Hey guys I just purchased a '66 Skylark 455 GS clone. I was blown away to find out that it has a 12 bolt 2.73 Eaton posi rear end in it. I've already decoded this rear end (CHC 016 2E, 3968278 NF, L170). It's nice running 2500 RPM at 65 with 245/60R15's, but I have plans to drop my 10 bolt 8.5 posi 3.42 in it. I haven't done any calculating yet, but I would imagine I will be in the 3000RPM range with 3.42's over 65 mph. Once a C-clip eliminator is installed in the 12 bolt, I've read that these two axles similar in strength???
Those rpms sound a little high for 2:73 gears. I run 3:73’s,with a TH400,and a 28” tall tire,and I am 2800 at 60mph. Are you sure there are still 2:73’s in it? As far as c-lip emliminators go,they are what I consider as a last-resort. I don’t like them,especially for a street application. I prefer to either keep them as c-clip axles,or cut the housing ends off,and weld new ones on,to make a true bolt-in axle. Yes,the 8.5” 10-bolt,and the 12-bolt Chevy are similar in strength. Your benefit is you own both of them. I don’t know which yoke is on your 12-bolt,but you would need a 3R u-joint for the 8.5”,and depending on what driveshaft is in there,and how accurate it is in length,you normally need to shorten it when installing an 8.5”.
336/tire height X gear ratio X MPH = RPM A P245/60R-15 is 26.6” tall 336/26.6 X 2.73 X 65 = 2241 RPM. Add 150 RPM converter slip and you are at 2400 RPM. Consider tachometer error ( especially stock tachometer) and 2.73 is about right. If you swap in the 3.42 rear, 336/26.6 X 3.42 X 65 = 2808 RPM. Add in 150 RPM for converter slip, and yeah, you are close to 3000 RPM.
The 12 bolt chevy should only need aftermarket axle shaft and be fine. I still have my Monte Carlo with just Summit axles in the rear and do 12 sec passes at the track with slicks on. IT was never made as a Speed demon but a respectable street car that could hold it own. 212 duration at .050 and is the crane HMV272 cam. Yes regular stock hydralic flat tappet cam. Edlebrock performance...NOT RPM, Stock GM 2.06 1.72 iron heads. 454 .030 over bore... NOT even balance or blue printed ...lol. Just lots of fun torque like back in th e good old days. SO all depends on where your going to spend most of your time as to how far you want to beef up. ALSO the choice of which rear you want people to see under your car is a choice too. BOTH rears will do the job well. If you install the 10 bolt 8.5 you will need to cut the driveshaft. The 12 bolt chevy was a direct cut nothing bolt in deal as there were late 60's cars that had the BOP yoke on the 12 bolt chevy s from Canadian built cars. same style as the 1964-1972 BOP 10 bot rears that use the u bolt and nut hold downs for the joint cups. As stated double check your gear ratio and be sure what you have. 2.73 should produce about 2300 or less RPM at that ratio ... So either you might have a slipping converter or CLUtch if manual trans or Speedo is off? Double check that ratio in the 12 Jim
I think we all agree that the RPM is a bit on th e high side and there might be something a miss... Yes?
No we don't, too many variables to say that with ANY certainty. He just bought this car. How do we know the speedometer is even accurate? Tachometer accurate? I'm not even sure how the OP determined that it had 2.73's.
Good point Larry. Best to start by verifying the speed with gps. I've had a 12 bolt in my blue 66 since the early 1980's..... that's just what we did in those days before we knew all the facts. The early Chevelle 12 bolts are a bit narrower than the Buick 8.2 for 1966. With stock Buick 15x7 mags, you can fit a wider tire, I had 275's on the rear. With the 8.2 or 8.5, a 255 is an iffy fit with the 4.375" backspace. If the 12 bolt has a chev yoke, you can use a conversion u-joint to join the Buick driveshaft (3R) to the chev rear. Upper control arms need to be modded for clearance when installing a 12 bolt... just some grinding will do it. I finally swapped out the 4.11's for a set of 3.42's. I really like them for back road cruising .
I guess my post was really just a conversation starter guys. I searched for a long time for a 12 bolt rear for my other 66 Skylark. This was before I had learned from you guys about the BOP 8.5” 10 bolt. With everyone’s help, I built an 8.5”. Its crazy that I stopped looking for a 12 bolt I end up with one in my new car. My speedo is off and I was using my GPS. I do have a spare driveline that I can build for the 8.5. Once I get some honey do’s done I will check the ratio. Thanks guys
GMMADMAN has a 1970 12 bolt so it is the same width as the 66 10 bolt and or the 10 bolt 8.5. 55.25 F to F or 61" over all D to D 10 bolt 8.2 to 12 bolt Chevy or any other 10 bolt or 12 bolt will be the same length drive shaft. (just need to watch the -u-joint which they have simple conversion joint for that ( Moog 372) ONLY rear end from 1964- 1972 that are trouble on an interchange in A-body GM and do not interchange clean are the 10 bolt 8.5... The 8.5 is the Bastard rear in this Muscle car era (1964-1972). 8.5 Pinion shaft is longer than all the other pinion shafts by at least 1" - 1.25" We swapped out tons of Chevelle and Monte Carlo 10 bolt / 12 bolt combos all the time NEVER did a driveshaft swap ever. 10 bolt BOP is the same length pinion shaft also we even had 1970 Monte carlos with the Tempest rears from the factory and swapped in 12 bolt Chevy rear and only did a (moog 372 conversion ) joint bolted it up and went. Jim J D
Finally got around to verifying ratio. I have 2.73’s. I believe I need a series 3 posi carrier or ring gear spacer when stepping up to 3.42. My car will be a 10.5:1 Stage 2 TA street car when I get my other 455 built. I’m going to sell off my 10 bolt 8.5 and regear the 12 bolt. Thanks guys
Do NOT use a ring gear spacer in a 12-bolt Chevy. They work fine in an 8.5” 10-bolt,but not a 12-bolt Chevy. Why? The reason is the ring gear journal on the carrier is rather narrow or thin. The spacer takes up all of that journal,which leaves nothing for the ring gear to press onto and stay true & centered. This results in the ring gear basicly floating out there,deflecting and distorting under load,which also flexes and twists the ring gear bolts until it fails.
