White_Knuckles

I caught the recent thread discussing 10-12 bolt diffs. Very interesting information which inspired me to expose my dumb questions.

What I have is - the original '67 axle with a healthy Olds 350 and soon a BOP 350T spinning the thing. Verified Posi but unknown ratio. The car stock had the 2-speed Jet-Away trans.

What I want to do - attack the rear-end for a fresh ratio upgrade where low end "pull" increases (hoo-hoo) but at 70mph the RPM is reasonable.

What I don't know - is the best move simply to change out the ring and pinion or would the subject matter experts swap the whole axle for some reason? I'm certain the clutch packs and other wear items need attention where a complete rebuild should be in order. What am I missing here?

Where I'm stumped - would be the ratio itself. I hope someone can advise the next step(s) up in ratios? Adding third gear in the new trans is bugging me. For example, if the next ratio up yields 3500 RPM @ 60, that would be disappointing. What I have in there may be fine as setup for a 2-speed where the new trans has a "real" 1st gear? I'm very curious how others solve for desired ratios?

need4speed

What RPM are you spinning at 60 MPH now? I ran 3.55s in my 69 which was around 2700 at 60 MPH. The height of your tire has an effect on this as well, so post that here too.

Octania

What if you were to determine your existing final drive ratio by one of the Approved Methods discussed here a few times?

I imagine the easiest route is to swap the entire rear end assembly, then you only mitaswell yourself into all new brakes, bearings, and park brake cables. But, a posi with a "better" gear ratio will be expensive too.

Changing the gears on your posi unit gets into some pretty serious difficult work- are your skills up to the task? Have you read thru the service manual procedures? Toolage? Beasty impact... inch lbs torque wrench? Then you have the issue that the carriers come in different flavors depending on the range of gear ratios they can utilize, so your posi carrier may not be suitable for the new gears you get.

https://classicoldsmobile.com/forums...-rear-end.html

In my limited experience, 3.08 is pretty lax, but 4.33:1 is no fun at all on the highway with a 3-spd TH400

mrolds69

Step 1 is to find out what you have. Jack up the rear with the tires off the ground, car in neutral...chocked. Turn either wheel 1 complete revolution and count the number of turns the driveshaft makes. The number of driveshaft turns is your ball park ratio. 3 + 1/4 turns =3.23, 3 turns or a bit less =2.93, or whatever. These are just examples, may not be actual ratios for your car. So...when you have the turn # match it up with known ratios for 67. You need to know this, because there are different carriers for groups of ratios, and no posi carriers are cheap. When you get the ball park ratio, ask on here what other gears fit. Like...under 3.00 is usually one carrier, over is another, over 3.42 or something may be another still. You can use spacers and stuff like that I think....maybe. Find out what you have, and the cost to change to a higher (Numerically) ratio.

White_Knuckles

Tires are 245X60X15 which equal the factory wheel/tire diameter.

The rear end would be described as, 10 cover bolts arranged in two groups of five (top & lower) with a "U" cutout parallel with the axle separating the bolt groups. There's a metal tag pinned under one of the bolts but I'm not sure a stamped number may be of value? The cover has a bulge on the ring side and the drain fill is on the passenger side. I'm pretty sure it's the type "O" stock unit.

The RPM at 60 is unknown as I'm not running a tach. By the drone of a dual and loud exhaust system, I'm thinking "cruise" rev around 2700 at 60? It's not revving high or lagging. Sorry, a proper RPM baseline would be helpful, I was speaking in general about gaining too many revs at freeway speed.

White_Knuckles

Octania, oops maybe some searches are in order? Thought I had this amazingly new question to explore? Wasn't thinking about prior "methods discussed". I hoped there may be numbers stamped on the gears that could be cross-referenced? The rotate the wheels trick is very cool to get a feel for it and a good starting point.

I really hoped our experts would nail the mystery gearing as original spec. documented somewhere? I can't find anything published. If I had the current gearing the next ratio options may come easy from suppliers and availability. Guess not.

And no, I'm not doing the work at adv. hobby level, this one will go to the pros!

mrolds69

There are numbers stamed on the gears if you want to pull the cover to know for sure. There is also a code stamped on the axle tube that would let you know the ratio. The tag just says use posi fluid, no ratio on it.

Gary M

The code on the axle will tell what came in it but someone may have changed them. The turn the wheels trick is good but you need to go 10 turns and then divide by 10, from what one of the moderators on this forum posted in the past if my memory serves me. That will give you a more accurate result. Or take off the cover.

mrolds69

I don't think you need to go 10 turns, at least not with a posi. I never have and I've always found the ratio. The code on the axle tube can be hard to find, you may have to wire brush the tube. And, like the OP said, it could have been changed.

