Rear-end questions

My 69 gto convert., purchased within the last year has a painful 2.56 rear gear. (painful to me while driving) I am pretty sure that it is the standard BOP 8.2, 10 bolt. It has an M-21 four speed and I am ready to upgrade the rear diff.

Everything I read recommends a medium-gear 3.23 or even 3.42. It will only see street action and probably nothing too serious at that. I think m-21's came w/ only 3.90 or 4.11 rear ends. I am leaning toward a 3.42 or even 3.55. Ideas?

Next question is that besides a ring & pinion, I'm pretty sure that it will need a new carrier & I will probably opt for a limited-slip carrier. I am pretty sure that the upgrade w/ parts/labor will be at least $1000 if not more. Is this rear-end worth that kind of money or should I upgrade to a 12 bolt moser unit. Keeping it fully original is nice, but not that important to me.

Before I drop a load of money, I would love to hear someone elses input w/ more experience.

Thanks, Lou

Are you sure that car has an M-21 and not an M-20? Was it changed at some point?

The PHS build sheet lists it as having the M-21 option and the tranny has the "B" at the end of the ID/date code (I think "A" was for the M-20 and "C" was for the M-22 (which I don't think was offered as an option on this car).

Hello Lou ;

I am in the same situation you are in , I have a 65 GTO with a 2:56 rear end with a 4 speed. A few years back my mechanic and I tried to put lower gears ( 3:73's ) in my ten bolt. The gear set was from summit racing and was thought to fit. The ring gear was ok but the drive pinion shaft diameter was bigger therefore the bearings were also bigger and wound not fit in the housing. So much for that idea, now I am looking for a GM 12 bolt from that era 64 to 67 even if I have to rebuild it or change gears . The rear ends in these cars were the weak link in the chain, strange they didn't put a good 12 bolt in right off the line. A good 12 bolt will handle 500 HP with no problems. You will never get that out of a ten bolt no matter how much money you dump in it, it is just a lighter set up as I see it.If you go through all the work of changing the carrier then you may just as well put a 12 bolt in and be done with it knowing that you have some beef under there then.
Hope this helps you , Kenny

If you want a versatile gear for power and highway use, a 355 rear would give you both.

I have a similar setup - 67 10 bolt rear, 2.56. I have heard 2 stories - the BOP 10 bolt is underrated, and, that it is crap get a 12 bolt. Since it already had a posi, I figured it was at least worth a try with a 3.55 gear. I have not seen anything but 3.55, 3.90, and 4.11 for the BOP 8.2 from the gear suppliers. I got a Richmond gear and I had to use a Mr Gasket 10 bolt spacer and Mr Gasket 1" ring gear bolts for pontiac 10 bolt. The gear was still supported on the carrier shoulder w/the spacer so I figure it may hold up. I am looking at a 400 HP max setup with a mild 400 build. So I have about $410 in parts including a Ratech install kit w/new bearings. I set it up myself but will not get it on the road for a while. So, if you must have a 10 bolt for the correct look, you can get different gears in there for less than a grand - I wish I could know if it will hold up?

My judge came with a 3:55 M21 close ratio. Although I do not know how it performed, my opinion is that a 3:55 is a nice inbetween highway and performance gearing. Fun out of the hole, yet not completely taching out at highway speeds.

Trying to find a 12 bolt of that vintage may sound easier than it is. From my experience, they have dried up long ago and are now quite rare. If you are going for correct look, that or getting a later model 10 bolt may give you added strength you are looking for. They tend to be a little wider (about an inch or so). Another option if look is not the deal breaker is to get a 9" and modify it to fit your application. Guarenteed not to break and parts are readily available as well as donor housings. You can also buy them pre-modified from Summit that will bolt into your application.

Good luck on your quest!

Jim

Trying to find a 12 bolt of that vintage may sound easier than it is. From my experience, they have dried up long ago and are now quite rare. If you are going for correct look, that or getting a later model 10 bolt may give you added strength you are looking for. They tend to be a little wider (about an inch or so). Another option if look is not the deal breaker is to get a 9" and modify it to fit your application. Guarenteed not to break and parts are readily available as well as donor housings. You can also buy them pre-modified from Summit that will bolt into your application.

Good luck on your quest!

