# aftermarket rear differential options for 69 428 swap



## Faeodan (Aug 22, 2014)

hello fellas. 
I'm currently working on getting the swap in 428 finished and mated back to the TH400, and then we are going to start refreshing the rear end. It doesnt have a differential currently, and haven't checked the gears yet, but i would imagine it's probably stock. The 428 is supposed to be around a 450hp build. it was built around 12 years ago by a local speed shop but never picked up by owner. It was later sold to my uncle (friends with the speed shop owner) to recoup what they had put into the engine at $2200. I checked with them to see if they still had a build sheet, but they didn't find anything. He said he had only built 2 428 Pontiac over the years, and believes both builds where in the 450hp area. Anyway, the engine set in my uncles shop for the next 11 years until he decided to sell it to me for my 69 goat. Its almost complete, TH400 is complete, so now i'm moveing on to the rear end set up. Car will be a weekend cruiser, car show, cruise night ride. I'm wanting to install a posi (safe-t-trac) and still havent decided on a gear selection either. What are some good aftermarket rear diff options. I am on a pretty tight budget at the moment, but i'm not going cheap on anything either. I don't have to have the very best, but i want good quality parts. Any help with rear diff and gear choices would be greatly appreciated, and sorry for the long winded post.
Here is the 428 currently, the Edelbrock carb is just a place holder. i have a 71 dated Q-jet being built by my uncle to replace it. thanks guys.


----------



## BearGFR (Aug 25, 2008)

I've been very happy with my rear (that sounds weird...  ) but it wasn't cheap.

I'm running a Moser 9-inch in mine, fitted with a nodular iron center section, Moser high-spline count axles, Wavetrac differential, and 3.50 gears. These things are built to order when you buy them, so I had mine built with an additional and easily accessible fill port/plug on the rear of the housing. It should be bullet-proof and way stronger than I'll ever need, even with the power I'm making at the moment or ever will make in the forseeable future --- which was exactly what I wanted. I wanted to spend this money one time and never have to worry about it ever again.

Bear


----------



## PontiacJim (Dec 29, 2012)

:biggrinjester:OK, this has been covered before. My Opinion.

The factory Pontiac 10-bolt is fairly stout and can be built to a handle extra HP. You can add a posi. But as horsepower levels go past factory outputs, you may want other options. Things to consider are tire size(width & "sticky-ness"), single track or posi. The things that make your car grip better, put stresses not only on the rear end, but transmission & u-joints, not to mention the rear suspension in the form of wheel hop. As you build bigger HP, it will find the "weak link" in your drive train/suspension(to include the possible cracking of the frame). 

You build a better rear, and then you may pop the trans or u-joints. So sometimes it is a better to compromise to allow the big HP to "vent" via smoking tires -ie single traction or smaller width tires.

OK, so you want a posi & want to upgrade. First choice over the factory 10-bolt is of course the 12-bolt rear-end. Pontiac used them as factory upgrades and of course a Chevelle 12-bolt will work. Problem here is cost in trying to get an original one as they command high dollars from the "restoration group." Then of course you want to rebuild or refurbish it. So the price of the rear plus rebuild can add up quickly. The next option is an aftermarket 12-bolt from one of the many big name rear-end suppliers. The benefit is it will be all new and you can spec it to your needs. I feel this is pricey, but done right, and if you plan to keep the car long term, a good investment. Replacement & rebuild parts are available, but will they be long term?

Second choice, and what I did, is the Ford 9" rear end. There are a number of rear end builders who can produce a Ford 9" housing with the Pontiac/GM rear mounts that will allow you to bolt it right up and use all the factory springs, shocks, and control arms. These can also be spec'd out with add-ons depending on your HP/torque outputs. I purchased a new housing with heavy duty axle tubes, big bearings, 31 spline axles with the Pontiac/GM bolt pattern, and 11" brake drum set-up(drilled for the GM bolt pattern) from Quick Performance. Now you have to decide what you want to do with the center section.

Options on the center section include purchasing a complete used center section (which must match your 31 spline axle ends) at a swap meet, craig's list, a buddy, trade paper, or wrecking yard. The neat part about the 9" is that the gears & housing are a unit. You can swap the housing/gearing fairly easy if you don't like one gear or just want to experiment. Next option is purchasing a new complete center section ready to go from any of the many suppliers. Again, many options/upgrades to include the posi (the Detroit locker which can be harsh in its engagement). In my case, I decided to purchase my parts individually because I wanted specific upgraded parts and I could pay as I go. Problem here is that I now will have to pay someone to assemble it which doesn't seem to be out of line with purchasing a complete center section with the upgrades I chose. I could probably do it myself, but you need some special tools which will add to the build, I only want to assemble it once, and if I mess up I'll have to re-do it; -more labor/time & possibly parts. Purchasing the new ready to run center section is in my opinion the way to go, but can be a big lump sum cost.

