Model A rear ends strength?

S

s1b

Senior Member
I read about these guy with Mildly warmed over engines and Over drive or 4spd trans going down the highway at 55-60 mph some say even higher.
So what I would like to know is how are rear ends holding up in these cars? Are there modification made to them to hold up? I was always told in the past, that stock Model A rears are worthless and to be only used in 100% stock As. A lot of these cars I read about appear stock on the outside.
 
I am having good luck (so far) with a modified "B" engine and the Model rear end. I have not dynoed the set up but literature says it will be about 90 HP or so.

I think a key is not to "pop the clutch". Others are to keep oil in it and make sure it is in good mechanical condition. Check the end play of the passenger side. Very much would indicate a worn carrier.

The Model A rear end has three shear pins. On is on each axle and one on the ring and pinion.
 
I am running a stock Rear End (3.78) behind a 4 sp Trans and a mildly "warmed over" engine. Specs and Literature says she is about 60 to 65 hp. I have 16,000 miles on this combination and lots of it is highway miles at 55 to 60 mph. I am not gentle with it.
Like Cajun says, the key is to keep engine oil changed regularly and greased like the book tells you to and good mechanical condition.

After 16,000 hard miles she has begun to leave a 6" dia spot every time I stop. This summer her engine bulider and I are going to tear the engine down and correct the shims, hopefully to stop the puddles.
 
Over the years I've seen hopped up model A's, A's running V8-60's and even mild flatheads. The key is NOT to abuse. Weak link I've found is axle key, sometimes before axle will snap. Like anything if it's not abused should be ok. JMO
Paul in CT
 
A steady pull at any speed is not going to hurt an A rear end.

As Cajun says, don't pop the clutch and you'll be fine. It's the sudden shock that does the damage, not a continuous application of the kind of power you can get out of an A or B engine, no matter how modified.

Joe
 
I would not worry about the rear end. I have used my speedster with a stock 3.78 rear for hill climbs for 8 years. That includes dumping the clutch at 4,000 rpms. The engine has a olds 455 head. The engine puts out well over 100 HP.

The only problem I had was separating the torque tube from the rear end. That was the first time I ran it. Fixed that with 4 vise grips, and won the hill climb.
 
s1b said:
I read about these guy with Mildly warmed over engines and Over drive or 4spd trans going down the highway at 55-60 mph some say even higher.
So what I would like to know is how are rear ends holding up in these cars? Are there modification made to them to hold up? I was always told in the past, that stock Model A rears are worthless and to be only used in 100% stock As. A lot of these cars I read about appear stock on the outside.
Click to expand...

Model "A" rear ends will handle most modified street engines. The weak link is no outer support on the pinion. Worn older rear ends often have too much side clearance. Under extreme pressure of hard starts and acceleration the ring will flex away from the pinion and it will flex enough that the pinion will try to "climb" the ring gear and break the casting or the gear. Model "A" rear ends were popular with sprint car racers because of all of the ratios available. One modification was to install a bronze snubber that limited the side movement of the ring gear. The axle nuts must be torqued to a minimum of 125#. I torque mine much tighter. Tried a heat treated key but it just sheared . Another trick is to lap the hub to the axle taper.
 
Not a problem as long as (like most other things) you do not abuse it. If you have it apart, I would definately have the axles magnifluxed as they tend to crack. Very good quality replacement axles are available.

I am running an early 32 rear end (which has the same internals as a Model A) in my Hot Rod with a warmed up 59AB Engine for about five years now. I have been known to "Smoke the Tires" a little and have not had a problem but I did go thru it before I started to run it.

Chris
 
JoeWay said:
A steady pull at any speed is not going to hurt an A rear end.

As Cajun says, don't pop the clutch and you'll be fine. It's the sudden shock that does the damage, not a continuous application of the kind of power you can get out of an A or B engine, no matter how modified.

Joe
Click to expand...

I agree and along the same lines (and this applies to Model A and modern) if the tires are spinning on loose gravel, mud, or ice and then grab on dry pavement, this can put a good shock on the driveline components.
 
s1b said:
I read about these guy with Mildly warmed over engines and Over drive or 4spd trans going down the highway at 55-60 mph some say even higher.
So what I would like to know is how are rear ends holding up in these cars? Are there modification made to them to hold up? I was always told in the past, that stock Model A rears are worthless and to be only used in 100% stock As. A lot of these cars I read about appear stock on the outside.
Click to expand...

Under normal driving conditions at any speed the rear axle assembly will last your lifetime as long as it is properly restored. ANY type rear end in good condition will do the same unless subjected to extreme loads or forces. No such load is present when crusing or even under acceleration.

As noted, the rear axle keys are generally the weakest link. However even this generally requires an additional problem or weakness to fail. An insufficiently tightened rear hub will place the stresses on the key instead of the axle taper as designed. If a key shears from this condition the drum will spin which now damages the axle taper as well as the taper in the hub. Once this occurs the axle and hub will never again have the desired fit or friction to carry the desired load. Instead a portion of the load is transferred to the axle key. The amount of load transferred to the key will be commensurate with the damage to the tapers. There are probably tens of thousands of Model A's out that with some level of damage here that still survive without further failures!

I've heard of a couple rear axle shaft failures over my forty years in the hobby but those I've seen had a "preexisting condition" like a groove worn in the shaft from a worn out shaft seal where the seal tension spring rode directly on the axle shaft for years.

Like most everything, the ring and pinion gears are designed to FAR EXCEED the needs of the vehicle. Again, there needs to be an additional problem for the gears to fail. Properly fitted gears just won't fail unless they shift off their centers under extreme loads. This only happens if the tapered roller bearings are badly worn or setup with insufficient preload. The bearings would need to be REALLY bad for the gears to fail under normal use.