Jim

As far as rearends are concerned, I believe that the G.M. 8.5" ten bolt would be an excellent compromise. It looks stock. The 8.5" ten bolt replaced the 8.2" ten bolt in the 1971 mid-size G.M. cars. The twelve bolt rears contain a 8 and seven eigths inch ring gear, but the axles are about the same size and quality as the ten bolt. Most 12 bolt stuff (gears, axles, carriers, etc..) is for the Chevy (c-clip type) rear. The B-O-P twelve bolt is different. The 8.5" ten bolt has a wide selection of gears from 2.41 all the way up to 5.57 ratio. The 1971 and 72 Oldsmobile Cutlass and Buick Skylark units have the bolt-in style axles and the housings are a direct bolt-in for the 64-72 mid-size coil spring chassis except that the u-joint may have to be changed. There also may be a slight difference in the lenghth of the driveshaft which is usually easily made up in the slip yoke. I have had great success with the 8.5" Oldsmobile ten bolt rearends in oval track competition on both dirt and pavement situations. The reliability and durability has proven to be better than the Chevy 12 bolt. The disc type posi units seem to hold up a lot better than the cone type. As for highway gear ratios...my experience has been that , unless you drag race, no more than 3.42 is necessary. I once had a 73 Camaro 4-speed that ( I had been told by a previous owner) had 4.10 gears. The car had tremendous acceleration, and tached a little higher than I wanted at highway speeds, so I considered switching it to a 3.73 or 3.55 ratio. Until I checked the gears in the rear and found that, in fact, I had been running 3.36 gears all along. With a good powerful V-8 a switch from 2.56 to 3.08 or 3.23 should put a smile on your face. Any more than 3.42 and you'll be wishing that you had an overdrive transmission. Most V-8 Cutlass' came with 2.73 or 3.08, while 6 cylinders came with 3.08 or 3.23 stock gears.

My 67 came with a 12 bolt posi, 3.31 gears. I'm pushing a little over 400 hp and run 15X8 wheels with 2.35-60r-15 tires, and the launch is incredible with the TH400, yet the rpm at 75 mph is around 3200. Certainly not stock, but not a bad combination. I may not set any 1/4 mile et records but I can run with most, yet still cruise on the highway. Today, crusing at 75 mph is a must if you don't want to get cussed out, so highway speed must be a concern unless all the driving is either on the track or the back streets. The ideal set up is with gear vendors gear box for TH400. Manual gear boxes are limited, 4th will always be 1:1 so the rear end will determine what you do. Anything higher than 3.5:1 will make the engine very busy at highway speed, anything less will slow launch capability. Wonder if anyone has slapped a 5 speed with OD on 5th into a GTO ?

I bought a 12 bolt from a 71 El Camino months ago and had it rebuilt w/ an Eaton carrier and 3.42 Richmond R&P. Very happy w/ this ratio. At 70+ MPH it still tachs a little.

All, I have never seen a GTO with a 2.56 axle. In 1965, GTOs came standard with 3.23 gears. The tallest gears I've seen in an original GTO is the 2.93 set that came on '67-'68 GTOs with automatic and air. I wouldn't be surprised that some of these GTO owners cars had the rear ends replaced. Due to the nature these cars were driven, it wouldn't be a surprise. My '65 GTO convertible has the OEM 3.23 "peg-leg" rear. I don't plan on dropping the hammer enough on this restored original car, but many of my acquaintances with these cars like 3.23 gears for overall use. The 3.55 gears seem to work well too.
If one was not too concerned with originality, there is a company called Peyton Performance Automotive in Waxahachie, Texas that builds custom-fit 9-inch Ford rear ends. I've seen other ads where a 9-inch Ford rear with all of the suspension brackets for GM A-bodies are welded on the Ford housing so you can bolt these right in a GM A-body.
It might be worth a look and best of luck with your GTO.

HEY I HAD A 69 JUDGE WITH A 355 POS AND IT COULD SMOKE THE TIRES IN THREE GEARS AND STILL HAVE A GOOD HIGHWAY.

THE 10 BOLT 355 IS THE WAY TO GO.
I MY OPINION. I AM RESTORING MY JUDGE AND STAYING WITH THE 355 POS.

I have a 4 speed 70 Judge with a 3.55 rear end. It is good thru the gears on the low to mid range and hums at about 3700 rpm at 70 mph. I also have an automatic 74 Super Duty Trans Am with 3.42 gears and I like that too. It turns about 3500 at 70 and hauls on the low end. I think you are in the right range with either one. Depends though on how you will drive it. I would stick with the 10 bolt and do it right.

Maybe someone can help me out. I've got a 71 lemans im cloning to a GTO but it has the peg leg rearend. I bought another 10 bolt 8.5 posi rearend that came out of a 67 camaro but I don't know if its going to work or not. They are the same length but my car has coil springs and this rearend was for leash springs. Could I just weld the brackets on from my old rearend and be alright?

I have been reading the forum with some interest, I have a 1966 Lemans with a 421 M21 trans and am changing the complete rearend from the current 2.96 to 3.55 I bought the complete setup minus the the drum brakes for $300.00 this is a posi unit, just curious as to the value of this rearend does anyone have any idea as to the average value of these? This rearend orignially came from a 69 GTO with a 4 speed.

I just converted my 2:56 peg leg to a 3:55 posi in my 68 Lemans. My God what a difference, like adding 100 more horsepower. Little tough on highway tho running close to 4 grand at 65MPH. Looking forward to seeing what it will do on the strip with the big block pushing 400 h.p.