I am looking to swap my stock 2005 3.55 diferential with a 3.73ls unit from an 04 axle. Both are 9.75 units.
Can it be done? Is it a direct swap? Recomendations like new bearing set?

 

i would imagine its a direct swap as far as bearings i personally would just get new carrier bearings and pinion bearings

 

Should be a direct bolt-in swap. Is the condition of the axle questionable...is that why you're asking about bearings, etc???

If so, and if you feel there's a need to go internal on the new axle, just keep your axle and upgrade the gears and all to 4.10's. 3.55 to 4.10 is a very subtle change, most would say not worth the effort. Swapping axle's makes it tolerable, especially if you're getting a good deal on the axle. But, if it's questionable and you need to go internal anyways, that benefit is now gone...IMO.

 

it is a direct swap.

couldn't hurt to go ahead and do axle seals and bearings. those are easy to do and should prolong the life of the axle. the seals have a tendency to go on the passenger side and it gives you peace of mind to go ahead and do the bearings while you're in there.

 

3.55 to 3.73 is subtle, 3.55 to 4.10 is noticeable.

 

Thanks for all the feedback. Swap is for towing, 3.55 to 3.73 should extend the range 1000# and still keep me good on 500 mile trips without the camper.
Both axles are in good shape. I do not have any experience with this so far and thought it was prudent to ask around. Thanks again.

 

if you dont have any experience with it and you plan on changing diffs i would find someone you know that has some experience or get a haynes manual it will take you through step by step on how to do it your gonna need to set the backlash and all that

good luck

 

What do you mean by this? It will not increase any towing or payload capacities.

 

If you've got 4x4, you'll have to swap the front too.

 

Why won't it increase towing? My 08 when equipped with 3.55 is good for 7700. when equipped with 3.73's. it's up to 8700.with 18" rubber on both.

 

There's more to it than just the axle ratio. The LEGAL towing capacity cannot be increased with ANY modifications.

You are going to reach other limits long before you exceed 7700# on the trailer anyway. There's GVWR, GAWR, and payload limits.

 

When I wrote "extend the range" I am referring to the maximum loaded towing weight listed that the truck can tow with listed engine/trans combo as per the Ford 05 RV trailer tow guide.
In this instance an 05 f150 4 x2 supercrew with 5.4 and 3.55 is 8,500, with a 3.73 9,500. Combined weight rating is 14,000 & 15,000 respectivly. Fifth wheel towing for the super cab is 8,300 and 9,300. Supercrew fifthwheel towing is not listed for this year as appropriate hitches were not available at that time. If we read the fine print it goes on to say weight ratings are decreased by 500# for trucks with 18" wheels. It's a bit tough to decipher when one considers all of the various and different combinations of vehicles.

I am open to corrections - thanks for helping.
Erik

 

Do you mean you want to change the entire diff with axle housing? If so, go for it and don't worry about bearings, etc. If you mean just changing the internals, I wouldn't bother just for that small change. This is a job for professionals. I'd go for a 4.10. With a 4.10 and overdrive the overall ratio is about 2.87 which isn't bad for the highway especially with today's large diameter wheel tire combination.

Another question: Have you already towed the 5th wheel with your Supercrew? I thought the Supercrew bed was too short for 5th wheel towing.

 

Yes I tow a 34' fifth. Lots of debate over this. Take a look at sidewinder hitches. Have several thousand miles in doing so. I have not informed my truck of this so don't tell it (it thinks we just have a heavy load). Trailer maxs out at 8800# which exceeds the trucks capacity, we do not load the trailer to that capacity however (stays around 8000 depending on burgers and chips). The whole combo is real close though, if not over a tad. Lots of fellas will be quick to point out the potential here. A bigger truck will do the job better, I understand. I of course will have difficulty in justifying my modifications. I'm not sure I could justify many of the modifications we make (tires, superchargers, fuzzy dice) other than I enjoy the hobby. By the way, I will be changing the differential unit itself and not the entire axle assy. Thanks for all of the advice.