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Old 04-07-2020, 02:46 PM   #1
nosoup4u
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Default Rear end ratio-gear question

I have a speedster with a model A rear axle that I am trying to slow down a little. The present rear axle is the standard 3.78. If I should find a 41 tooth ring gear, is it a direct replacement? Is the pinion the same or does anything else need to be changed?
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Old 04-07-2020, 03:18 PM   #2
BRENT in 10-uh-C
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Default Re: Rear end ratio-gear question

No, the 9:34 (3.78), 9:37 (4.11), and 9:41 (4.33) gear sets all have a different gear pitch even though they share the same number of teeth. The Pinion will need to be matched to the correct Ring Gear for them to work correctly.
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Old 04-07-2020, 03:19 PM   #3
jimvette59
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Default Re: Rear end ratio-gear question

No you change a matched set of what gear ratio you want to run. JMHO
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Old 04-07-2020, 03:24 PM   #4
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Default Re: Rear end ratio-gear question

Ring and pinion are always replaced as a set. Not only will the gears from different ratios not fit together, ring and pinion gears are machined to match each other as a set.


If I understand correctly, you don't want a speedster, you would prefer to have a slowster.



Try smaller diameter tires.
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Old 04-07-2020, 03:35 PM   #5
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Default Re: Rear end ratio-gear question

To slow down you let up on the gas peddle a little. Cheaper than a R&P.
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Old 04-07-2020, 04:01 PM   #6
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Default Re: Rear end ratio-gear question

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To slow down you let up on the gas peddle a little. Cheaper than a R&P.
Try driving a car on a trailer in second or third gear and you will get an idea of what I'm talking about.
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Old 04-07-2020, 04:27 PM   #7
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Default Re: Rear end ratio-gear question

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Originally Posted by nosoup4u View Post
Try driving a car on a trailer in second or third gear and you will get an idea of what I'm talking about.
What's wrong with your transmission ? Use 1st gear and let it idle up, or put it reverse and back it up.
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Old 04-07-2020, 05:41 PM   #8
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Default Re: Rear end ratio-gear question

If your motor won't pull a light car like a speedster onto the trailer with a 3.78 rear end gear set, you have a sick engine, IMO. I pull a trailer weighing about a ton up a hill near my home which is no less a grade than you are trying to climb. I use a 3.78 rear end!
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Old 04-07-2020, 06:10 PM   #9
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Default Re: Rear end ratio-gear question

I don't know about you guys, when it comes to a trailer, I use a winch.

I cannot tell you how many I have seen damaged by driving on the trailer.

For far less that a R&P, you can get a small electric winch and bolt it to your trailer.

just my $0.02, J
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Old 04-07-2020, 07:57 PM   #10
David R.
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Default Re: Rear end ratio-gear question

Is it just me or does none of this make any sense? Driving on a trailer in high gear?
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Old 04-07-2020, 09:34 PM   #11
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Default Re: Rear end ratio-gear question

some call it a trailer,others a ramp...I say send it!!!
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Old 04-08-2020, 08:34 AM   #12
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Default Re: Rear end ratio-gear question

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Originally Posted by Ak Sourdough View Post
Ring and pinion are always replaced as a set. Not only will the gears from different ratios not fit together, ring and pinion gears are machined to match each other as a set.


Can you elaborate on this machining process of "machined to match"? I have not found any mention of this on the factory prints of the A-4215 Differential Pinion and the A-4210 Differential Driving (Ring) Gear. On the A-4209-* print, it shows a drawing that lists both the Driving Gear and Pinion.

I think where some of this mis-information originated from is in the Parts Lists books, it mentions "Matched" on the A-4209 sets however it appears this was done because of all of the Ford ratios used a 9-tooth Diff Pinion (9:34, 9:37, 9:39, & 9:41) however none of the four pinion gears were interchangeable. This kept confusion down when the agency parts clerk was issuing parts. It didn't mean they were specifically lapped or honed to match as a set.


Therefore it is my opinion these sets were NOT matched on the Model-A differential gears but maybe you knowledge of this operation that counters my thoughts.
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Old 04-08-2020, 09:02 AM   #13
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Default Re: Rear end ratio-gear question

When Ford put sets of stuff together, they usually had some way to quickly test the fit & function. In the case of ring & pinion, they likely used a gear marking compound and turned them together in a test fixture to make sure of contact. Now I don't know what Ford did in the model A era but later on in the 30s, sets were marked with a match number or serial number. If one ring didn't look right with a pinion then they would try another one. If a good match couldn't be establish quickly, they likely chucked the offending part in a scrap bin.

