1970 GMC 1 Ton Front Disc Brake/14 Bolt Conversion

[Sheepdip]

I've been working on this for about 4-5 months now getting all of my Internet Engineering Calculations checked and re-checked, parts bought etc.

Wednesday of this week my buddy Curtis called and said he was ready to dive into it with him doing the work in his shop, with me mostly in the way and lending a helping hand when and where needed, for the most part it went pretty smooth with no major hiccups. Finished it up yesterday

THANK YOU CURTIS!!!!

I made notes along the way to pass on with Otis to my 2 sons for reference in the future, they will have to sock fight it out as what to do with Otis as I play no favorites.

I for one find a lot of these threads on conversions (not all) really lacking in detail so I thought I would pass along my notes rather than retype it all with hopes that someone can use the info. It may be a little too much detail for some of you but my real intent is for this info to follow the truck into the future. Hope its not too long of a read for you guys n gals.

1970 GMC C35 1 Ton Cab and Chassis Flatbed Converted to Front Disc Brakes And Corporate 14 Bolt Rear Drum Dual Rear Wheel Differential. 3/20/2020


All conversion components are from a 1986 Chevrolet C30 Dual Rear Wheel Cab and Chassis 1 Ton front disc brake suspension cradle and Corporate 14 Bolt 10.5" rear differential that I purchased unless noted.

Conversation Mileage was 96,500 Odometer Miles.

What I used to complete the front disc brakes transition is as follows:

86 Stock C30 Complete Front Spindle left and right Rotor Assembly’s separated at the upper and lower ball joints and tie rod ends.

They were disassembled, cleaned, checked and bearings re-packed with hi-temp bearing grease.

86 C30 Front Disc Brake Pads: O’Reilly P/N - BB2 MKD149 (1) set of 4 pads required.

86 C30 Front Disc Brake Calipers: O’Reilly P/N’s -

BC 18-4166 (1ea) Right Side

BC 18-4167 (1ea) Left Side

1972 GMC C35 front disc brake flex hoses were used from the caliper banjo bolt to male fitting line connection at the frame. The frame end of the hose had to be turned down to fit thru the securing ring at the frame. Other than that they fit fine and are the correct length.
O’Rielly P/N BH71305 (2) required.

Brand new Mev-Tech (Original Grade) Upper and Lower Control Arms to fit 86 C30 Cab and Chassis.

They came supplied with Moog upper and lower ball joints and upper and lower control arm shafts and shaft bushings already installed

These bolt right on the existing 1970 GMC C35 front suspension cradle no modifications required and the 1970 GMC C35 front coil springs were re-used with no issues, they fit perfectly into the new lower control arm spring saddle bucket.

The 86 springs were a little taller and a little beefier and I did not want to raise the existing ride height if possible.

The existing 1970 GMC C35 front shocks bolt right up with no issues.
Monroe front shocks:
P/N 34817( 2) Required

Mavtech Control Arms are as follows:
(1) Left Side Upper/Lower required

(1) Right Side Upper/Lower required

Lower Arm P/N GS9703 left side

Lower Arm P/N GS9702 right side

Upper Arm P/N GS9701 left side

Upper Arm P/N GS9700 right side

86 C30 Upper Ball Joints: Moog K6122 (2 Required) These are listed here for future maintenance purposes when replacement is required.

86 C30 Lower Ball Joints: Moog K6129T (2 Required) These are listed here for future maintenance purposes when replacement is required.

I used the existing 1970 GMC C35 steering cross control bar/link, I installed a new Pittman Arm and new Idler Arm to fit the 1970 GMC C35. Parts are as follows:

1970 GMC C35 Pitman Arm (steering box splined shaft to steering cross link)
Moog P/N K6131 (1) Required
1970 GMC C35 Idler Arm (frame bracket to steering cross link)
Moog P/N 57192JV (1) Required

86 tie rod ends must be used, they bolt right to the 1970 GMC C35 steering link and are a factory fit for the 86 spindles parts are as follows:

86 C30 Outer Tie Rod Ends: Moog ES409RT (2 req)
Use 1 each Side

86 C30 Inner Tie Rod Ends: Moog ES409LT (2 req)
Use 1 each Side


86 C30 Tie Rod Adjuster Sleeves (2req)
ACDelco P/N 46A6004A or
GM P/N 88876894
Use 1 each side


What I used to complete the 1986 14 Bolt Rear Differential swap is as follows:

The 86 14 bolt was opened and inspected for wear, it was found to be a 4.10 ratio in excellent condition with good tooth contact pattern and backlash within specification. A new pinion seal was installed everything re-checked again and the rear cover reinstalled with silicone sealer.

The 86-14 bolt rearend bolts right into the existing 1970 GMC C35 H072 rear differential factory mounting points using the existing U-Bolts.

I had earlier relocated the shock absorber mounting locations on the 14 bolt to accept the existing 1970 GMC C35 rear shocks (both forward facing) These weld on shock brackets can be sourced thru ebay or Amazon.
RuffStuff Specialties R1988 Jeep Lower Axle Shock Mount (Pair)

Note: I chose these mounts because they pitch sideways about 10 degrees.

The center section carrier housing of the 14 bolt is wider than the H072 and also has pressed in axle tubes unlike the welded smoother transition H072 housing.

The right or passenger side you can get right on the money, the left or drivers you can get within 3/4” and still leave enough weld area next to the axle tube sleeve for proper attachment.

You also have to tilt them a little to get the shock mounting bolt level with the axle tube centerline or parallel with the ground, however you want to look at it.
A good wire feed welding machine is the ticket here to keep fitment to a minimum amount of work.
Monroe Rear Shock P/N 34803 (2) Required

A new brake line center flex hose was installed:
Dorman P/N H80967
ACDelco P/N 18J2119

No modifications to the driveline length were required.

2 new U-Joint retainer straps to fit the 14 bolt pinion yoke were required for the existing U-joint to bolt up to it.
U-joint retainer strap kit:
ACDelco p/n 45U0508

U-Joint at the rearend/driveline yoke was almost new and the correct application so it was re-used.
A/CDelco U-Joint P/N 45U0123
Listed for future maintenance

Wheel mounting surface to Wheel mounting surface of drum faces is 63”
Spring pad mounting pads are 40” center to center

Drums are 13”x3-1/2” Dual Rear Wheel configuration

Gear Ratio is 4:10
Open differential (no positraction)

Truck tachometer now reads 1950-2000RPM @ 55MPH in 4th Overdrive.

Brake Drums were turned and trued up.

New brake shoes: A/CDelco
P/N 14452R 13”x 3-1/2”

New rear wheel cylinders: (2) Required
API P/N 53623MY-13x3-1/2” Brake Shoes

New Axle Flange Seals/Gaskets:
FelPro P/N 55350


New master cylinder and proportioning valve for a 1972 GMC C35 original equipment Disc/Drum set up was used.
Master Cylinder: O’Reilly P/N NMC11302

Proportioning Valve GM Style Disc/Drum - PV2- Classic Truck
eBay sourced.

The only modification required was the push shaft out of the existing dual diaphragm booster was for a drum/drum master cylinder so it was to long and was shortened 5/8”+/- and tested for proper clearance with contact putty and contact grease.


In a future maintenance application replace the Vacuum Booster with a 1970 GMC C35 dual diaphragm Original Equipment application and replace the Master Cylinder with the O’REILLYS P/N as above then re-use the existing modified push shaft and there should be no fitment issues. Another option is to replace as a matched unit both the master cylinder and brake booster for a 1972 GMC C35 disc/drum assembly.