My '53 F100 is equipped with flathead 8, light duty 3spd and 3.91 diff.. I have a 9" diff. with 3.25 gears that I want to swap with the stock rear. Also I might be running 16" stock wheels.(currently 15").
I recently acquired a '53 parts truck that has the 3spd heavy duty transmission. Would there be any advantage to running the HD transmission with my proposed setup? What is the drivability difference between these 2 transmissions?

 

First remember that 3rd gear in both is 1:1 ratio
The difference will be in 1st and 2nd gears - I don't have the ratios at hand but the term Heavy Duty usually means that 1st gear will allow you to drive up the side of your house or to take off with maxed out weight and get up tp 5mph before needing to shift into 2nd gear.
Unless you intend to work your Truck like Trucks back then were built for - the standard 3spd should be your choice for a driver.

 

Hi Dick,
That HD 3spd does not have a granny low gear. I will look it up and list it for comparison.

 

Gear Ratios
Side by side comparison of light and heavy duty 3spd transmissions.
Source FOMOCO '53 Shop Manual.

Transmission--------Gear Type----------1st-----2nd------3rd------Rev.

3spd light duty------All Helical----------2.78---1.62-----1.00-----3.63
remote shift

3spd heavy duty-Helical-2nd&3rd-----3.71---1.87-----1.00----4.59
remote shift-------Spur- 1st & Rev.

Does anyone have any experience with the HD trans.?

 

Without doing any of the math, I'm thinking that if you change from a 3.91 to a 3.25 you just might want the low first gear that the HD model offers. Check it out on any of the number of gearing calculators that have been posted here.
I will say this, I had a '55 Fairlane with 272 and 3 speed, and going from a 3.89 to a 2.90 was a HUGE difference. I had to slip the clutch a lot more with the 2.90 to get it moving.

 

That is what I was thinking. I wonder if the spur cut 1st gear would be noisy?

 

You are right on the spur gears being noisy. You will need ear plugs if you are going to be diong a lot of low gear work.

 

I sure hope so!

 

i have the heavy duty 3 speed in my 49 with 239 flatiron. i've never driven with a light duty 3speed so i don't have any comparison. my 1st doesn't seem particularly loud (among all the other noises, ok, so i can barely hear the radio) but the spur gear non synchro part is obvious, no shifting into first unless you are virtually at a stop. dick r

 

That shift into first is the same for the light duty 3 spd, have to be at a dead stop. Dick, do you have a 2 piece drive shaft on your truck? The one I have has th E brake mounted to the bak end of the trans. Link to picture.https://www.ford-trucks.com/forums/p...ictureid=13095

 

mert, your tranmission looks quite different (heavier?) than mine. mine is in a '49 not sure if it's original but it does look like the one in my motor manual (manual says that the f-1 had a HD trans as an option). i didn't know i had a heavy duty until i looked for a speedometer gear, the part number on the old one was id'ed as from a heavy duty transmission. i have one piece drive shaft and the e brake on the back wheels. just learning about it now, trying to figure out which rearend gears are in it. dick r

 

Thank you for the replies, I am still kicking around the idea of using this trans. behind the flathead with a 3.25 rear.

 

MTFLat has this combo in a BonusBuilt, although he may have a 3.50. He also lives at a mile+ altitude and in the mountains, but he can give you an idea if you send him a note.

 

Merten, do you have an update to this? Heavy duty vs LD.

bob