C10 12 Id and issues need help. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Toby Hines · 04-15-2016, 03:58 PM

Im having issues with a C10 12 bolt and I need to pin down the year. Casting code is 3856525 and Axle tube # HA 02 18W. I know its a 3.73 open diff.

This rear has barely any room outside the Main caps for shims. I know 12 bolts usually have thick shims. This one had only a .083 on right side and a .073 on left and the main caps dont have a little tab on the upper out side to hold the shim down. The problem is I purchashed this rear and went to do a fluid change and noticed the thin shims had slid out and hit the rear cover and ground into the Axle. I want to freshen this up. I even went as far as pulling off a carrier bearing to check the part number to verify it had the correct bearings. Ive never seen these small shims in these. I was told 60-61 rears were bastard years and used shims under the carrier bearings and also told these early year 12 bolts had pressed on backing plates, this one has bolt on backing plates. Im hoping to use this rear. the other issue I cant find replacement shims and even if I do theres nothing to hold them down other than a tight fit at install. What gives????

**aerotruk63** · 04-15-2016, 04:39 PM

HA starts in 65 , coincidences with the bolt on backing plates.
3.73 ( exc. positraction )
1966-69 all state the same info.
3856525 does not appear in any of my parts books. I believe this is a part number, but is not posted as it is part of an assembly.

Toby Hines · 04-16-2016, 05:25 PM

Ok thanks now I gotta figure out the shim issue. any takers?

**aerotruk63** · 04-16-2016, 07:19 PM

I don't know if this slideshow will help.

**aerotruk63** · 04-16-2016, 07:20 PM

https://www.youtube.com/watch?v=miCa9cDNKgI

vince1 · 04-17-2016, 11:06 AM

I took my trashed 66 12 bolt apart. Don't remember much except that the shims were not very thick. On the other hand I seem to remember 10 bolt types at the junkyard with very thick shims like maybe almost a quarter inch.

HackJob65 · 04-19-2016, 12:01 AM

All the early 12B truck rears have the thin shims I recently had a 64 and a 66 apart and used a couple of the thinner original shims along with some from install kit to set up new gears. As for not having tabs to hold shims in place I didn't encounter that problem. Have the caps possibly been removed before and turned upside down? The tabs are fairly small. If I can find the info I do remember someone sells shim kits specifically for these.

04-15-2016, 03:58 PM	#1
Toby Hines Registered User Join Date: Apr 2009 Location: Titusville Fl. Posts: 47	C10 12 Id and issues need help. Im having issues with a C10 12 bolt and I need to pin down the year. Casting code is 3856525 and Axle tube # HA 02 18W. I know its a 3.73 open diff. This rear has barely any room outside the Main caps for shims. I know 12 bolts usually have thick shims. This one had only a .083 on right side and a .073 on left and the main caps dont have a little tab on the upper out side to hold the shim down. The problem is I purchashed this rear and went to do a fluid change and noticed the thin shims had slid out and hit the rear cover and ground into the Axle. I want to freshen this up. I even went as far as pulling off a carrier bearing to check the part number to verify it had the correct bearings. Ive never seen these small shims in these. I was told 60-61 rears were bastard years and used shims under the carrier bearings and also told these early year 12 bolts had pressed on backing plates, this one has bolt on backing plates. Im hoping to use this rear. the other issue I cant find replacement shims and even if I do theres nothing to hold them down other than a tight fit at install. What gives???? __________________ 1959 short fleet Camaro clipped, Shockwaves, Strongarms, Musclebar, 4 link explorer rear. LY6 6.0 and 6l90. Current project. 1958 50th Anniversary Golden Jubilee next project Last edited by Toby Hines; 04-15-2016 at 04:09 PM.

04-15-2016, 04:39 PM	#2
aerotruk63 Check The Champ, Demonstrator Join Date: Jun 2009 Location: Montreal,Quebec Posts: 6,625	Re: C10 12 Id and issues need help. HA starts in 65 , coincidences with the bolt on backing plates. 3.73 ( exc. positraction ) 1966-69 all state the same info. 3856525 does not appear in any of my parts books. I believe this is a part number, but is not posted as it is part of an assembly. __________________ 1963 *Chevrolet Truck* Literature LINK: https://picasaweb.google.com/113840717762774560215 YouTube Channel with 20+ Original *Chevrolet Truck* Salesmen, Mechanics & Service Department Training Slideshows. https://www.youtube.com/results?sear...otruk63&page=1

04-16-2016, 05:25 PM	#3
Toby Hines Registered User Join Date: Apr 2009 Location: Titusville Fl. Posts: 47	Re: C10 12 Id and issues need help. Ok thanks now I gotta figure out the shim issue. any takers? __________________ 1959 short fleet Camaro clipped, Shockwaves, Strongarms, Musclebar, 4 link explorer rear. LY6 6.0 and 6l90. Current project. 1958 50th Anniversary Golden Jubilee next project

04-16-2016, 07:19 PM	#4
aerotruk63 Check The Champ, Demonstrator Join Date: Jun 2009 Location: Montreal,Quebec Posts: 6,625	Re: C10 12 Id and issues need help. I don't know if this slideshow will help. __________________ 1963 *Chevrolet Truck* Literature LINK: https://picasaweb.google.com/113840717762774560215 YouTube Channel with 20+ Original *Chevrolet Truck* Salesmen, Mechanics & Service Department Training Slideshows. https://www.youtube.com/results?sear...otruk63&page=1

04-16-2016, 07:20 PM	#5
aerotruk63 Check The Champ, Demonstrator Join Date: Jun 2009 Location: Montreal,Quebec Posts: 6,625	Re: C10 12 Id and issues need help. https://www.youtube.com/watch?v=miCa9cDNKgI __________________ 1963 *Chevrolet Truck* Literature LINK: https://picasaweb.google.com/113840717762774560215 YouTube Channel with 20+ Original *Chevrolet Truck* Salesmen, Mechanics & Service Department Training Slideshows. https://www.youtube.com/results?sear...otruk63&page=1

04-17-2016, 11:06 AM	#6
vince1 Registered User Join Date: Jan 2013 Location: Calgary Alberta Posts: 1,166	Re: C10 12 Id and issues need help. I took my trashed 66 12 bolt apart. Don't remember much except that the shims were not very thick. On the other hand I seem to remember 10 bolt types at the junkyard with very thick shims like maybe almost a quarter inch.

04-19-2016, 12:01 AM	#7
HackJob65 Registered User Join Date: Apr 2015 Location: MO Posts: 42	Re: C10 12 Id and issues need help. All the early 12B truck rears have the thin shims I recently had a 64 and a 66 apart and used a couple of the thinner original shims along with some from install kit to set up new gears. As for not having tabs to hold shims in place I didn't encounter that problem. Have the caps possibly been removed before and turned upside down? The tabs are fairly small. If I can find the info I do remember someone sells shim kits specifically for these.



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