Four speed to three speed switch.

1crosscut

1crosscut

Well-known member
I asked this over on the AA site and haven't received a definitive answer. I think it is probable but...
Is it possible to pull the engine and four speed transmission out of a 1929 worm drive and replace it with an engine coupled to a three speed transmission without doing any modifications?
The truck the four speed would come out of does not have the dual high auxiliary transmission.
 
You will need a different U joint and I think the housing the U joint fits into on the back of the transmission and of course the 3 speed bell housing. I am not sure about the pedal setup. Now the fun part, you will need to get the charts and check to see what the gear/rear end ratios will end up being doing this. As 3rd and 4th are direct drive in their respective transmissions, you will need to see where 1st and 2ed for the 3 speed and 2ed and 3rd for the 4 speed put you. The difference between the 2 might not warrant the work. Guys putting the 4 speed in cars/pickups are doing so to get the granny low which would not be used for normal driving anyway(with the 4 speed).
 
Would your truck have had a 3-speed originally?

You do not gain much on ratios...

1st is 6:4 in the 4 speed

2nd is 3:09 in the 4 vs 3:12 in the 3-speed 1st gear

3rd is 1:69 in the 4 vs 1:85 in the 3-speed 2nd gear

high gear, 4th and 3rd respectively are both 1:1

I wonder about getting a U-joint that will match the smaller 3-speed output shaft to your existing AA driveshaft..
 
Model AA's built before Oct or 1929 would have used the 3 speed car transmission/bell housing/pedal setup/u-joint. The difference is which jack shaft that goes between the transmission and main drive shaft and U-joint that goes between the two shafts, you would need to the correct shaft and U-joint to be used with the 3 speed setup.
For the amount of searching for parts and work, you net gain will be not that great as shown above. Your better bet would be to find a set of hi-speed gears for the rear end.
 
1955cj5 has a very good point. The swap would in essence be the same as if you currently just started in second gear and shifted to 3rd and 4th. This would pretty much be the same driving experience.

For that big truck, you may occasionally appreciate having that super low gear if you ever do pull something big.
 
I should clarify a bit on this. The truck currently has an engine with a 4 speed in it. The engine is stuck and he wants to rebuild it eventually since it is a matching number engine. I have a good used engine with three speed transmission that he would like to put into the truck for the time being. By switching transmissions one would not have to switch bell housings and go through the process of dialing in the bell housing to the flywheel. (I think there are issues with his transmission too)
Not worried about getting any more speed out of it by going with the three speed. Loosing the ultra low 1st gear is not a big issue as he is not going to be hauling heavy loads by any means. I think that his u-joint should match up to the three speed. I think that it will work but didn't want to overlook something obvious.
 
As far as dialing in to the flywheel, I don't believe you'll have to worry about the bell housing (the housing to which the transmission is bolted). The housing that you have to be concerned with for that is the one bolted directly to the engine (the flywheel housing). While the A 3-speed and AA 4-speed bell housings are different, the flywheel housing to which they bolt is the same.
 
Support the transmission/bell housing unbolt the bell housing, pull the engine then reverse the process.
 
You will need to determine if the AA drive shaft is the same diameter as the A drive shaft. I don't have the right parts book to look it up..

I know the 4-speed transmission output shaft is larger than the 3-speed, and there is a special u-joint for adapting the 4-speed to a single A driveshaft....

Of course you could just swap engines as Red suggests...and maybe do bearings in the 4-speed at the same time...you can change the input bearing and the main gear shaft pilot bearing without disassembling the transmission, just take the front bearing retainer off...
 
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Hello Dave,
If someone has installed a 4 speed to a worm axle I am thinking that they had to have modified a coupler to fit, does it look stock,? if it has not been modified then maybe a Hi-Low unit will fit, the early trucks did have a coupler on the high speed axle before the Hi-Low was used. can you get a pic of the coupler that's in the truck now.
 

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The gent I'm working with on this is in Texas and I'm in Nebraska so I don't have first hand access to the truck so it makes this a wee bit more challenging.
Looking like just swapping the engine along with the flywheel housing is going to be the way to go. Was just trying to avoid the necessity of dialing in the relocated flywheel housing to the flywheel after the switch. I'm not sure he is up to doing that along with a possible transmission rebuild.
Doug - your right about not dialing in the bell housing. My brain was in neutral and I was typing bell housing when I meant flywheel housing.
 
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