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Swapping from BB to SB, what's involved?

68stang289

68stang289

Member
Yes, you did read that right. My 70 Mach currently has a built 428 SCJ/Toploader 4 speed in it that is not original to the car. I would like to swap out the 428 for a 351 so I can put the car up for sale to help pay some of my incredibly large law school loans that will be coming due in the next 3 months. I'm fine with leaving the Toploader in there as I believe that trans was originally with the car (it is a factory 4 speed car) but what all will I be looking at to swap it over? Here's the list that I could think of off hand:

Engine
SB Bellhousing (or could I reuse the BB bellhousing?)
Z-Bar, Z-Bar brackets, upper and lower clutch rods
Exhaust system

Am I missing anything? Is there anything from my BB that I could use to transfer over to the SB?
 
The trans won't work, at least not worth your while. 428 uses big input toploader, you would need a special pilot bushing at a minimum.

Don't forget all the engine pulleys and brackets.

A new trans means new trans mount too.

I don't think this swap adds value upon sale so what am I missing?
 
I was unaware of that. Would there be any way to use the small block trans on a big block? Is there any way to tell the difference without removing the trans?

If I swap a BB toploader for a SB toploader wouldn't the trans mount be the same?

You are correct, it does not add to the value by swapping back to the SB. That being said, I only paid $7300 for the car. I want to use the 428 for the next project (a '68 vert) and since it is non-original to the car, I don't feel bad taking it out. I also have so little into the car that the value the car loses by doing the swap doesn't affect my bottom line.
 
After doing a little more research I see that you are right and that I cannot use the trans crossmember from a big block toploader. That being said, the guy who did the conversion over to a 428 cut a lot of corners and I'm wondering if he just tried to jerry-rig the small block 4 speed into the big block. So I am looking at:

351 motor
Motor mounts
SB 4 speed
SB trans crossmember
SB bellhousing
SB flywheel
Z-Bar and all the related stuff
Exhaust
SB power steering bracket

Did I miss anything?
 
I will swing over tomorrow to where the car is currently being stored and crawl under it to try and get some pictures to post up for you guys. I don't recall the trans having a tag on it unfortunately. I also seem to recall that the frame mounts for the motor mounts did not sit flush on it.

One other question. I have had some issues with getting a starter motor that fits properly. Is it possible to use the small block bellhousing on a big block?

Thanks guys!
 
No. The small block bell housing will not bolt up to a big block. Look for the metal tag on the transmission usually riveted at the front near the bell housing flange to ID the trans.
 
Ok good to know. I will pull as many part numbers as I can tomorrow and hopefully will be able to sort all of this out. Thanks for all the insight guys! I am sure more questions will follow tomorrow.
 
Removing the 428 and replacing it with a lighter weight 351 may effect the front stance/ride height of the vehicle.... especially if the PO had to install new BB coil springs when he installed the 428.
 
Another point I thought about last night actually Dave. The PO had cut the original small block springs to get that rake look. I installed a new set of BB springs on it and those would need to be switched out as well.

Unfortunately, the guy who painted my engine and trans for me painted over the trans tag despite my explicit instructions not to and my already having taped it off. I can't seem to get it off and don't really want to take sandpaper to it for fear of filing down the numbers. I did try to count the splines but without having a grease pen to mark my starting point I counted between 26 and 29 each time (I wasn't drinking (yet), I swear!) so I'm guessing with a 28 spline tailshaft that it's a small block toploader.

I also looked for the motor mounts on the car and there is a gap on the frame side motor mounts which I believe indicates they were the original small block mounts? Would anyone be able to get me a picture of their 428/FE frame side motor mounts on a 69/70? I of course left the camera sitting on the table and my cell phone camera is pretty worthless.
 
"68stang289" said:
I did try to count the splines but without having a grease pen to mark my starting point I counted between 26 and 29 each time (I wasn't drinking (yet), I swear!) so I'm guessing with a 28 spline tailshaft that it's a small block toploader.
Click to expand...
This could still be a Big Block trans but just a small input/output shaft for a 390 car and this will still not work with a small block car without a special pilot bushing.
 
"02Lightning" said:
This could still be a Big Block trans but just a small input/output shaft for a 390 car and this will still not work with a small block car without a special pilot bushing.
Click to expand...

