Disc Brake conversion problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

69worktruck · 10-06-2014, 02:41 PM

Hello Board,
I am hoping that someone can help me with a problem I have with my disc brake conversion on my 69 GMC C2500 8 lug.

Here is what I have done. The donor is a 78 C30 Chevy.

Upper and lower ball joints
Tie rods
spindles
rotors
calipers
MC

The upper and lower control arms are the original.

The brakes work good, I assume even better with a 1 ton booster (I kept the original) which I will eventually do.

My problem is that the truck won't align. I have too much positive camber and the shims on the upper control arm have all been removed so I have no more adjustment.

Are there differences in the control arms other than ball joints and bushings? Are the mounting points on the cross member different from 69 to 78? Or?

Any help will be greatly appreciated.

Thanks, Eric

viking427 · 10-06-2014, 09:19 PM

67-87 should all have the same locating points. The lower control arm is the difference b/n 1/2 ton and 3/4 or 1 ton (larger ball joint) but your 69 C20 and 78 C30 upper & lower arms should be interchangeable. GM went to metric fasteners in 78 but that shouldn't effect anything relating to your problem. Short of the frame being bent I cant see why it wouldn't align.

69worktruck · 10-07-2014, 08:55 AM

Thanks Viking,
I have owned the truck since 01 and the alignment has never been a problem till I did the conversion. Perhaps the spindles from the donor truck are the issue.

Eric

69worktruck · 10-29-2014, 11:23 AM

The only differences I could find were with the 78 C30 spindles...the geometry is a bit different from the originals. What I ended up doing was to pull the convex spacers from between the upper control arm shaft and the frame mount. Then I ground off the concave portion of the upper control arm shaft to make it flat. Then remounted the upper control arm without the spacers. This gave me enough room and now the camber is spot on with the addition of a couple of shims. Truck drives great and the only drawback I can see is that this will not allow any caster adjustment (using different thickness of shims front to back) which, in my case anyways, was not being utilized as the shim stack front to back has always been equal.

10-06-2014, 02:41 PM	#1
69worktruck Registered User Join Date: Oct 2014 Location: Daphne, AL Posts: 10	Disc Brake conversion problem Hello Board, I am hoping that someone can help me with a problem I have with my disc brake conversion on my 69 GMC C2500 8 lug. Here is what I have done. The donor is a 78 C30 Chevy. Upper and lower ball joints Tie rods spindles rotors calipers MC The upper and lower control arms are the original. The brakes work good, I assume even better with a 1 ton booster (I kept the original) which I will eventually do. My problem is that the truck won't align. I have too much positive camber and the shims on the upper control arm have all been removed so I have no more adjustment. Are there differences in the control arms other than ball joints and bushings? Are the mounting points on the cross member different from 69 to 78? Or? Any help will be greatly appreciated. Thanks, Eric

10-06-2014, 09:19 PM	#2
viking427 Registered User Join Date: Aug 2011 Location: Dallas TX Posts: 225	Re: Disc Brake conversion problem 67-87 should all have the same locating points. The lower control arm is the difference b/n 1/2 ton and 3/4 or 1 ton (larger ball joint) but your 69 C20 and 78 C30 upper & lower arms should be interchangeable. GM went to metric fasteners in 78 but that shouldn't effect anything relating to your problem. Short of the frame being bent I cant see why it wouldn't align.

10-07-2014, 08:55 AM	#3
69worktruck Registered User Join Date: Oct 2014 Location: Daphne, AL Posts: 10	Re: Disc Brake conversion problem Thanks Viking, I have owned the truck since 01 and the alignment has never been a problem till I did the conversion. Perhaps the spindles from the donor truck are the issue. Eric

10-29-2014, 11:23 AM	#4
69worktruck Registered User Join Date: Oct 2014 Location: Daphne, AL Posts: 10	Re: Disc Brake conversion problem The only differences I could find were with the 78 C30 spindles...the geometry is a bit different from the originals. What I ended up doing was to pull the convex spacers from between the upper control arm shaft and the frame mount. Then I ground off the concave portion of the upper control arm shaft to make it flat. Then remounted the upper control arm without the spacers. This gave me enough room and now the camber is spot on with the addition of a couple of shims. Truck drives great and the only drawback I can see is that this will not allow any caster adjustment (using different thickness of shims front to back) which, in my case anyways, was not being utilized as the shim stack front to back has always been equal.



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