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Old 04-08-2020, 03:37 PM   #21
BRENT in 10-uh-C
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Default Re: Rear end ratio-gear question

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Originally Posted by Ruth View Post
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".
I sure don't know about that. I have ran several 4.33 gears in my speedsters with plenty of success. In hill climbing, I can start in 2nd gear and shift to high which is faster than shifting from 1st to 2nd. I raced my speedsters on dirt with 4.33 gears running at WOT on -mile dirt tracks without an engine issue. I guess it is all in what you like with a speedster, but 4.33 gears were fun for me. My son is restoring the old 'Crash' Brown Model-A sprint car, and we will be putting a 4.33 in that too.



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Originally Posted by Ak Sourdough View Post
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.

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.

Yes, I agree with the hypoids sets being lapped however nothing that I have ever found in researching this has ever given any proof that Ford lapped them during Model-A production. I would expect this would have been an expense he was unwilling to include, and I am certain this is the major contributor to why 600wt. lube was used in the transmission & rear ends. With the higher viscosity, they would not have needed to have a lapped tooth to still be quiet.



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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 ???.
Yes John, I have on a couple of different occasions. In one instance, my father drove a '29 Phaeton to the MARC Nationals in Jacksonville, FL. En route, the pinion bearings came apart and killed the R&P gears. We sourced used parts from several vendors in the swap meet and the mis-matched gears were assembled. That assembly was as quiet as a mouse. Another time was on the Great Race when a team from Iowa broke a tooth on a drive gear. Parts came from everywhere that night from local hobbyists and we assembled it and it did well. With the proper weight lube, and the backlash correctly set, you should not experience any issue unless one of the gears is severely worn.
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Old 04-08-2020, 04:01 PM   #22
john charlton
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Default Re: Rear end ratio-gear question

Thanks Brent ,good info, so he should keep rolling until he does a full rebuild .

John
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Old 04-08-2020, 04:10 PM   #23
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Default Re: Rear end ratio-gear question

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Originally Posted by Ak Sourdough View Post
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.

Or see your local GM dealer for some Gear Marking Compound. It's a yellow paste of the right consistancy.
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Old 04-08-2020, 05:06 PM   #24
ursus
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Default Re: Rear end ratio-gear question

Helped a fellow with a noisy rear end some years ago. On inspection, the pinion was bad because the pinion bearings were shot. He found a rear axle lying in the weeds in a vacant lot and pulled the driveline/pinion from it. I was skeptical, having become accustomed to seeing matching ring/pinion sets in every rear end I opened. We assembled it with new bearings, checked the contact with Prussian Blue paste, and installed it in his coupe. To my surprise, it ran quiet and smooth. Maybe we got lucky?
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Old 04-08-2020, 05:08 PM   #25
rotorwrench
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Default Re: Rear end ratio-gear question

A person can run a higher numeric gear ratio and install a Mitchell overdrive if you want the best of both worlds. Back in the day they would have likely used an under/over gear box but that would have been more for pickups or trucks. With a speedster, the sky is the limit.

The gear marking compound is made from yellow pigment (pastel chalk as was mentioned) and synthetic oil mixed to form a paste. I started making my own back when red lead was no longer available. In packaged form, it's available from gear shops, Summit, and Jeggs plus many others. GM has a part number and there is an AC/Delco number but I don't worry too much about where it comes from. I just don't want to pay a lot for it. It works pretty well. I liked red lead paste better but such is life.

Last edited by rotorwrench; 04-08-2020 at 05:28 PM.
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Old 04-08-2020, 06:53 PM   #26
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Default Re: Rear end ratio-gear question

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Quote:
Originally Posted by BRENT in 10-uh-C View Post
I sure don't know about that. I have ran several 4.33 gears in my speedsters with plenty of success. In hill climbing, I can start in 2nd gear and shift to high which is faster than shifting from 1st to 2nd. I raced my speedsters on dirt with 4.33 gears running at WOT on -mile dirt tracks without an engine issue. I guess it is all in what you like with a speedster, but 4.33 gears were fun for me. My son is restoring the old 'Crash' Brown Model-A sprint car, and we will be putting a 4.33 in that too.
I'll admit I was thinking more of a street drivien speedster. Still doesn't explain why he needs 4.33 gears to put it on a trailer...
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Old 04-09-2020, 04:41 AM   #27
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Default Re: Rear end ratio-gear question

Hi John,
I have also replaced a 4.11 set with an unmatched 3.78 pair in my RPU. I got the two parts from two different guys, they looked like a match with no apparent wear. It runs smooth and quiet apart from the other noises a Model A makes.
Take care and be safe guys.
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