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Joined: Aug 2016
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Hello,
New member here, I have a 68 400 convertible on it's way from California. It has a 2.56 gear rearend that i'd like to change to 3.55 gear. I need to know the axle spline count and if the rear has c clips or bolt in axles, also what trans gear do I need to correct swap so speedo is correct?
This is something i'm looking at
ebay ad 391355941901
Thanks, Todd.
Last edited by T-FATTY; 08/15/16 10:22 PM.
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Joined: Sep 2000
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Welcome..
That's quite a jump from 2.56 to 3.55. Guess it depends on what trans and diameter tires you have?
From what I've seen all Firebird BOP 10-bolt axles are 28 spline. When I had mine apart, the axles did not have c-clips and they're not shown in the factory manual. If I recall only the GM 12-bolt used c-clips.
re: speedo gears: search google for pontiac speedo gears by color - numerous sources come up.
PeteJD
[69 400/400]
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Maybe best to wait to see what you've got. But I know you're wanting to hit the ground running.
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I don't think the same carrier can be used without some sort of spacer. May be easier just to swap out the whole rear with one that already has the wanted ratio.
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Joined: Sep 2000
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I don't think the same carrier can be used without some sort of spacer. May be easier just to swap out the whole rear with one that already has the wanted ratio. Just the carrier needs to be changed. The small carrier goes up to 3.31 and the large carrier starts at 3.55. [source: factory manual]
PeteJD
[69 400/400]
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Large carrier starts at 3.36
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Joined: Aug 2016
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I was looking at removing carrier intact and put aside and buy a new complete carrier/gears and install kit. I tried to post what I found on e bay but couldn't link it but I posted auction number above.
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The e-bay "kit" contains the rebuild parts (bearings/bolts) new ring & pinion and a new Eaton posi unit. You cannot upgrade to a 3.55 ring gear with the OEM 8.2 BOP small gear carrier without changing it out or getting a spacer. The 3.55 ring gear will not physically fit in the housing. We have a member here with a shop that specializes in rear axle components, Monaz. You should really talk to him. J & D Race & Restoration
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement
2006 Mustang V6 Pony, Vista Blue. Factory ordered.
2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it)
Old projects, gone but not forgotten:
1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it.
1980 Turbo Trans Am
1970 Mustang fastback, 351C 4Bbl, auto
1988 Mustang GT, 5 speed
1983 F-150 4x4, built 302
1994 Chevy K2500 HD 4x4, 454 TBI
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Thanks, he's on the hbody forum too. Just sent him a message.
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I was looking at removing carrier intact and put aside and buy a new complete carrier/gears and install kit. I tried to post what I found on e bay but couldn't link it but I posted auction number above. That should be all you need to convert it over. The Easton posi is the carrier. So that is all you need.
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Yeah, there are several assemblies on Ebay. http://www.ebay.com/itm/GM-8-2-BOP-10-Bolt-Rearend-Posi-Gears-Bearing-Kit-Package-3-55-Ratio-NEW-/391057584540?hash=item5b0cd9319c:m:mPu9ajDcrPDZYWCBXVAa1Dw&vxp=mtr http://www.ebay.com/sch/i.html?_from=R40...amp;_sacat=6000Don't know why some links don't work, on this site. ??
Last edited by oldskool; 08/16/16 11:45 PM.
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How often do you drive 55 or higher? I don't. I rarely drive in a zone 55 and above. I mostly drive in zones 45 and slower. My 3.55 turns the engine around 2600 rpm at 55. If you're in the same boat as me, consider a 3.90 or 4.10. I'm planning on a 4.10.
I'm interested in street performance. For this you're generally looking for a first gear final drive ratio of 10:1 or a steeper. Take the M22 cars for example. Cars with this transmission usually came with a 4.11 because of the high 2.20 first gear. 2.20 x 4.11 = 9.042. Even this setup fell a little short. My 2.56 x 3.55 = 9.088 fares a little better. But with a 4.10 I'd end up at 10.496. Perfect. A 3.90 gear would put me at 9.984. This would do really well too. Given that I don't drive fast, I can run a steeper gear.
To bring the revs back down when needed I'll get a Gear Vendors OD. With its .078 OD, it'll bring the gear down to around 3.20. This would be a great highway gear.
68 Sprint
Chevy 350 with 4-speed
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When I had the M21 in my car with the 3.73's and a 26" tall tire it was perfect with the 400 engine I had at the time.
Pontiac power
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How often do you drive 55 or higher? I don't. I rarely drive in a zone 55 and above. I mostly drive in zones 45 and slower. My 3.55 turns the engine around 2600 rpm at 55. If you're in the same boat as me, consider a 3.90 or 4.10. I'm planning on a 4.10.
I'm interested in street performance. For this you're generally looking for a first gear final drive ratio of 10:1 or a steeper. Take the M22 cars for example. Cars with this transmission usually came with a 4.11 because of the high 2.20 first gear. 2.20 x 4.11 = 9.042. Even this setup fell a little short. My 2.56 x 3.55 = 9.088 fares a little better. But with a 4.10 I'd end up at 10.496. Perfect. A 3.90 gear would put me at 9.984. This would do really well too. Given that I don't drive fast, I can run a steeper gear.
To bring the revs back down when needed I'll get a Gear Vendors OD. With its .078 OD, it'll bring the gear down to around 3.20. This would be a great highway gear. Wow, First I've ran into on the forum that understands gear final drive ratio ! Here's to you!
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Joined: Jan 2010
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I agree with Bob S. wait until you see what you have under the car. My 68 came with a 69 Camaro 8.2 C-clip rear not a BOP. I didn't realize it until I noticed the track was one inch narrower on the rear.
Al
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Joined: Feb 2015
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I agree with Bob S. wait until you see what you have under the car. My 68 came with a 69 Camaro 8.2 C-clip rear not a BOP. I didn't realize it until I noticed the track was one inch narrower on the rear. I feel your pain.. I have a Chebby Chevelle rear on mine. I don't even know how to figure out what gear is in it.
1957 Thunderbird 289
1967 Firebird Base 461
1968 C-20 327
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I was rather disappointed as the Pontiac 8.2 10 bolt is stronger than the Chebby but unless I put some slicks on I will be OK. Tire traction is the relief valve, tires spin and nothing breaks, tires grab and??
That said, there are piles of Camaros with good power either racing or just being driven hard that have no problem with rear breakage.
One advantage is I have an extra half an inch per side extra clearance from the fender lip than the BOP.
Al
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I'd also consider waiting until you drive it a while before you think about gears. Mine has a 373, every time I got out on the road I'd be in the right lane, engine spinning and traffic passing me, pissed me off. You should have enough torque to get the thing moving fast with a 323-342 ratio and still not rev the heck out of it above 50mph. But as oneinch says, it depends on how you want to drive it. 373 to 410s are great for drag racing but suck if you drive to the track, 100 miles with 410s would irritate me but I use mine for more than driving around town. A 355 would be a good compromise, I was going for that until I changed the transmission.
Man that attachment looks good, I'm going to pop a cool one.
Al
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Or you can use the 3.73 gears like I have and put in a 2004R OD transmission and have the gears and cruising RPMS!!  That brought my cruising RPMs down from approx 3200 RPMs at 60-65 MPH to 2200 RPM at the same speed!
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I have a 3:23 and rarely take it on the freeway. At 60-65 with a 26" tire and T400 trans, the engine is at ~3K rpm.
If anyone knows where to find a 2:93 carrier complete, I'd be interested.
If I change out the trans, I'm afraid I'l reduce the value of the car.
PeteJD
[69 400/400]
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