weren't any tabs. It was cheaper for them to fix the $0.25 piece than to change the process of making the $30 piece- a case of economics making the engineering design decisions. After all, they just need it to last past the warranty period, so when things are older and have more "slop" from bushing wear, it isn't their concern...heck, it might even be to their benefit because you'd need to buy another (truck or transmission). So here is a picture of their "new" design that fully surrounds the outside and bottom (actually outside and front) of the snap ring (used on 5R's such as Jefe's '99 model, which is where this one came from):
27133A-022F.JPG


So that solved their problem - right? NO IT DIDN'T!! :fire: So instead of doing the "right" thing, they again redesigned the cheap part (spring retainer) to enclose the snap ring even more, this time with a small ridge on the top, but they added bumps in the bottom! They obviously haven't learned from past mistakes, because here they added bumps that could potentially catch on the edge of the snap ring cutout, and unravel the whole thing again just like in the first design. Now to their credit, at least this time the edges of the bumps aren't sharp, and are more likely to serve as a ramp for the snap ring edge. Here are pictures of the "newest" spring retaining ring:
27133A-023F.JPG


and a closeup of the top (back) lip
27133A-024FB.jpg


So here are the three iterations that I'm aware of at this point side-by-side (newest on left):
27133A-025F.JPG


Now, in Frankentranny, what I'm going to do is what Ford refused to do, and that is remove a little bit of the material on the inside of the planet carrier (the $30 part) so that there is more clearance between the retaining ring and the wall. This is going to be done on the lathe and I'll post an update when I get this operation finished (waiting on a carbide tool for the lathe, probably two/three more days). I'm going to tape up quite a bit to protect the bearings and gears on the planet carrier to keep them from being contaminated with metal shavings, but if Ford had done this originally, it could be done without those concerns (prior to welding the splined basket to the planet carrier) and associated costs.

I apologize for the rant, but this is why bean-counters get a bad rep...they ended up with a $5 solution to a fifty cent problem. Bearings/bushings are designed to be bearings/bushings, parts that aren't bearings/bushings shouldn't be exposed to this kind of wear. Fix the core problem, not the symptoms.
 




Brain

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This brings us to the overdrive ring gear, overdrive sprag, and intermediate shaft. The intermediate shaft fits through the center hole of the center support and has a small snout that fits into the end of the output shaft. The biggest changes that I see are an increase in the size of the intermediate shaft, a taper of the shaft where it decreases in size, locations for bearings in the 5R unit instead of thrust washers for the A4 (including a raised face), and a differnt housing for the sprag. The "tin plate" that retained the sprag is gone and is now a solid piece.

A4 ring gear and sprag (and "tin plate" sprag reatiner)
27133MV-006F.JPG


5R ring gear and sprag
27133MV-007F.JPG


A4 intermediate shaft
27133MV-008F.JPG


5R intermediate shaft
27133MV-009F.JPG


In this picture you can see the increase in thickness of the shaft and the raised face for a Torrington bearing (A4 on left, 5R on right)
27133MV-010FB.jpg
 




Brain

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Next up is the center support. There have been at least four versions of the center support from the A4 through the 5R. There is a thrust washer or bearing on each side of the center support, one goes on the front (#3) and the other on the back (#4), and the one on the back is selective (available in different thicknesses to get the right amount of play or "slop"). The only version used on the 5R is the one that has thin Torrington bearings for both positions in addition to a roller bearing on the snout that support the intermediate drum (like the pump supporting the overdrive drum). I have three of the four versions on hand, so I'll show pictures of each of these along with the part number.

Here's the front of the original 89GT-7G033-AA that uses only thrust washers and has a sleeve to support the intermediate drum (started production of this part in '89 from the part number).
27133MV-011F1.JPG


and close up
27133MV-012F1.JPG


Here's the back of that part.
27133MV-017F.JPG


and close up
27133MV-018F.JPG


I don't have the updated 95GT-7G033-CC that uses a bearing in the #3 position, a thrust washer in the #4 position, and has a sleeve to support the intermediate drum. Ford started production of this part in '95 from the part number, but I'm figuring this was a short production run because the newer updated one also started production in '95.

