4 spd/3 spd interchange?

[Small Block]

Looking at a slant six Demon that has the factory 3 speed in the floor so all the pedals and such are in place. My questions to the more knowledgeable is this. If i were to consider, which i am, changing to a 4 speed what problems, if any, will i run into? Will the same bellhousing work? Is the spline configuration the same on the 3 speed as the 4 speed? Pressure plate the same? Shifting fork location the same? Shifter hole in floor the same location? You get my drift so if you know please clue me in as to the magnitude of this swap. Thanks guys
Small Block

 

[dgc333]

If your bell housing has another set of mounting holes that are outboard of the ones holding the 3spd to the bell housing the 4spd will bolt up. The clutch and all the clutch linkage will work just fine with the 4spd.

Depending on what tranny you get the input bearing retainer that holds the front bearing into the housing maybe bigger or smaller than the one on the 3spd which means it won't fit the registration hole in the bell housing. You may be able to swap the one off the front of the 3spd or you may need to get another on of the right size (Year One has them) or if the tranny you get has the bigger one you could get it machined down.

The output shaft on the 4spd will be bigger than the 3 spd requiring a new slip yoke. You can get these from a drive shaft shop, Brewers, Year One or off a car with either a 4spd or 727 auto. If you get one used it may very likely have the yoke for a 7290 size universal joint. You can get a conversion joint at NAPA so it adapts to the smaller 7260 yoke on the drive shaft.

Of course you will need a 4spd shifter.

NOTE: the 4spds came in two lengths; A/F-body and B/E/C/Truck/Van. The longer B/E will work but it will require having the drive shaft shortened and you will need to make an adapter to mount the shifter, bigblockdart.com has a drawing of the adapter.

 

[Blownfish]

If your bell housing has another set of mounting holes that are outboard of the ones holding the 3spd to the bell housing the 4spd will bolt up. The clutch and all the clutch linkage will work just fine with the 4spd.

Depending on what tranny you get the input bearing retainer that holds the front bearing into the housing maybe bigger or smaller than the one on the 3spd which means it won't fit the registration hole in the bell housing. You may be able to swap the one off the front of the 3spd or you may need to get another on of the right size (Year One has them) or if the tranny you get has the bigger one you could get it machined down.

The output shaft on the 4spd will be bigger than the 3 spd requiring a new slip yoke. You can get these from a drive shaft shop, Brewers, Year One or off a car with either a 4spd or 727 auto. If you get one used it may very likely have the yoke for a 7290 size universal joint. You can get a conversion joint at NAPA so it adapts to the smaller 7260 yoke on the drive shaft.

Of course you will need a 4spd shifter.

NOTE: the 4spds came in two lengths; A/F-body and B/E/C/Truck/Van. The longer B/E will work but it will require having the drive shaft shortened and you will need to make an adapter to mount the shifter, bigblockdart.com has a drawing of the adapter.

Not all 3spd's have the smaller yoke, I have a 3spd out of a 74 Duster and it takes the 727 yoke. I thought the smaller (904) yoke was only on the older 3spd's? Just drop the driveshaft and see what yoke you have before buying the parts you need.

I would stick with an F or A body tranny, just my opinion though.