# Boxed Rear Control Arms



## rcrboy (Dec 28, 2020)

Has anyone boxed in the upper rear control arms for strength or not a good idea. I was thinking of boxing in my lowers and uppers on my 67 GTO. I see in the book
"Pontiac GTO Restoration Guide" they mention a different upper control arm part number(9788993) for the lower HD rear axle gear ratios.


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## Mine'sa66 (Oct 30, 2019)

It's a great idea. You can box them yourself or buy them.


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## Sick467 (Oct 29, 2019)

Ames sells the inserts that you can weld into box the originals. I have not done it, but for $30...seems like a no-brainer. I may end up going with adjustable rear arms OR DIY boxed arms. I guess it depends on the budget as I get closer to it.

*Ames part #* - S139CB 1(964-72 Rear Lower Control Arm Inserts) ....... 30.00/pr.


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## ylwgto (Oct 27, 2009)

Makes a huge difference. I woulda boxed mine, but when I went to do it both lower arms were bent and twisted so I just bought some tubulars to replace them.


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## armyadarkness (Dec 7, 2020)

I'm a huge fan of it... All of my stuff is tubular, front, back, upper, lower. Handles like a pro with no hop at all.

Im a welder and have my own shop, and I wouldn't/ didnt box them. You can buy UMI's cheap and it's less work.

Bur regardless of how you go, it helps tremendously


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## armyadarkness (Dec 7, 2020)

Sick467 said:


> Ames sells the inserts that you can weld into box the originals. I have not done it, but for $30...seems like a no-brainer. I may end up going with adjustable rear arms OR DIY boxed arms. I guess it depends on the budget as I get closer to it.
> 
> *Ames part #* - S139CB 1(964-72 Rear Lower Control Arm Inserts) ....... 30.00/pr.


For a littler over $100, you can get UMI with new bushings.


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## O52 (Jan 27, 2019)

He's asking about the uppers. And no, it's not a requirement unless you're into autocross. 
The lowers need to be boxed if you're installing a rear sway bar.


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## coyote595 (Dec 4, 2019)

Better to get an aftermarket arm with a flex joint in the frame end for the upper to avoid inducing bind during rear axle articulation.


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## maw2078826 (May 1, 2020)

Manual transmission GTOs in 1967 came with "Arm Assembly, Rear Axle" Gr. 5.382, Part # 9788993 for all "A" Body cars with Locking Differentials (better known as "Saf-T-Track" -or- POSI's)...These Rear Axle Upper Control Arms were reinforced from the factory with a welded plate. Additionally, Manual Transmission cars received "Brace, Reinforcement Cross Member" Gr. 7.039, Part Numbers: 3909670 (RH), and 3909669 (LH)...As GM received reports of frame cracking due to the torque put out by these cars. These heavy duty parts continued on to all later production GTO's. As previously noted, AMES and other vendors sell the reproductions.


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## Machinest-guy (Jul 19, 2019)

I boxed my control arms, both upper and lower, a long time ago. Likely in 1971 when I was auto-crossing this car in SCCA H/P class. It helped control wheel hop. So did Gabriel adjustable "E" shocks set on the stiffest setting provided, but they are long obsolete now. I also ran heaver duty shortened springs. I boxed my arms using 1/8 band iron welded on the lower edge and formed around the end leaving clearance for suspension motion. I used a high density rubber bushing in the vehicle end and an OEM bushing in the differential housing eye. My bushings were not tack welded to the control arms as some guys did back then.

Now, many years later, I wanted to replace the rear axle with a new unit from Currey so it became time to disassemble and inspect the rear suspension. All the control arms were stripped to bare metal, crack inspected for defects (none found), then powder coated for a better appearance. I reinstalled MOOG bushings in all positions. I also replaced the bolts, spring insulators, and hardware holding the springs in place.

My suspension (convertable w/ factory HD frame rails) had gone about 70 thousand miles between 1971 and December of 2021 without any damage occurring to the modified arms or OEM fasteners, but the bushings were cracked and worn out. The frame is also undamaged, but this may have been prevented by support from a roll bar mounting pad. Total vehicle miles on the frame are about 160,000. I have now installed AMES reproduction frame support brackets and reworked my roll bar welding. I don't expect any issues with the rear suspension to form within the remaining expected life of the car. 

One unusual modification to the rear suspension I made, because of the GM "A" body's exceptionally conflicted rear suspension geometry, was to articulate the upper shock mount with a ball joint. This allows the shock absorber to pivot freely and not add additional resistance to already binding travel arcs generated by unequal arm lengths placed angularity to resist side loading, torque moments, as well as vertical forces. Articulation of the upper shock mount allows the reinforced control arms to function better. 

One caution would be, if you are going to use a rear sway bar and a Currey housing be sure the sway bar mounting holes are as low as possible in the lower control arms. The center section is larger so it can interfere a very small amount as the suspension moves around.

Best regards, Ladd


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## lust4speed (Jul 5, 2019)

Worth noting that forward acceleration pulls on the upper arms and not much stress on them. Lower control arms are pushed against and the bracing really helps -- although the stock bracing included with a sway bar was probably more for preventing bending of the arm with the added twisting force.

I've seen several cars with popped quarter panels that didn't have the frame stiffeners installed, and one damaged car that had the braces but the bolts were loose allowing the bar to slide around. My first purchase would be the frame brackets if my car didn't have them.


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## armyadarkness (Dec 7, 2020)

lust4speed said:


> Worth noting that forward acceleration pulls on the upper arms and not much stress on them. Lower control arms are pushed against and the bracing really helps -- although the stock bracing included with a sway bar was probably more for preventing bending of the arm with the added twisting force.
> 
> I've seen several cars with popped quarter panels that didn't have the frame stiffeners installed, and one damaged car that had the braces but the bolts were loose allowing the bar to slide around. My first purchase would be the frame brackets if my car didn't have them.


Yep. I got the Global West ones, cheap. It's also worth noting that it's much easier to do it all at once, since most of the same stuff has to come apart.


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## Baaad65 (Aug 29, 2019)

rcrboy said:


> Has anyone boxed in the upper rear control arms for strength or not a good idea. I was thinking of boxing in my lowers and uppers on my 67 GTO. I see in the book
> "Pontiac GTO Restoration Guide" they mention a different upper control arm part number(9788993) for the lower HD rear axle gear ratios.


Are you concerned because you have a big torque motor or big sticky tires? If so make sure you install tubular triangulation bars not the cheap stamped ones that were stock, and yes boxed lowers or lift bars which are solid. I semi boxed mine you could say by using the sway bar bushings I didn't need, drilled two holes evenly spaced in the upper arms and installed the bushings in the channel with grade 8 bolts so they are more rigid and don't flex and bend.


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