Why the 9"? They are still used today by all the Nascar guys. They are still very much used in drag racing & circle track racing. Hot rodders still use them -and have used them. They are still available used, but supplies like all older things is running out......but there are a big number of aftermarket suppliers who make them new or can supply you with an older unit refurbished. Ton of parts, used and new. I think the 9" will have a greater long term life span in the future over the 12-bolt.

OK, so there are some basics. The aftermarket rears are not cheap if you have a tight budget. Once you have that bullet-proof rear, now you have to be concerned with the trans, u-joints, and rear suspension. Look to spend additional money at some point to upgrade these if you want to really exercise that 428CI to its maximum potential........and you will.

Many of us (*inexperienced or irresponsible drivers *) on this forum will try to steer you away from a posi rear end for a number of reasons, mainly the loss of control in anything less than a straight line hard all out acceleration. A posi can throw your car out sideways in a heartbeat on damp/wet roads, in a curve, loose sand, snow, ice, oily road, etc.. Many cars have been wrecked or totaled by the unexpected kick-out a posi rear can provide(this can also be done with a single legger and big HP with the right conditions:biggrin2. Even a good driver can be overpowered into a situation you can't get out of. Posi will indeed add stress to your suspension & driveline assuming you have some nice biting tires. Use factory skinnies and you'd be better off, but you still may find yourself going sideways at a time you would rather not. Now if you plan to drive it like grandma, then everything I just said is unimportant and the 10-bolt/posi won't pose any problems.:rofl:

My bottom line choice if on a budget would be the factory Pontiac 10-bolt, non-posi, slightly wider than stock tires, an air bag in the rear springs to help with any wheel hop. Next might be to locate a complete good used 12-bolt which will handle more traction/power/wider tires. And finally a new aftermarket piece, either 12-bolt or 9" Ford spec'd for kill.

And that's my opinion.:bannana:


----------



## Goat Roper (Oct 28, 2014)

Thanks for this write up on rear ends PJ, this is the last thing I need to do on my car.
I was thinking about a Eaton posi unit but after reading your post I think I am just going to blast the original 10 bolt pegleg and paint it.
I drive mainly in the mountains and don't want to be drifting it around the curves.
O*o


----------



## geeteeohguy (Feb 2, 2008)

A newer GM 8.5 ten bolt is stronger than the old 12 bolt, and can be modified to fit your car. That said, there is no rear end stronger than the Ford 9", mainly due to its fully supported pinion gear. If you want to be bulletproof, and do it once, the 9" is the way to go. For your 428 with a TH400 and for your purpose, I'd go with a 3.00 or 3.25 rear gear ratio. Good all around performance. 3.50's and up will limit your highway cruising ability and cost fuel mileage.


----------



## ALKYGTO (Mar 29, 2010)

Here you go OP - http://www.strangeengineering.net/d...ation-kits/gm-10-bolt-chevy-olds-pontiac.html For parts for your 10 bolt.

Or- Complete Rear End Assemblies - Street / Track

I run a 12 bolt Chevy rear in my car. It has survived over 10 years channeling over 750 rwhp to 12.5" wide tires with no problems. Of course it is fitted with all Strange Engineering internals to do that. From the factory 12 bolts are not that strong really but with aftermarket internals now the weakest point in my rear axle is the ring gear.

For *Ultimate* strength, if I had to do it again I would run the Strange S60. Based on the Dana 60 used in trucks and in Mopar musclecars of the 60's it is the strongest passenger car rear ever. It makes a 12 bolt look like a tinker toy. 

The 9" Ford is also a great rear but again, you are looking at a complete aftermarket rear if you want all the good pieces, nodular center section, good differential, strong axles. The advantage here (and why NASCAR runs them) is that they have a drop out center so it's easier to swap gear ratios. The 9" also has the greatest selection of gear ratio's to choose from.

Any of these axles will work fine behind your 428 OP, but do it once and do it right.