White_Knuckles

It's becoming clear the carrier size must be able to support the new gear set size. The below table listing viewed on http://www.oldsmobility.com/old/reardiffs.htm indicates a '67 Holiday with an automatic is likely to have "3.08" as standard but will fit ratios up through 3.91. If I'm I reading this right, 3.91 might be fun with a 3-speed and fit nicely in the housing?

3.08 3.21 3.23 3.42 3.55 3.90 3.91

Gary M

Before spending money on the new trans mrolds69 had the best advice. Pull off the cover and count the teeth. I turned my wheel once and driveshaft to get a 2 3/4 which to me says 273 but the guy that sold it says he put 323 in. I still need to pull the cover to solve my mystery. Only one true way to see. That's about all I can contribute. Let us know what you find out.

White_Knuckles

I'm afraid I already spent the $$ for the TH350 just waiting to get it installed and test the existing ratio which I'm sure will propel the car both forwards and backwards.

It's going to be interesting seeing how a real first and third gear will affect RPM and low-end pull? I'm thinking the old gearing will work but continue to lack the low-end I want? Whole other problem but I'm still procrastinating about the stall RPM! Probably go safe with near stock 2000 stall. Ugh, stupid cars!

Octania

I did a search for "count turns" and found several threads right away.

e.g., post #5 of
https://classicoldsmobile.com/forums...ial-ratio.html

wherein it is written, that you can find numbers stamped into the ring gear. Due to the nature of the beast, the smaller number will be oh around 9 to 15, and the larger number around 30 to 40. At least one of them is generally a prime number- 11, 13, 39, 41, etc. in order to ensure even toothwear and extend the life of the gearset.

If you wish to count turns, refer to the above post and others, and note that the driveshaft has a handy built-in feature- the U joint. It marks each 1/4 turn easily. So, turn your elevated wheel[s] until the U-joint is nice and aligned with something, like say horizontal. Then mark the brake drum and backing plate somewhere. Then turn the apparatus until you have attained one turn of the RING GEAR as described in said other post, meanwhile counting turns of the prop shaft. When you get near the end, go by 1/4 turns of the prop shaft- 2 and a half, just a teeeeensy bit more, is 2.56:1 ratio. More than three but less than 3& 1/4 is 3.08... etc. You can for sure tell the difference between 2&3/4 and 3.23 = 3 & 1/4. One is on one side of 3.00000 turns, the other is on the other side. Telling 3.08 from 3.23 may require the 10 turns trick, or remove the cover and read the actual numbers. Or, count teeth if you have extra time to spare.

Your diff cover "with a V" sounds like Chevy C type 10-bolt to me. look for ribs between the cover bolts. Look for differential ID how-to already existing on the interwebs. Look for a Supercars Unlimited catalog with the pictures of the basic types. Maybe on their web site.

PS I think if you removed a Jetaway 2-speed auto trans and put in a TH 350 you are in for a less than impressive improvement. The Jetaway had a switch pitch converter, in effect giving it a lower low gear, much like a 3 or 4-speed auto trans, but w/o overdrive... much like a TH350. Only, now you have the kickdown cable to attach properly and set right....

Gary M

From what you found on Olds-mobility it won't be a problem to change gears if needed. Small potatoes when looking at the benefits of finally getting what you want. Some aftermarket gears are noisy though. I don't know yet because I haven't put in the gears I got from WestCoastDifferentials.com but they told me the OEM gears I got from them don't whine. Ask before you buy if you change gears. Good luck with it and definitely share your experience with us on where you got em and if they are quiet.

White_Knuckles

Yes, I've considered this where my switch-pitch is functional noticed when it's kicked down from 2nd switching the T-convertor to a passing gear affect. Off idle, it likely has a higher 1st than the T350 offers? Thrid may be higher then the Jet's second as well?

It's cool how the 2-speed had a near three speed operation. I'm getting rid of it as it leaks (no matter what) and is really non-serviceable anymore.

I see some good methods to figure out where I'm at but still wonder how far up in ratio should be considered for street fun?

Fun71

The TH350 transmission gear ratios are 2.52, 1.52, and 1.00.
The Jetaway transmission gear ratios are 1.765 and 1.00.

So simply switching to a TH350 will be equivalent to having a lower rear end gear ratio in 1st gear.

For example, if you have a 3.08 rear gear, the overall ratio in 1st gear with the Jetaway would be ~5.44:1 and with the TH350 it would be 7.76:1. That's the equivalent a 4.40 rear gear with the Jetaway! So do the trans swap first and then see if you think you need a lower rear gear.