The only way you are going to make a GOOD rear end assembly fail is popping the clutch HARD on a regular basis. There are plenty of guys out there running real strong engines and racing without any signs of rear end problems.

P.S. When I was a kid I DID shear an axle key in my '29 Roadster. I was spinning donuts in the mud in Carson City, NV. :o

 
The axles and keys accept v/8 brakes , if lapped and the axle has GOOD key ways, it will handle 150HP, the axle nuts need to be torqued! The ring and pinion I think is the problem, if any, because it is missing the third pinion bearing, on the nose, like the 34 and later rears! Dudley
 
With tapered rear axles, ideally the hub and axle are "locked together" by the interference of the shaft & hub taper and the axle nut. The key is intened to be more of a locating device than to be the sole element of torque transmission between the axle and the hub/ wheel assembly.

With my pre-1962 MoPars, if the axle-nut is not tight enough, you can hear a very audible "crink!" or "crunk" when changing direction ( forward to reverse / vice-versa ) - that is the axle shifting inside the hub; if not corrcted, the key will eventually shear. MoPar axle nuts are supposed to be torqued to a minimum of 150 #ft.

SC Frank
 
I have seen three cases of the pinion climbing the ring gear and breaking the teeth. In all three cases the pinion bearing had failed first. Not sure if the bearings failed from excessive load but two of the engines were dead stock.
 
Most of the rear ends I have pulled apart that had a ring & pinion failure were caused by lack of lubrication. In other word the owner ran it dry. In several rear ends I found that someone had tack welded the three spider gears to their hubs. This eliminates any differential action and creates a poor man's posit-traction. It was probably do by some kid back in the 40's or 50's who wanted to lay down a strip of rubber from each back wheel. The tires probably chirped when turning a corner. The ring & pinion seemed to have suffered no ill affects.

Tom Endy
 
Higher number of miles

I have a '29 Phaeton with over 46,000 miles on a mildly modified inserted engine that I estimate should be putting out 55-60 HP with the parts used. The rear end ratio is standard with a 26% Mitchell overdrive. I put the Mitchell and re-built rear end in use at the same time as the present engine. My favorite speed is 35 to 50 mph but I often run at 60-65 mph on the interstates when we are returning from events and are traveling alone.
Of the total 46k miles traveled I would estimate the following: 70% of my miles are run at or below 55mph, 20% are run between 55 and 65 mph, 10% between 65 and 75 mph. I do not hill climb or slam through the gears. I have not changed the rear end oil all that often but I do check the overdrive and rear end oil levels on a regular interval. The times I did change the oil, I have heated the oil and filtered it to look for metal and did not find much if any at all. I have had no problems to date with the engine, Mitchell or the stock rear end. I hope this helps answer your question regarding the stock rear end. I think if you keep them lubed with the proper "heavy & sticky" oil they can stand a lot of hard use.
Good Day!
Dave in MN
 
Last edited:
s1b said:
I read about these guy with Mildly warmed over engines and Over drive or 4spd trans going down the highway at 55-60 mph some say even higher.
So what I would like to know is how are rear ends holding up in these cars? Are there modification made to them to hold up? I was always told in the past, that stock Model A rears are worthless and to be only used in 100% stock As. A lot of these cars I read about appear stock on the outside.
Click to expand...

The maximum torque applied to the pinion does not increase with higher road speed. It is the max engine torque multiplied by the ****** gear ratio. If you have a really hot 4 banger capable of 100 ft-lbs, in high gear. 1:1 ratio, you can put 100 ft-lbs to the drive shaft and pinion. In overdrive, say 0.72:1, no matter how fast you go, at the max you put 100 x .72 = 72 ft-lbs to the rear.

The maximum continuous load is actually delivered in low gear. Stock, that would be about triple the engine torque.

Axle torque is the pinion torque times the ring/pinion ratio divided by two (two axles). A 3.27 rear ratio is easier on axles than a 4:11.

If you wind up a 50lb flywheel with a totally stock engine and pop the clutch you can easily get a temporary 500 ft-lbs from an engine not capable of 1/10th that. If you have an S10 T5 trans with 4.03 low, a 4:11 rear, and pop the clutch, each axle could see 4,141 ft-lbs if the tires do not break traction!! That's 57 times more load than driving a hot four 65mph in O.D. Goodbye axle keys (or gear teeth:D)
 
90d25499.jpg


Speaking of popping the clutch, I wonder if that's what took out this keyway. At least the differential case and gears are good.
 
JoeWay said:
A steady pull at any speed is not going to hurt an A rear end.

As Cajun says, don't pop the clutch and you'll be fine. It's the sudden shock that does the damage, not a continuous application of the kind of power you can get out of an A or B engine, no matter how modified.

Joe
Click to expand...

Joe ,I agree with you 100 %. Popping the clutch is like putting a chisel on the teeth and hitting it with a hammer! I'm sure we all did it as kids and paid the price for "burning rubber". I know I broke many of transmissions , axles and rears when I was a kid.
 
In the photo of the axle with the key area broken away, notice that the break starts at the inboard end of the key. There is apparently a stress riser at each side of the key at this point. I have seen a number of axle shafts with a crack just starting at either side of the key at the inboard end. This is an area that should be inspected very carefully when rebuilding a rear end. I usually bead blast or wire wheel the axle shafts and when clean in this area it is easy to see if a crack is starting to eminate. Certainly a reason to reject the axle shaft if even a slight crack is detected.

My opinion is that what starts the cracks is the axle nuts not being tight enough. This allows the hub to move on the shaft and put pressure on the side of the key slot. It will also wallow out the key slot as it rocks back and forth.

Tom Endy
 
You must log in or register to reply here.
Back
Top