Slant Six Forum
https://slantsix.org/forum/

Recommendation for 7.25" gear swap?
https://slantsix.org/forum/viewtopic.php?t=55313		 Page 1 of 1
Author:  banter [ Fri May 09, 2014 5:34 pm ]
Post subject:  Recommendation for 7.25" gear swap?
Hi All,

I have a 77 Aspen used exclusively as an in-town cruiser. It currently has 2.76 gears in the 7.25" rear, and I was wondering if anyone had recommendations about brands for a new set of gears and pinion? Since this car does at most 10% on the highway (and short drives, at that), I am thinking 3.55 as the new ratio.

I've heard Chrysler OEM are the best.. but just looking for other opinions. Anything I should be aware of during the install? Still trying to figure out whether my brother the mechanic is up for this :P or if I have to take it to a shop.

Thanks in advance!
Author:  Joshie225 [ Fri May 09, 2014 5:55 pm ]
Post subject: 
See if you can find an 8.25 with 3.21 gears. Putting money into a 7.25 is often a losing proposition.
Author:  banter [ Sat May 10, 2014 12:02 am ]
Post subject: 
Thanks. I understand the 7.25 is weaker than an 8.25 and rarer nowadays, but they seem pretty durable. Right? I just want to get a little more oomph, and never abuse it. Isn't a whole new diff a much more involved process?
Quote:
See if you can find an 8.25 with 3.21 gears. Putting money into a 7.25 is often a losing proposition.
Author:  Junior [ Sat May 10, 2014 12:27 am ]
Post subject:  x2
Quote:
See if you can find an 8.25 with 3.21 gears. Putting money into a 7.25 is often a losing proposition.
for what its going to cost to get new ring and pinion and associated parts you should be able to find an 8 1/4. the 7 1/4 is not durable. i went through a bunch of them with a stock motor before going with a bigger rear. swapping out rears will be easier then the gear change. you will most likely have to shorten your drive shaft though.

the swap is easy. undo the break line and the emergency break cables, universal joint and the 4 ubolts and the rear comes right out.
Author:  SpaceFrank [ Sat May 10, 2014 5:41 pm ]
Post subject: 
I looked into doing a gear change a while back, and it seemed to be beyond my capability. I wasn't willing to trust my ability to properly align everything and adjust the backlash. A local shop wanted about $300 labor to do the job, and a new gear set costs around $300 already.

If you can find an 8.25 rear axle from an F-body (any Aspen or Volare) with the gears you want, it's almost a bolt-in swap. Because the 8.25 axle tubes are larger, you'll need 3" U-bolts and shock plates. The shock plates can be junkyard-sourced from any 8.25 or 8.75 car for probably less than $50, or you can get them brand new and powder coated from FirmFeel for $145. I doubt you'd even need a new flexible brake line. You'd definitely need to remove your drive shaft and have it shortened 1.6", which can be done for $60-80 at any competent machine shop. I'm pretty sure all the F-bodies had the same bolt pattern, regardless of rear axle, so you shouldn't have to worry about switching wheels. Someone correct me if I'm wrong.

If you try to install an 8.25 from a different chassis, you'd be in for a much more complicated job. It would include all of the above, plus relocating the spring perches and making sure the pinion angle is correct, which is not as easy as it sounds. Of course, you'd also have to make sure the axle isn't much wider than an F-body axle so your wheels will still fit; I don't know how much room you have to spare right now. You can compare the widths on this table, but keep in mind that the dimension given there is flange to flange width, not overall width. The differences only carry over if every chassis used the same brake drums, and I don't know if that's the case.

FYI, an 8.25 out of a Jeep Cherokee is very close to an F-body 8.25 on width- about 1/2" wider per side. It's also easy to find in a junkyard and almost guaranteed to have 3.55 gears. Just make sure you can identify the Chrysler axle as opposed to the Dana 35 that was also used in those. This is the direction I'd recommend taking if you can't find an F-body axle and you can handle doing the spring perches.
Author:  banter [ Wed May 14, 2014 12:58 pm ]
Post subject: 
@spacefrank - thanks for the info. I'll check my local yard for an f-body, but they are hard to find. And then I'll take some measurements and compare to the table of centerline/flange measurements.
Author:  MarcD [ Thu Aug 07, 2014 10:35 pm ]
Post subject: 
I'm in a similar situation with a 2.76 7 1/4 large bolt pattern axle from a '75 Valiant in my '63 Dart; but worse because my car has a close-ratio A833 four-speed installed by a previous owner...launches like second gear!

The Cherokee as an 8 1/4 axle source is new info to me and very interesting as I've had no luck in finding one ( I live in SF about 20 miles from the OP); do I understand correctly that the spring perches have to be relocated?
Are there any years in particular to look for?

Thanks, Spacefrank!
Marc
Author:  mpgFanatic [ Thu Aug 07, 2014 11:48 pm ]
Post subject: 
Quote:
I looked into doing a gear change a while back, and it seemed to be beyond my capability. I wasn't willing to trust my ability to properly align everything and adjust the backlash.
Exactly. I've seen many, many instances where someone yanked out an entire rear axle assembly to replace with a different junkyard item. It's not only the parts cost and the time and finesse required... getting those gears set up properly in the housing requires several hundred $ worth of tools to measure everything. I must say, it's extremely satisfying to finally get the courage and to overcome the challenge, though. :D
Quote:
It currently has 2.76 gears in the 7.25" rear, and [...] I am thinking 3.55 as the new ratio.
No one has mentioned, they've only slightly hinted.... that is a huge change, my friend. :shock: Huge! Every time I have changed axles, each tiny 10% step makes a very noticeable difference in how it drives-- from a 2.93 to a 3.23 (or vice versa)... from a 3.55 to a 3.31... I don't think I would want to make a jump bigger than two common "sizes". In your case, from a 2.76 to a 2.93 to a 3.23 qualifies as two jumps. 3.55 would be a third jump.

With one jump you'll notice a difference, with two it'll be quite impressive, with three I expect it would be somewhat shocking.

What's the current tranny? What first gearset does it have? For a manual transmission, multiply the 1st gear by the rear end to get an overall reduction ratio-- I've really enjoyed numbers in the range of 10:1 or so-- extremely satisfying to drive. 9:1 can be fine... in fact that's plenty for a V8 daily driver unless your goal is to smoke tires at the track... but if you get over about 11:1 or even 12:1, you'll discover that you wind out of first gear so soon it's almost useless. The light turns green, you get halfway through the intersection and it's already time to shift.

All of those guidelines would change if you're talking about an automatic, due to torque converter slippage. I'm afraid I don't have the same seat-of-the-pants experience to guide you with that.

- Erik
Author:  sandy in BC [ Fri Aug 08, 2014 9:17 am ]
Post subject: 
You need to know tire size....what speed you cruise at and what rpm the engine is happiest at.

I bet you are happier with 3.23 gears. 3.55 works nice with an overdrive.
Author:  ski [ Sat Aug 09, 2014 8:10 am ]
Post subject: 
I have a complete 7.25 with 3.23 gears and suregrip sitting out back probably never to be used. Only issue for you would be its out of a 65 Dart and has SBP. I know there are some differences between the LBP and SBP 7.25 but I ASSUME you could swap center sections? (and pinion) Let me know if you're interested in it. It's in socal and I need to get rid of some stuff.

ski
Page 1 of 1 All times are UTC-08:00
Powered by phpBB® Forum Software © phpBB Limited
https://www.phpbb.com/