I doubt it. The motor was pulled from a wrecked 69 or 70 Mustang. The car was originally equipped with a toploader 4-speed and since he cut a lot of corners (he hacked up a 351 Z-Bar to "work", made his own lower clutch rod that was a hybrid of 64-66 lower rod parts and 351 lower rod parts, used a 66 upper clutch rod, you get the idea) I wouldn't be surprised if he kept his SB 4 speed and just used that. I will head back with some sandpaper to where the car is being stored in the coming weeks and try to get the numbers off the painted over tag.
 
Not sure how you would get a small block trans to fit a big block because the input shaft is too long. Unless you cut the input shaft down or put an adaptor plate between the bell housing and the trans.
 
"02Lightning" said:
Not sure how you would get a small block trans to fit a big block because the input shaft is too long. Unless you cut the input shaft down or put an adaptor plate between the bell housing and the trans.
Click to expand...

People do cut the shaft down but that itself isn't pretty!

Sell the car as is!
 
I do not doubt that it will bolt together, but have you checked the pilot input on the transmission after running it for a while? I got to believe there has to be a reason for the difference in transmissions as far as the pilot length goes otherwise Ford would not have made them different. Maybe a difference in SOME BB crank shafts? I don’t know. That’s good information to know that they will bolt together though.
Here’s one for you, my car is a BB car but it had a 302 in it when I purchased it. Well, in my search for BB specific parts I purchased some BB motor mount brackets to go with my 428 I was putting in the car. Well after looking at my old mount brackets I figure out they are BB mount brackets too? So my car had a 302 in it with BB motor mount brackets. I’m not sure how well it fit together because I didn’t pay any attention to it when I pulled the motor, nor did I know the difference at the time.
 
Yea, I know the only difference is the bolt hole location but both my sets of mounts are in the BB position. I just wondered if someone just pried on the rubber mounts to make them fit a SB.
 
I just don't understand the logic of what you're trying to salvage when you owe money on your college/school loans! You just need to get out of debt. Just sell your car, and go on! you can buy another mustang when you're practicing getting your life on track. You don't make money off of Mustangs~ it will just continue to cost and cost and cost and you'll be lucky to sell your Mustang for what you want anyway~ it's a tough market the way the enonomy is right now.

"gtscode" said:
People do cut the shaft down but that itself isn't pretty!

Sell the car as is!
Click to expand...
 
Wow, I spend a few days in New Orleans and this thread has exploded with some great information! I will be out of town for conferences and other vacations for the next two weeks so I won't be able to get back under the car to sand the paint off the trans tag to figure out just what trans is in there. I am curious about the differing lengths of the pilot though. How much of a length difference are we talking about? What about diameter of the pilot, is it the same for BB and SB?

To your question Dne', I'm not looking to sell the car because I absolutely need the money to pay off my loans. I stashed a good amount of money (enough to keep my afloat, loan payments and all, for about a year) from my summer associate job last summer and most first year associate jobs at firms pay more than what I need a year to pay off my bills/loans. What I did not want to give up is this built motor as I was contemplating putting it in my next project. However, after more contemplation and looking at what it would cost me, I am going to sell the car as is. And I have actually made money off of my last 2 mustangs and should make money off of my 70.
 
Another question that just occurs to me, would it be possible to swap in a BB shaft to a SB trans and still have all of the SB trans parts bolt in and spin freely? Please forgive the stupidity of this question if it is in fact a stupid question. And yes, I will blame it on all the booze my liver has had to contend with for the last several days, haha.
 
The trans mount will bolt in but it will change your driveline angle, not sure on the lenght diff. and a BB trans mount and a 6 cylinder mount is the same thing. My BB trans. mount has 250 stamed in it, I assume 250 ci. 6 cylinder.
Just re-reading your question again and I think you are talking internal trans. parts and that, I have no idea.
 
"02Lightning" said:
The trans mount will bolt in but it will change your driveline angle, not sure on the lenght diff. and a BB trans mount and a 6 cylinder mount is the same thing. My BB trans. mount has 250 stamed in it, I assume 250 ci. 6 cylinder.
Just re-reading your question again and I think you are talking internal trans. parts and that, I have no idea.
Click to expand...

Yes, I was talking about internal trans parts.

But you bring up another interesting point about changing the angle. Any idea just how off that angle would be? What kind of problems might I experience if that angle is wrong? Just a lot of driveline vibrations? My biggest concern at this point is that there is a hodgepodge of parts that were used to assemble the car and I would be selling someone else a mess (which I absolutely will not do).
 
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