Initial machining would have been as precise as they could maintain it so that the percentage of scrap would be at a minimal level. Gear hobb cutters and machine bit tooling wears out so it had to be closely monitored by whomever ran the machines or by the folks that inspected the parts after they came out of the machining process.
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Old 04-08-2020, 09:07 AM   #14
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Default Re: Rear end ratio-gear question

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Originally Posted by David R. View Post
Is it just me or does none of this make any sense? Driving on a trailer in high gear?
I was trying to make a comparison. The way it is geared and the RPM of the engine make driving it on a trailer comparable to driving some other cars on a trailer in high gear. In my estimation, if you tried to drive a standard model A on a trailer in second or third gear, you'd be slipping the clutch a lot and/or trying not to kill the engine.
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Old 04-08-2020, 12:19 PM   #15
Jim Brierley
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Default Re: Rear end ratio-gear question

I don't know if R&P gears were matched at the factory or not but I put together a 4.11 rear end one time using gears that had not originally been run together. They had a slight howl at around 35-40 mph, otherwise ran quiet, in my tudor. Hypoid gears as used today are a different story.
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Old 04-08-2020, 12:27 PM   #16
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Default Re: Rear end ratio-gear question

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Originally Posted by nosoup4u View Post
In my estimation, if you tried to drive a standard model A on a trailer in second or third gear, you'd be slipping the clutch a lot and/or trying not to kill the engine.
I am still confused. Why would you try to "drive a standard model A on a trailer in second or third gear" in the first place. unless you have a lightened flywheel or something a Model A should be able to chug up a reasonably steep ramp onto a trailer in 1st gear.

But if you really want to run 4.33 gears, I say go for it. Just be aware your engine will be screaming at 35-40 mph in high. Not much fun in a "speedster".

Last edited by Ruth; 04-08-2020 at 12:39 PM.
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Old 04-08-2020, 12:56 PM   #17
Ak Sourdough
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Default Re: Rear end ratio-gear question

Brent, I don't know anything about the manufacturing process used in the Model A assembly plant. However, you are right in your first post that the gear tooth shape could not be the same for different ratios even though the tooth count for the pinion is the same.


The ring gear retains the same inside and outside diameter, which it would have to do to bolt to the carrier and fit inside the case. To fit more teeth on the same size gear they would have to be smaller teeth. So, even though the pinion still has 9 teeth, they would have to be a different shape to match the shape of the teeth on the ring gear.



My experience is entirely with the more modern hypoid gears not the early spiral bevel gears used in the Model A. They are always sold and installed as sets because they are lapped together in final manufacturing steps. That according to a movie shown in automotive trade school in the early 70's about rear axle manufacturing and assembly.
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Old 04-08-2020, 02:50 PM   #18
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Default Re: Rear end ratio-gear question

Just wondered if someone has put together a 3.78 rear end out of "orphan" parts and did it work OK ??? . Friend has just lunched his rear end going over an unexpected "speed bump" I do have ring and pinion that I am fairly sure came out of the same axle but a little doubt lingers . He just wants to put the axle together quickly so the vehicle is mobile for short trips he is planning on a total rebuild later so now he wont bother with new bearings etc etc .I usually mark parts but these ones I didnt .

John in totally locked down and the spring weather has been perfect for weeks for taking my English 28 phaeton with no top around the country lanes DRAT Suffolk County England .
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Old 04-08-2020, 03:15 PM   #19
J Franklin
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Default Re: Rear end ratio-gear question

If they are paired the only problem might be a bit more noise but they should work ok. The new R&P I put in my A was a matched set and the tag said they were lapped in together to run without noise.
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Old 04-08-2020, 03:32 PM   #20
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Default Re: Rear end ratio-gear question

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Originally Posted by john charlton View Post
Just wondered if someone has put together a 3.78 rear end out of "orphan" parts and did it work OK ??? . Friend has just lunched his rear end going over an unexpected "speed bump" I do have ring and pinion that I am fairly sure came out of the same axle but a little doubt lingers . He just wants to put the axle together quickly so the vehicle is mobile for short trips he is planning on a total rebuild later so now he wont bother with new bearings etc etc .I usually mark parts but these ones I didnt .

John in totally locked down and the spring weather has been perfect for weeks for taking my English 28 phaeton with no top around the country lanes DRAT Suffolk County England .

I'd say try them, it there are no tight spots open it back up and put something on the teeth to read the pattern. They used to use white lead, but along with being a hazard, you probably can't get it anymore. A good substitute can be made by mixing chalk dust with gear oil to make a thick paste.


I hope it works out for you.
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