Here's the front of the updated 95GT-7G033-AF that uses Torrington bearings in both the #3 and #4 positions with a sleeved snout to support the intermediate drum. This one shows the sensor (copper colored sensing element in view) mounted on the support.
27133MV-013F.JPG


and a close up view
27133MV-014F.JPG


I've got the #3 bearing, but I didn't ever get a #4 selective bearing for this one so it is missing from my pictures of the rear. It is the same center support as Glacier991 has in his rebuild, so if you want to see pictures of this bearing, search for it in his A4LD rebuild diary. The #4 selective fits onto the end of the snout.
27133MV-019F.JPG


and a closer view of where the bearing sits
27133MV-020F.JPG


It appears as if a bearing could fit here too, but there isn't one that I'm aware of.
27133MV-021F.JPG


Now comes the best (most robust) design- the center support from the 5R. It has base mounted Torrington bearings in the #3 and #4 positions, as well as having the roller bearings on the snout that support the intermediate drum (another huge improvement). It also has a steel sleeve in the inside of the snout to keep the steel sealing rings from wearing against the native aluminum bore (as it does in earlier center supports, but there is an aftermarket kit to sleeve this bore as is shown in Glacier's posts).
27133MV-015F.JPG


a closer view
27133MV-016F.JPG


Here is the back of that part-> notice the inner sleeve (for bearings!, up next)and roller bearings on the outside of the snout.
27133VC-001F.JPG


and a close up
27133VC-002F.JPG


In all of the models, there are three fluid channels (holes) that run from the bottom of the center support up to the snout. This is most evident on the 5R (the three lumps on the bottom spoke of the support). One channel carries lube to the bushing or bearing supporting the intermediate drum, one carries fluid/pressure to activate the reverse/high clutch (fed through the outside of the support snout), and one carries fluid/pressure to activate the forward clutch (fed through the inside of the support snout). Where the bottom of the center support meets the case, fluid is transferred, but there aren't any seals in any of the designs. Forward clutch failure has been a problem in the past, and I can't imagine why this potential leak wouldn't be sealed. I'm thinking of adding some small o-rings to correct this, because I can still see where fluid leaks by at the center support / case interface. All that losing fluid to those leaks means is that the clutches won't have as much pressure to activate and hold the clutch plates together. I'll update this as I figure out what to do about it.

Now a word or two about the proximity sesnor. The sensors started showing up on the 4R, and were carried though on the 5R. Not all of my pictures show this sensor in place (the middle one does), because I removed it from the 5R before taking the pictures, and the original support for the A4 didn't have one. This is the device that senses the passing of the spokes on the overdrive planet carrier and sends a pulse to the computer to let it compute the input shaft speed. If you are attempting to upgrade a 4R with 5R guts, you will need to keep this sensor working.
 




Bronco638

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This is awesome and I know very little about slush boxes. Thanks for all the research and hard work - Dave.
 




Jefe

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Thats strange that the starter pocket is different, because the starter mounts in exactly the same place between all versions.
Brain said:
The 5R bellhousing was the same as the 4R, except the pocket for the starter was shaped differently (which makes sense because it was behind a different motor).

5R bellhousing front (notice the different starter pocket shape)
27133MVC-018FB.jpg
Oh man. . you could have at least cleaned up my tranny before taking pics :fart:

:p
 




Brain

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Jefe, at least the inside was clean! :D
I'm not sure that the starters don't bolt right up, but the shape is different, so I'm erring on the side of caution. I learned to look for the slightly different starter pocket shape from this site that Glacier991 posted as a source to get the front bushing bored out:
http://www.omegamachine.com/html/bell_housing_guide.html

I think they have some numbers confused/backwards, but at least it showed me what to look out for.

Jefe, again - a huge thanks for the 5R so that this project can teach me and others what can be done for the A4 and 4R to make them better trannies and the earlier Explorers more reliable beasts that people can feel more comfortable with in the back country. :thumbsup:
 


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Jefe

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Brain said:
Jefe, at least the inside was clean! :D
I'm glad the 30,000 mile oil/filter changes & aux filter helped somone. :)

Brain said:
I'm not sure that the starters don't bolt right up, but the shape is different, so I'm erring on the side of caution.
The adapter I used to put the 700R4 in is designed around the A4, and my stock starter bolted right up and works fine with my stock flexplate. Maybe the starter was revised and required less room, or maybe the SOHC has a different bolt pattern? Or maybe someone at Ford just had to make a little change so that we could easily tell the difference between parts. :D
 




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