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Discussion Starter · #1 ·
So way back here it's said:
Big block toploaders have the shorter pilot shaft. (the part that fits into the pilot bushing)

The BB unit is only around 3/4" long, while the SB unit is a little over an inch.

You CANNOT use a BB trans behind a SB. ...
What I want to know is can I put a SB transmission, (with the longer pilot shaft) behind my 390?
 

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Yes you can. All you need to do is to cut the pilot shaft down to the 3/4" length so that it mates properly in the crank bushing. Taper the end some to make insertion into the pilot bearing easier.

BTW, the factory 390BB's toploader did not use the 1 3/8" diameter input shaft that the 427 and up toploaders had, nor the 31 spline output shaft. The 390 used the same input shaft and output shafts as the small block, with only the pilot shaft being different.

The input shaft shown on the bottom is authentic 390:



On the street in most conditions the SB tranny will work fine as the toploader is a tough beast. Put on sticky tires and hit the dragstrip and you will have issues with the small 1 1/16" input shaft. Luckily it can be swapped to the better 1 3/8" BB input stuff as the other toploader guts are identical.
 

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I will add that some combos will fit without modification

You need to measure from the tranny mounting surface to the crank pilot hole for overall depth, then from the same surface to the pilot bushing

Then measure the tranny, overall pilot length and length of the splines

If nothing bottoms out when you compare numbers, you can slide it in without modification.

Larry (Monstermach) was able to do this on his 460, I have not tried it on a 390.

Additionally, a 390 truck bellhousing will NOT need this done, because the 390 trucks actually used a different bell with a SBF length input

Bottom line - Measure what you have and it is easily figured out
 

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Wow some great info in this thread...I always wondered why the 390 toploaders would work with the small block input shaft without bottoming out in the crank..I see now that the 390 input shaft is indeed shorter just the same dia as a smallblock toploader...Dennis answered that one with a great picture....I used to run a smallblock toploader behind a 390 when I was a teenager and wondered how I got away with it...But now that My427Stang mentions it,I was running a truck bellhousing....
 

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Kind of off topic but does anyone know if a TKO600 will bolt to a 428 with truck bell housing? The suppliers can't tell me for sure, they only say they have two different input shaft lengths they can install.
 
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