Hello again folks. So I'm in the midst of a transmission swap on a 1981 F-150 XLT Ranger with the 300 inline-6, and I've hit a bit of a road block. I just got the old transmission removed, and in the process, I learned that I did not have a C6 3-speed automatic like I thought I had when I set out on this mission. Instead, my '81 has the lowly C4. Its square transmission fluid pan and removable bellhousing gave it away.

Problem is, I've already bought a C6 3-speed from a junk yard bound parts truck. Supposedly, the donor truck was also a two wheel drive 1981 model, but it was equipped with the 351 Windsor. Which supposedly should not matter in terms of bellhousing patterns, as I've read from multiple sources that the C6 used in these trucks had the same pattern for 300, 302, and 351 equipped trucks.

NOW, first off, can I swap a C6 into a truck that was originally equipped with a C4? They look very similar, and if the bellhousing patterns are the same, I feel like it should be possible.

Second question; if the swap is possible, what modifications will I have to make? The most glaring difference seems to be that the C6 is slightly longer. Here's a picture of the two side-by-side:


Does this mean I'll need a different driveshaft? Furthermore, another noticeable difference is the input shaft. On the C4, it is recessed. On the C6, it spans out to the very edge of the bellhousing. Here's another picture:


Finally, my C4 equipped '81 was modified at some point with a floor mounted shifter. This made removal quite easy, but I noticed in my donor C6 there is a set of wires routed at the shift linkage. What function did these wires serve?


Any help or advice is greatly appreciated! I'm really hoping to get this thing back in action. Its in rough condition, but that 300 is a heart of gold with plenty of life left to give. I really don't want to see it go down because of a bad transmission.

 

If it was me, I think I'd sell the C6 and rebuild the C4. They're a good tranny and just right for the 300.

 

I may still do that, but I'd like to at least know my options. I have no experience with the C4, but I am a pretty big fan of the C6. I've had them in all of my Ford trucks, with the exception of my E4OD equipped '90. The C6 is extremely durable. I'm more wary of the C4, partly because this one failed, and partly because I'll be using this truck for pretty heavy work. Its had its payload capacity significantly raised, and I'd like to know I've got the transmission to get some work done, and a C6 is that insurance.

 

The C6 input shaft has been pulled out.

Wires are for the neutral safety switch.

 

So the input shaft shouldn't be freely moving, as it is in my C6? The input shaft in my C4 is pretty firmly lodged in there.

Would you say this conversion is feasible?

 

I don't believe it is a conversion, but a restoration. The 1981 F150's did not come from the factory with a C4, as shown in the screen grab from Ford's master parts catalog, below. So, your truck came with a C6 and you can put one back.

Let's take this up back on the 80-86 forum where there are lots of guys that have worked on these trucks.

 

Input shafts on C4 & C6 are easy to pull out and replace.


As for feasibility? Anything can be done given enough patience. Not for me to say.