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Old 09-13-2018, 02:32 AM   #1
tow rig
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3/4 ton LWB needs front discs and a 2.5 - 3" drop.

I have a 1970 GMC C2500 Pickup with leaf spring rear suspension. I am trying to find the best way to add disc brakes and drop the front 2.5"+. My critical parameters on this project are:

1) I don't want to push the front wheels out 3/4" per side.
2) I want to retain 8 lug wheels all of the way around, and the ability to use the truck for serious work and towing. (This vehicle will tow my 23 foot boat - heavy load and substantial tongue weight.)

One problem is - that any combination of factory parts will change the track width significantly. Trying to find a set of 8 lug wheels that look at home on a 2 wheel drive truck is difficult enough without dealing with offset issues.

Another problem is that the only modular (maintain drum brake front track width) drop spindles for the 3/4 ton - seem to be designed to convert the front end to 5 or six lug wheels. The press-in axle on the spindle will not fit an 8 lug disc.

Is there a way to get everything that I want? Can I press out the 5/6 lug axles (from the spindle,) and machine a set of 8 lug disc axles to press into the resulting hole? Are 8 lug disc brake axles available for purchase? Do factory 8 lug disc brake spindles have press-in axles? Can I just take a pair of 5 or 6 lug discs and redrill them for 8 lug studs? Is there a control arm based solution available?
If anybody has experience with this combo, an opinion, or an idea that hasn't occurred to me, please point me in the right direction.
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Old 09-13-2018, 08:50 AM   #2
Mike C
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Re: 3/4 ton LWB needs front discs and a 2.5 - 3" drop.

I wouldn’t even think about modifying a spindle. But you can get 8 lug disc conversions with drop spindles. Here is a link to the Performance Online version. I’m sure others make them but that was what I found first using Google.

https://www.performanceonline.com/19...-8-Lug-Deluxe/
If you want 3” or more drop you will need it. You can do a GM front conversion and 2” springs with no troubles and not affect your load carrying capability.

You are correct on rims, though. The way around that is run narrow wheels and tires in the front for sure. The GMT 800 Alcoas are 6.5” I think and they hav 4” backspace.

I did a disc conversion on my 70 GMC 2500 with factory parts. I ran Alcoa 7” classic polished rims with the holes. I ran 265 75 16 tires.

Do you have a coil or leaf rear? That is probably a bigger concern getting a drop in the 4-5” range with leafs. I have a half ton short step with leafs that doing a flip on netted 7” drop. I didn’t want to spend the $$$$ for drop leafs. On my truck I am quite happy with the result, but I won’t be hauling anything real heavy....
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Old 09-13-2018, 11:27 AM   #3
tow rig
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Re: 3/4 ton LWB needs front discs and a 2.5 - 3" drop.

Thanks Mike C. This is a good time to add some more detail on where I am trying to go with this build. I would like the truck to have the largest wheel and tire combo that will fit - for the performance and look that I desire. If I can find a way to keep the track narrow in the front, I would like to run 18X9 Ultra 235 Maverick Chrome rims (8 lug, all four corners) with 255 50 18 or 255 55 18 in the front and 275 55 18 in the rear. I plan to run XL (load) rated low profile ultra high performance all season tires. I have already lowered the rear about 4" using drop shackles and drop hangers with the factory HD leaf springs and new polyurethane bushings throughout. (The cab and bed are currently off of the frame.) The truck will be used primarily for towing a race car trailer or large boat trailer, but I also would like to be able to have it handle as well as available parts allows, and do an occasional run down the quarter mile. I am currently building a 512 ci Cadillac motor (500 - 525hp and 600+ peak torque) to power the truck. The trans will be a TH 400 with a manually operated switch pitch converter. Rear will be a DANA 60 full floater with 4.10 gears and a Power Lock limited slip differential. I should have a disc brake conversion kit arriving today for the rear of the truck.
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Old 09-13-2018, 12:28 PM   #4
Mike C
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Re: 3/4 ton LWB needs front discs and a 2.5 - 3" drop.

Wheel offset is the easiest way to accomplish what you want. The bigger the rim diameter, the more backspacing you can run. I haven't shopped for 8 lug wheels with a positive offset so not sure what is out there. You may not be able to run the same size wheel all the way around and I'm not sure if that is concern for you. But a 5" back space 8" wheel should make up for any displacement caused by the disc conversion.

It was not for me as I ran 235 55 17 on front of my 69 and 275 55 17 on the rear on 8" and 9" rims respectively.
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Old 09-14-2018, 02:43 PM   #5
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Re: 3/4 ton LWB needs front discs and a 2.5 - 3" drop.

Following. I just started my research on doing basically the same thing to my 70 C20 except built 350/tremec 5-speed. It's going to be my son's daily driver and occasionally pulling a race car trailer also. Very interested in your results.
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Old 09-15-2018, 02:07 AM   #6
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Re: 3/4 ton LWB needs front discs and a 2.5 - 3" drop.

The two wheel drive guys are almost totally ignored by the aftermarket when it comes to 8 lug wheels. Nearly all of the available wheels seem to have the busy, fake beadlock look that is currently the flavor of the month for the four wheel drive and off-road guys. I think that I have found a decent looking, 9x18 wheel that is available in two offsets (12mm and -6mm) The difference in the two offsets is exactly 3/4". This may be the solution that I have been looking for. Once I get the cab and bed back on the frame, I will have to do some careful measuring to check fitment. The factory based, or Performanceonline conversion kits may work for me after all. Thanks for the input. Cheers Mike C.

JustKIDn, I am am in the early stages, and am still feeling my way through this. Do you have the leaf or coil sprung rear suspension? (If you have leaf springs I may be able to offer some help.) Do you have the wide or the narrow rear axle? Both were available in 1970.
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Old 09-15-2018, 07:55 AM   #7
justKIDn
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Re: 3/4 ton LWB needs front discs and a 2.5 - 3" drop.

I've got a leaf spring rear, tow rig. I don't know if it's narrow or wide, don't know the difference yet. I'll have to research that one. It's a DANA 60 I do know. I haven't made the time to start a build thread, but I fully intend to soon. Not too much building yet, just a lot of unbuilding!
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Old 09-15-2018, 03:25 PM   #8
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Re: 3/4 ton LWB needs front discs and a 2.5 - 3" drop.

Thanks for the kudos.

I don't know there is a wide/narrow 8 lug setup. I think they are all the same? That's an interesting point and I will be wathcing to see what others say.

I have a couple of Dana 60's, both 1970 models and both leaf spring. I have a HO5200 in a 72 sub but it has wheels on it sunk in the mud so I don't see an easy measurement solution. LOL
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52 M38A1
64 Corvette Coupe
68 Camaro 'vert LT1 & TH700
69 Z/28 355 12.6's @110
69 Chevy Short Step 4 1/2"/7" drop
72 Jimmy 4WD 4spd 4" & 35's
02 GMC 2500HD 4x4 Duramax
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Old 09-16-2018, 12:29 AM   #9
tow rig
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Re: 3/4 ton LWB needs front discs and a 2.5 - 3" drop.

I could be wrong about wide VS narrow as far as the Dana 60 is concerned. I had heard of the 1970 narrow and wide 12 bolts. I measured my Dana 60, and found it to be about 1/4 to 1/2" wider than the wide 12 bolt IIRC.
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Old 09-16-2018, 06:32 AM   #10
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Re: 3/4 ton LWB needs front discs and a 2.5 - 3" drop.

Porterbuilt can do narrowed control arms for c20’s. Not the cheapest option but still an option.
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