Hi, I'm looking to get my 68 Skylark a new diff gear ... and a new cover as well. For the cover I only have one question : Chrome or black ?? For the gear I actually have a 3.08 and looking for 3.23 to 3.42 to replace. It still have the ST300 2spd in place , I do not made a lot of highway ,use my car more for cruisin . It's a Canadian built A-body so it does have the 12 bolts Chevy . I'd like to know what to check to get the correct gear and if you get some examples (w/url link) Thank you
@ToddsGS has (had?) a '70 3.23 posi for sale. Personally, I'd go higher - At least 3.42. I'm supposed to be getting a '68 GS 400 w/a 3.64 posi that I'll be listing for sale (out if a manual trans car so I believe the driveshaft would work too). Of course you could go to an 8.5 (71-up) & get whatever gear you want. Not sure but I assume if you go to anything but a 12 Bolt or Chevy 10 bolt that you'll have to change the driveshaft length.
The 12-bolt Chevy car has a 3:07 ratio,and a 3:31 ratio,in that range. Nothing in between there. I think Buickcub had a used set of 3:07’s on here. As far as the cover goes,that is your choice if you want chrome,black,or a fancy aluminum one. I have some nice originals that I had powder-coated black.
I answered your question in another thread, https://www.moserengineering.com/12-bolt-331-p-s-gear-us-gear.item https://www.strangeengineering.net/product/gm-12-blt-3-31-standard-gear-set.html/ https://www.ebay.com/itm/222343310691 https://www.summitracing.com/search...n-ratio/3-31-1/ring-gear-diameter-in/8-875-in https://www.motivegear.com/auto-par..._viaSearch=pathart%3Aring-and-pinion;page:6
Thank you so much for the answers ! I assume now that the carrier is 8.875 and not 8.5 (since it's a 12 bolts) . So it will be a "direct fit" so to speak, Am I correct ?
Is there any formula to know the engine revolutions differences from 3.08 to 3.31 ? or aprox For those who got the chrome 12 bolts diff cover, did it stay shinny for long with minimal cleaning ? Thank you
Yes, 336/tire height X gear ratio X MPH = RPM, Add 150-200 RPM for torque converter slip. Stock tire height is about 26.6" Some tires are taller, some are shorter. 336/26.6 X 3.08 X 60 = 2334 RPM + converter slip = 2484 - 2534 RPM 336/26.6 X 3.31 X 60 = 2509 RPM + converter slip = 2659-2709 RPM
It would help to understand what you don't like about your current gear ratio and what you want to gain from a gear ratio change. -Bob C.
we are always asking those questions....What do you want from your car? WHere are you driving it. What RPM do you like at freeway speeds. ALL these things help us help you figure out how to help suggest the correct ratio. \ Chevy ratios 3.07, 3.31, 3.42 (aftermarket) 3.55, 3.73, 3.90 (can be 3 or 4 series aftermarket) 4.10, 4.33, 4.56 ....and on....
2.56 and 2.73 will be on a 2 series carrier and harder to find these days ...posi units are pretty tough to find too. Use to have them all the time... JD Jim
I ended up with a 2:73 and matching posi last week,but yes,you don’t see as many these days. What you do see is a gradual trend back to the highway friendly gears due to the increased speed limits and people wanting to drive anywhere just like any other car. I’ve done three or four 2:73 posi rears in the last year,when I almost never did any. I did a matching pair of 3:23 rears for a Chevelle and Elcamino,both small-blocks,that will be towing an old Airstream,and so far they are perfect for that owner.
I guess it's like most of us want ... add some pep and fun with minimum cost and parts change. It also could be reversed at any time . If it add about 150-200 RPM at 60MPH fuel efficiency (and engine noise) would not be much a factor . I have to replace the diff cover so .... the mecanic will allready have his hands there Thank you guys for the answers
In my Canadian made 12 bolts Chevy (8.875) do you think that this gear/pinion, diff cover and bearing set would fit in my Skylark ? Found in Amazon : Yukon (YG GM12P-331) High Performance Ring and Pinion Gear Set for GM 12-Bolt Passenger Car Differential https://www.amazon.ca/gp/product/B00AWE59NM/ref=ox_sc_act_title_1?smid=A3DWYIK6Y9EEQB&psc=1 USA Standard Gear (ZK GM12P) Master Overhaul Kit for GM 12-Bolt Passenger Car Differential https://www.amazon.ca/gp/product/B0078U90BK/ref=ox_sc_act_title_2?smid=A3DWYIK6Y9EEQB&psc=1 SPE Spectre Performance 6071 Chrome 12-Bolt Differential Cover for GM https://www.amazon.ca/gp/product/B000BPUTEG/ref=ox_sc_act_title_1?smid=A3DWYIK6Y9EEQB&psc=1 PS: Do you see the US$ or Canadian$ on your screen ?? (136$ + 202$) Other than that I know I will have to change the speedo gear . would I need anything else to do the job ? Thank you
Yeah, the specialized knowledge to set up the gear pattern and back lash. It is not as simple as bolt out, bolt in.
