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#1 |
Registered User
Join Date: Jan 2018
Location: TN
Posts: 263
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Rear end gear swap help!!
I知 running a stock 3:07 open rear in my truck with my th350. Since I am swapping to a 700r4 those gears will not work well with that transmission for a number of reasons. I have a spare 3:73 72 12 bolt and a unknown posi 77 12 bolt (both 5 lug). Would I be able to just have a shop swap the gears or is the 3:07 a different carrier and I would need to swap the whole rear end to get the gears I want. Thanks guys.
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#2 |
Registered User
Join Date: Jun 2015
Location: @
Posts: 890
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Re: Rear end gear swap help!!
you should be able to swap the carrier from the 12 bolt into yours as long as yours is a 12 bolt too . the different size series 2 to 4 I think it is all fit the same housing
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#3 |
Post Whore
![]() Join Date: Feb 2016
Location: Portland Oregon
Posts: 11,117
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Re: Rear end gear swap help!!
Short answer is no your 3.07 carrier (commonly referred to as a 3 series carrier)will only go up to a 3.42 with a standard factory ring gear.
The medium answer is the 3.73 carrier will, go in to your original housing. You will have to do a positive identification on the 77 rear end to be sure you can use the posi carrier from it with the 3.73 gear set. The long answer is below. https://www.crawlpedia.com/thick_gears.htm I have a 3.73 thick gear on a 3 series carrier in my Burban.
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Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. ![]() RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377 Last edited by HO455; 06-27-2020 at 07:29 PM. Reason: -2 spelling |
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#4 |
Registered User
Join Date: Jan 2018
Location: TN
Posts: 263
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Re: Rear end gear swap help!!
So if I swap the carrier as well would it be possible. I知 thinking of installing a posi unit so I would need to change the carrier anyway. Thanks.
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#5 |
Registered User
Join Date: Jul 2012
Location: Whitehorse yukon
Posts: 1,218
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Re: Rear end gear swap help!!
Is it a posi or one of those gov bomb lockers ?
Never seen a gov locker that hasnt blown up |
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#6 |
Registered User
Join Date: Jul 2009
Location: Space Coast, Fl.
Posts: 1,050
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Re: Rear end gear swap help!!
[quote=Awann99;8763379]I知 running a stock 3:07 open rear in my truck Since I am swapping to a 700r4 those gears will not work well with that transmission for a number of reasons. Thanks guys.[/
I'd try it with the 700r4 before you change the rear gear. My'71 GMC stepside has a mild 350/700r4/3.07 rear combo and it drives fine. A 700r4 has a low 3.06 first gear and my truck isn't lazy off the line at all. It'll burn the right rear 275/60/15 T/A if you slap it too hard from a standing start. Mine goes into O.D. at about 50mph. Sometimes I bump the shift lever down one so it doesn't go into O.D. ... third gear on the 700r4 is a 1 to 1 ratio same as your current 350 turbo in top gear. Depends on what you do with your truck. I've had mine about 7yrs and it's a fun driver. EZ cruise on the Interstate and keeps up w/traffic good in town.
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Dad always said, "Son, WISH IN ONE HAND, and ......." -------------------------------------- -------------------------------------- Current toy trucks: '71 GMC 1500 SUPER CUSTOM short/step, orig. dk. blue, 350/700r, ps,pb, A/C '72 Blazer 2wd, ochre & white, 454, tremecTKO 500 5spd. ps, pb, A/C, tilt Last 10yrs of hobby vehicles, had a FEW more in the 50yrs. before these: '66 Plymouth Belvedere City of Miami cop car clone. '70 Nova 406"sb, 13.5-1, solid roller, Brodix, p.glide/t.brake, back halved, 9" Ford, spool 4.88, cage,ladder bar/coilover, 10.5 tire....... SOLD!! '67 C10 short/step side mount spare -- SOLD!! '72 Jimmy 2wd, 350/350, ps, pb, fun driver, lots of bondo & a shiny red paint job..... SOLD!! '69 Nova 350, 4spd, A/C, ps, p.b, ...SOLD!! |
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#7 |
Registered User
Join Date: Jan 2018
Location: TN
Posts: 263
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Re: Rear end gear swap help!!
I'd try it with the 700r4 before you change the rear gear. My'71 GMC stepside has a mild 350/700r4/3.07 rear combo and it drives fine. A 700r4 has a low 3.06 first gear and my truck isn't lazy off the line at all. It'll burn the right rear 275/60/15 T/A if you slap it too hard from a standing start. Mine goes into O.D. at about 50mph. Sometimes I bump the shift lever down one so it doesn't go into O.D. ... third gear on the 700r4 is a 1 to 1 ratio same as your current 350 turbo in top gear. Depends on what you do with your truck. I've had mine about 7yrs and it's a fun driver. EZ cruise on the Interstate and keeps up w/traffic good in town.[/QUOTE]
What HP does your engine make. I知 using a good wrench 350 that makes 195-260 HP. I will be using the truck as a comuter so mostly 65 mpg driving. What kind of mpg do you get? I was more worried about the pulling power and around town driving with the 3:07 rear. |
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#8 |
laying low
![]() Join Date: Mar 2001
Location: Searcy, Ark. USA
Posts: 13,232
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Re: Rear end gear swap help!!