And do you think I run an open diff? :skep:


----------



## PontiacJim (Dec 29, 2012)

*Car and Driver*, Feb. 1968. "Project Car: Pontiac GTO 428." The Royal Bobcat upgrade. 390HP 428CI, TH400, 3.55 posi in a new 1968 GTO. 

From their road test. "In the hands of an inexperienced or irresponsible driver the 428 GTO would be as dangerous as a basket of hair-trigger hand grenades. Too much throttle at the wrong time* will spin the car, or send it rocketing off the road* and into the farmer's field. You can light up the car's tires like it was an AA-fueler anytime the notion seizes your fancy.":yesnod:


----------



## ALKYGTO (Mar 29, 2010)

Are you saying you are an inexperienced or irresponsible driver Jim? :rolleyes2::smilielol:


----------



## Pinion head (Jan 3, 2015)

...and a Royal Bobcatted '68 428 GTO would have had a nodular hsg HD Safe-T-Track 8.2 Pontiac rear with hd forged axles and the car would have had narrow G78x14's. Also, good chance spun the tires on the street... As its a much more expensive and ultra rare rearend that can hold up better than std center hsg, stock axle deal, not really not a good comparison to how a base gray iron 8.2 rear struggles when under high torque loads in an A or F body that can HOOK, whether on the street or at the track. 

Having built up gobs of 12 bolts and now well over 300 tapered bearing axle 8.5 A body rears and installed them behind heavily built Pontiacs, Buick's, and BBC's (nearly all were considerably stronger than a 425 horse 428HO), as a builder it makes little sense to throw $$$$ into a foundation that lacks pinion support. Always caution the owner of the street/strip build that there is a place for the gray iron Pontiac 8.2 10 bolts and that is under moderate torque level drivers with stock tires, and of course under high end showcars. 

Now, if one bleeds $$$$ and are the guy with the 13- 12 sec car, but is blowing the bank acct & has dreams of running 9 sec 1/4 miles on a transbrake in a 3600 lb A body and also wants to try and drive the car on the street, by all means invest in a custom 35 spline 9" Ford rear with nodular aftermarket case/daytona pinion support. Time ones all done, with a custom hsg and axles, aftermarket case built up with spool and then another nodular case built with street gears, a detroit locker, then brakes to fit, & finally the needed adjustable upper arms to get the pinion angle right with the different geometry upper perches, will be lighter by about 4K, that's if can work some deals.

For 99% of the rest of Abody owners that have upgraded to a much higher torque level engine and often stronger transmission, & are rightfully worried about the lack of strength of their gray iron 8.2 rearend, need to realize all the upgrading to the 8.2 will not solve the inherant lack of pinon support. A well built bolt-in axle 8.5 rear, or an upgraded 12 bolt will fit the bill perfectly. The inherent strength of the 8.5 A body rear has been proven over and over and over for the last 25 years. On original 12 bolt posi rears, a minimum of a carrier rebuild with new heat treated side gears & spiders, along with new HD axles will eliminate the first weaknesses. Strange has come out with a street/ strip c-lip elim kit for 12 bolts which mimics the A10 tapered bearing set-up of the best 8.5 A-body rears. Def something to consider if a stock hsg 12 bolt is in the works.

On the 8.5 A-body rears, have built several that have held up well to the rigors of 1.40 60 foot times at the track. Most, however, were built and live in 12 sec to low 11 sec footbraked 3500-3800 lb A body street/strip cars. Axle strength of the stock A9 tapered bearing axles does need to be addressed. Though quite a few have been repeatedly driven down into 1.60 60foot times on slicks in 3500 lb early A bodys, even performing this on shimmed up S spring GM posi carriers, at that level one is tempting the laws of physics. By upgrading to A10 tapered bearing 30 spline axles, a HD Eaton carrier, and only housing mods being a steel LH cap, end up with a much stronger and long term dependable stock appearing and fitting rearend. For the quicker guys, I also TIG weld the center hsg to the axle tubes in my line up bar, something I nearly always do on leaf spring housing builds.


----------



## Faeodan (Aug 22, 2014)

Thank you for all the great information. I have no plans of building this into a street/strip car. It really will be just a weekend cruiser. We already have a build for the strip when we make a trip to the track in a 84 S10 blazer with SBC. All the money invested in the S10 is in the engine, so if I really want to go out and flog something, the little yellow devil fits the bill, and i don't have to worry so much about jacking up the body, or the paint, or the rims, etc. If i have 400 rwhp with the 428, I will be perfectly fine in that reguard, and if i need to upgrade the rear, and ditch the posi idea, I can live with that too. This is my first build, and my father has built many from the 50's-70's but he has been out of it for a long time. We are both learning as we go. We just want to get her built right, and back on the road. The 428 was our best engine option for the cost, as the 400 we pulled would have taken longer, and more money to get back to a decent enough build to even warrant putting it back in the car. It was pretty trashed, had the horrible 4x heads on it, and would have required a full rebuild, with new heads.