White_Knuckles

^ Wow! That's interesting Fun71, excellent points. I assumed 1st on the 350 may be well received but guessed it would only be slightly lower?

I have never looked at trans gear ratio specs and am embarrassed to see high gear is 1.00 on both. Therefore, probably all cars without over-drive gears are 1.00? I thought they had varied final drive ratios.

Now if I could just figure out the stall speed, the column linkage mod (to get it into low) and a speed-o-meter drive gear part source, life would be good! Thanks for all the comments you all have shared. I'll buck-up and get that trans stuffed in and check back.

MDchanic

Yes. 1:1 final drive ratio has been a standard for probably about a hundred years.
That's why transmissions with higher final drive ratios are called overdrive transmissions.



Also note that when confirming the final drive ratio by turning the driveshaft, don't forget that both wheels have to be turning.
If you ack up one wheel and turn the driveshaft, that wheel will rotate twice as fast, so you will need to divide by two.

It's always better to rotate through ten revolutions of the wheels.
What's easier to count, 2.73 turns vs 3.08 turns, or 27 turns vs 31 turns?

- Eric

mrolds69

If it's a posi like he said, you're going to have a REAL hard time turning one wheel if the other wheel is on the ground!

MDchanic

Ha ha. Good point.

Which brings up the other simple detail:

Open: Turn one wheel, the other turns the opposite way.
Posi: Turn one wheel, the other turns the same way.

- Eric

hookem horns

As far as what ratio to pick, a TH350 with a 3:42 rear end will spin ~3000 RPM at 65-70 MPH depending on tire height. IMO, a good compromise between perfomance and highway manners if not going with an OD transmission

pmathews

Now's the perfect time to upgrade the converter.

During my recent engine rebuild, I should have swapped the converter to a higher stall converter. The old converter was literally staring me in the face during the install of my rebuilt engine, but I had tunnel vision, and the thought didn't even cross my mind. Now, I wish I'd paid the extra $150.

It's so easy to swap during the trans install, much harder afterwards.

1964f-85

I ran 2:90 posi with a 455 and a muncie 4 speed and it was fine. I would install the transmission first and drive it, you may like it. With olds torque, it wil run well. I currently have a mosier 9" with a protrax posi unit and 3:25's nice set up with the 5 speed. I may swap 3:50's in at some point as the over drive is a highway gear only now, it basically idles at 1000 rpm in 5th grear at 49 mph.

White_Knuckles

Nice! That's what I hoped was out there. A little more kick off the line if ya know what I mean?

White_Knuckles

This was interesting. I looked in curiosity what the options are available with a Mosier, and was surprised to see 3:50 at the "pro/street" starting point. This baby looks like an awesome product if your serious. I always thought guys were hacking in these legendary Ford 9", lockers by welding mounts, brackets etc. Nope, you order 'em GM ready. Unfortunately we're talkin' 3 grand. Wife might spot that trick - yikes!

I remember the intimidating "4:11's" often touted back in the day. Man, those are mid-way on Mosier's chart meaning TALL. I don't know what those guys were doing running that setup in the 70's? They had to be awful at speed.

MDchanic

When I was in high school, my Chevelle came with a 4:56 Dana posi with ladder bars.
Damn thing would lay two strips of rubber if you as much as looked at the gas pedal.
If my memory is correct, it'd turn 4,000-4,500 RPM keeping up with traffic on the highway.

I swapped for a 2:73 posi that a guy sold me for $10, and I was much happier, especially as a high school kid without much money who had to buy his own gas, even though, through the M21, it was like starting out in 2nd gear.

- Eric

Koda

Simplest solution to finding out the answer to the gearing is to get the two letter code off the backside of the passenger side axle tube.

Some codes for 67:

Open rear, standard
2.41 SS
2.78 SA
3.08 SC
3.23 SE
3.42 TL
3.91 TN

With posi, standard equip
2.41 TA
2.78 SB
3.08 SD
3.23 SF
3.42 TM
3.91 TO

The 67 csm calls for 108 differential combos, but I bet you have one of those posi ones listed there, and I bet even further it's probably a 3.08

White_Knuckles

Eric - hey, whatever happened to those $10 complete posi deals?

Koda - thanks. That would be the second resource I've seen with data matching the specs I found. The oldmobility chart indicated the 3.08 SD as "Standard" as well. They didn't show the trailing letter codes you posted - nice.

Still seems strange there were so many options where station wagons, 3-4 speed manuals and autos may vary but still that many ratios?

I'm still liking 3.42. Throwing a dart that is.

MDchanic

They went away with the $500 Chevelle SS deals and the $400 '59 Caddy deals.

- Eric