My old 88 GMC had a 305/700r4 with a 2.73 rear end. That low HP combo ran ok overall. With that low HP I would have preferred a 3.42 but never bothered to change it. I learned to run it in 3rd gear around town and OD on the highway.
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Boog 69 Chevy stepside, 358/T350, 4.11 posi, 4.5/4 drop, rallys, poboy driver primer is finer 91 Chevy sportside, Tahoe, Yukon & GMC Crewcab All GM..'nuff said. I stand for the flag and kneel at the cross |
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#9 |
Registered User
Join Date: Jul 2009
Location: Space Coast, Fl.
Posts: 1,050
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Re: Rear end gear swap help!!
[/QUOTE]
What HP does your engine make. I知 using a good wrench 350 that makes 195-260 HP. I will be using the truck as a comuter so mostly 65 mpg driving. What kind of mpg do you get? I was more worried about the pulling power and around town driving with the 3:07 rear.[/QUOTE] I'd say somewhere between 195-260hp ... I'm assuming it's a prev. owner rebuild, I didn't do it. Original engine, cam change, H.E.I. distributor, 600 Holley on a Performer manifold, iron ex. manifolds... no hot rod motor. Seems like it uses a lot of gas, haven't checked mileage. Probably 10-12mpg. Truck has ps, pb, after mkt. A/C. In reality I'm not sure if the O.D. transmission is saving anything. First gear is a little stronger 'cause of the low trans. ratio.
__________________
Dad always said, "Son, WISH IN ONE HAND, and ......." -------------------------------------- -------------------------------------- Current toy trucks: '71 GMC 1500 SUPER CUSTOM short/step, orig. dk. blue, 350/700r, ps,pb, A/C '72 Blazer 2wd, ochre & white, 454, tremecTKO 500 5spd. ps, pb, A/C, tilt Last 10yrs of hobby vehicles, had a FEW more in the 50yrs. before these: '66 Plymouth Belvedere City of Miami cop car clone. '70 Nova 406"sb, 13.5-1, solid roller, Brodix, p.glide/t.brake, back halved, 9" Ford, spool 4.88, cage,ladder bar/coilover, 10.5 tire....... SOLD!! '67 C10 short/step side mount spare -- SOLD!! '72 Jimmy 2wd, 350/350, ps, pb, fun driver, lots of bondo & a shiny red paint job..... SOLD!! '69 Nova 350, 4spd, A/C, ps, p.b, ...SOLD!! |
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#10 |
Senior Member
![]() Join Date: Feb 2004
Location: Williamsburg, Ohio
Posts: 1,781
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Re: Rear end gear swap help!!
I am not sure what tire you are running but if 29-31's using lockup in od will have a low (too low) rpm. I run 29's on my 42 and I ended up changing the rear gear from 3.08 to 3.73. The engine is about a 220hp 350 with a 700R4 and lockup.
I would move any perches and mounts and use the whole 77 rear end. All brake parts including the backing plates should interchange. As far as correct carriers, I thot the breaking point was up to 3.73. 3.92 up takes a diff carrier. But HO455 may be correct.
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1942 Chevy 1/2 ton Rod, 1959 GMC 1/2 ton Fleet Stepside, 1967 Chevy SWB K10, 1985 Corvette Coupe 4+3, 2015 Chevy Duramax reg cab 4WD W/T |
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#11 |
Registered User
Join Date: Jan 2019
Location: St Peters, MO
Posts: 439
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Re: Rear end gear swap help!!
I'm with Rusty & Awann and would suggest trying the 3.07 gears before swapping. I also run 3.07s with a 700R4 with hydraulic lockup and like it, especially on the highway. Sometimes for low speed local driving less than 45-50MPH, I just put it in D, which keeps it in the sweet spot for torque without dropping the RPMs too low. I have a fresh GM SBC 350 short block with mild Comp roller cam, Brodix IK-180 aluminum heads, Edelbrock EPS intake and AVS2 carb, full-length headers...maybe 280-300HP?
With that said, I plan on building a posi rear end, and *might* change the 3.07s to 3.42s (not 3.73s) as I'd still like to cruise the interstate without pulling too many revs. 3.07 w/26.5" tires ~ 1900 RPM @ 70MPH 3.42 w/26.5" tires ~ 2113 RPM @ 70MPH 3.73 w/26.5" tires ~ 2304 RPM @ 70MPH
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1967 C20, 350 SBC w/Comp roller cam, 700R4 w/3.42 posi, PS, 4-wheel PDB, Old Air AC & GM Tilt column. Last edited by tim_mc; 06-26-2020 at 09:25 AM. |
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