----------



## geeteeohguy (Feb 2, 2008)

Whatever you do, put a limited slip in it. Not a locker, not a spool, but a clutch type limited slip. Even with a 326 engine, these cars will spin a single tire forever. Just don't get crazy in the rain.


----------



## PontiacJim (Dec 29, 2012)

ALKYGTO said:


> Are you saying you are an inexperienced or irresponsible driver Jim? :rolleyes2::smilielol:


That all depends on which side of the court bench you're sitting on.:bannana:


----------



## Faeodan (Aug 22, 2014)

an LSD was definitely what I was looking for. Not interested in a locker. Alot of the events in the local area like cruise nights, car shows, LBRA club get together, and similar usually end up with a closed Street burnout competition. Our club recently attempted a Guinness world record of the largest simultaneous burnout. They missed it by a few, but thats the kind of car culture we have in this area. I just really didn't want to build a torque monster, and have a peg leg. I'm an experienced driver, and biker that takes safety absolutely serious. The s10, probably around the same HP lvl as the goat, is about as squirrely as it gets. 355 roller motor with Dart heads, and a light little s10 blazer body is a fun setup, but if you don't respect it, it will throw you sideways in a heartbeat.


----------



## PontiacJim (Dec 29, 2012)

Faeodan said:


> an LSD was definitely what I was looking for. Not interested in a locker. Alot of the events in the local area like cruise nights, car shows, LBRA club get together, and similar usually end up with a closed Street burnout competition. Our club recently attempted a Guinness world record of the largest simultaneous burnout. They missed it by a few, but thats the kind of car culture we have in this area. I just really didn't want to build a torque monster, and have a peg leg. I'm an experienced driver, and biker that takes safety absolutely serious. The s10, probably around the same HP lvl as the goat, is about as squirrely as it gets. 355 roller motor with Dart heads, and a light little s10 blazer body is a fun setup, but if you don't respect it, it will throw you sideways in a heartbeat.


Don't let me scare you.:biggrin2: Again, I presented my opinion not knowing you personally. Better to be safer than sorry in giving advice. If you can handle a car, and an S10 is pretty light in the rear, then you already know what HP is capable of. The limited slip unit works fine and is very controllable as it will disengage as soon as you let off the gas.

The 8.2 in my opinion is still a good rear end, but has its limits as I feel I pointed out. I can't speak for an 8.5 rear, but you mentioned "inexpensive" and "inexpensive" is a relative term; it means different things to different people. I personally would shy away from beefing up an 8.5 rear unless I had to have "stock". With all the money it would take to purchase an 8.5 replacement, add all your HD components, you will have a pretty sizable bill......and there was no mention from Pinion Head as to whether the 8.5 rear was a nodular iron housing -my guess is that it is not and you'll still have the inherent weakness of the gray casting.

If you do add a limited slip and the rear grenades, then you have an investment loss unless you get another 8.2 case -assuming you did not damage the limited slip. But, if you go that route again and locate another 8.2 replacement, would you have the same results? Again, not trying to scare you or tell you what to do here, but personally, I would want to invest my money into something that had a much lesser chance of grenading and hopefully only had to do it once.:thumbsup:

I have not owned but a couple cars out of the many I did own that had the limited slip. I like to hammer(use) my cars and never had any problems blowing up rear ends. Had the trans clutches burn up because they could not hold the wider gripping tires, universals break, even a cast iron front pinion yoke grenade -its all in fun. So for me, I'm not afraid to break things and live by the saying "if it breaks, re-build it better." But, that can be costly and it always breaks where it is the most inconvenient and right after you paid all your monthly bills and there ain't nothin' left in the savings.:smilielol:

That said, you could certainly add a limited slip, and never ever experience a failure.......and have a ton of fun melting tires at the burnout contest.:yesnod: The TH400 is a lot easier on the driveline in general than a 4-speed, so that is a plus. If you grenade the rear, then you get the bigger one. And if you grenade the bigger one, then you get a rear end out of a Mack truck and be done with it.:bannana:


----------

