The Stovebolt Site Forums The Shop Area Engine Shop best trans to bolt up to my inline 250 motor - sm 420 swap

Ok, I've looked through the Search and Tech Tips but didn't find anything that really applied to my situation. I have a '49 3/4 ton pickup with a inline 250 with a sm 420 tranny behind it. I want to make this truck more highway friendly. I'd like to find out what are my best options as far as trannys go, like highway gears, ease of adapting the clutch and make sure the shifter will not end up in the seat. I know about the T-5 swap but didn't know if that will bolt up to my engine and don't know how to adapt a clutch and bellhousing. I already have a 60's model rearend that I plan to put under the truck as well. I'm not trying to break speed records but would like to cruise 60- 70 without the engine screaming. I don't care to put an auto tranny unless I had to either. Any help is really appreciated!
Jeremy

Also if I do swap the tranny, can someone give me just an idea of what a new driveshaft would cost?

There's a heavier duty O.D. trans called the NV4500 from modern bigger trucks. I just don't know where the shifter would end up in your cab with that trans.
Someone that's done one should chime in soon,
Scott

The T-5 is the easiest, cheapest way to go with a trans. but if you are changing rear ends why not find a ratio about 3:23 -3:55 & leave the same trans in it.

Where there is an SM420 a T5 can replace it, and it is bolt in with the proper parts, clutch disc being the most difficult to get right, everything else should work as is. This has been covered any times here, try a search.

If you have a 250, then does that open you up to different bell-housings and any transmission that was as behind a small v8? I don't know..

What ratio is in the rear you have to swap?

Use this site to figure speeds and ratios to get what you want
http://www.onlineconversion.com/bigger_tires.htm I like the last one on the page
Do some checking, but I think you will find that the 250 likes to be run about 2500 rpm at cruising speed (you say 60-70) when empty and on the highway. So that speed, or whatever you find out is best, is what you are shooting for when you plug in the numbers into the calculator link above, the OD ratio of the trans, or direct drive (1; like for the SM420), rear axle ratio, and tire outside diameter.

Grigg

The rearend I have to swap in I beleive is a 3.27 ratio but I honestly didn't think that would make that big of a difference considering how slow it runs now.
The clutch linkage isnt going to be much of a problem to work out then? only the disc itself?

''''''' Ok, I've looked through the Search and Tech Tips but didn't find anything that really applied to my situation.'''''''''
OH contrare' ... there is TONS of info on this subject
best
is a hard defination to answer
what is it you want to do w/ your truck is the question
then ask about possibilities because you have just asked a boatload of questions
search rearends
search T5
search tramsmissions
search drivelines
search clutches
read
you live in relatively flat terrain ?? a 3:20-1 rear could (not would) work. if you head to hill country go to a 3:70 or more. towing is also an issue, so is torque of the motor, tire size, clutches......
$225 for a driveline
T5 from $95.00=1000.00 ... NV's $100.-$2300.00
clutch,T/O bearing, correct fork and pressure plate $ 100.00- $225.00
your driving pleasure ............. priceless
so read read read and do the conversion once.
my .02
ask more question

And What did you find?

To me it looks like you can switch rear axles and leave the transmission alone, this gives you approximately 65 mph at 2500 rpm.

Grigg

With a 3.27 rear end a o-drive would be worthless with a 250 engine unless you were running some silly low pro tires.

What wrenchbender says.

To get back to the NV4500 that was mentioned earlier... If you use the 94-97 dodge 2wd version it will push the shifter back about 6 1/2 inches from where the sm420 shifts if you use an adapter plate. You could run a different bellhousing behind the 250 and mount the gm version of the Nv4500, but you would have to convert an electronic signal for the spedometer.

I would rule the T5 out for a 3/4 ton truck. I don't believe that transmission is built strong enough to do the work.

Overdrive transmissions work pretty well with lower gears. 3.73 gears with a 27% overdrive would effectively become 2.73 gears. 4.11 gears would become 3.00.

My preference and one I haven't ween mentioned is keeping the SM420 and adding a Gear Vendors overdrive. This allows you to keep the granny 1st gear, as well as split all the remaining gears since it is a true overdrive. They are expensive new, but can be found used for around $500 in my neck of the woods.

Devin