Is this a rear ratio code? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bobs409 · 08-19-2003, 03:34 PM

I just found this stamped on top of the pumpkin on my 71 C-20. This is the Eaton style rear. Can anyone decipher it? It's JBC2521

Thanks,

Bob

**COBALT** · 08-19-2003, 04:10 PM

If you get an answer for this that makes sense let me know. I have every number off my HO52 axle, and they don't mean jack-squat. They're mostly casting numbers, the casting clock for when the housing was made, etc. Nothing about the ratio...

bobs409 · 08-22-2003, 08:41 AM

I guess it doesn't mean anything, no responses.

Maybe someone can help me figure out the ratio anyway. I jacked up the rear and put a chalk mark on the tire and driveshaft. This is not a posi rear-turn one tire and the other tire turns opposite.

When I turn the tire one full revolution, the driveshaft turns just a tad over 2 turns. What the heck is that???

Thanks,

Bob

SLS · 08-22-2003, 09:39 AM

On NON-posi units the tire needs to be rotated TWICE instead of once...with one tire blocked (or left on the ground).

71GMC_3/4T · 08-22-2003, 09:59 AM

That number in the pic is only for the casting. The number you are looking for is stamped on the top of the tube, somewhere between the punkin and the brake plate. Takes a little scraping sometimes, but it's there. Sometimes there's even a little tag that has the ratio under one of the bolts on the cover plate.
Sounds like you have a 4.11.
If you were to turn the one tire two exact turns, your pinion would rotate the same if one tire turned one full turn. This applies to conventional rearends.

bobs409 · 08-22-2003, 11:50 AM

I just checked and there is nothing stamped on the tube. I have the box off so I can see very clearly. I scraped and wire brushed the passenger side top and front and even a bit of the back. No tag either.

I will be cleaning it in more detail soon so maybe I will find it yet.

I have a Chiltons book that says the 69-73 has a serial number stamped on the bottom flange of the differential carrier housing and on the front of the right axle tube for 1974 and later. Does that sound correct?

Will have to try turning the tire twice yet too.

Thanks for the help so far...

**Classic Heartbeat** · 08-22-2003, 12:24 PM

Those are not casting numbers. They are the numbers that are supposed to identify the rear end. They are hand stamped letters that are in the correct spot. They just don't make sence. There must be something missing. According to my GM Master Parts Catalog, all of the "70" and "71" rear ends started with a "T" and "72" they started with an "R". For all three years the letter codes were 3 letters long and prior to "70" they were 2 letters long. WES www.ClassicHeartbeat.com

**Classic Heartbeat** · 08-22-2003, 12:28 PM

If it were missing the "T" and the first 3 letters were TJB then it is a 4.10 non possi. That is the only way I can make sence of it. WES www.ClassicHeartbeat.com

bobs409 · 08-22-2003, 04:56 PM

That sounds like a possibility then Wes. Thanks! If I turn the tire two revolutions, it most likely will turn the driveshaft over 4 times so sounds about right and it is non posi. (will try it yet)

I've noticed mention of a 4.11 and now 4.10. Isn't that the same thing?

08-19-2003, 03:34 PM	#1
bobs409 1969 Custom 30 wrecker Join Date: Sep 2001 Location: Pottsville, Pa Posts: 2,611	Is this a rear ratio code? I just found this stamped on top of the pumpkin on my 71 C-20. This is the Eaton style rear. Can anyone decipher it? It's JBC2521 Thanks, Bob Attached Images __________________ Bob 63 Impala 283/PG/3.36 (under restoration) 66 Impala 327/PG/3.36 66 Chevelle SS 409/M22/3.55 69 Chevelle 307/PG/3.08 (future restoration) 69 C10 short stepside 454/TH350/3.73 69 Custom 30 tow truck 350/4 spd/4.10 (resto done, CRUISIN TIME!) 71 Cheyenne 20 fleetside 350/4 spd/4.10 71 Chevelle 6/PG/3.08 72 Chevelle 454/M21/4.10

08-19-2003, 04:10 PM	#2
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	If you get an answer for this that makes sense let me know. I have every number off my HO52 axle, and they don't mean jack-squat. They're mostly casting numbers, the casting clock for when the housing was made, etc. Nothing about the ratio... __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

08-22-2003, 08:41 AM	#3
bobs409 1969 Custom 30 wrecker Join Date: Sep 2001 Location: Pottsville, Pa Posts: 2,611	I guess it doesn't mean anything, no responses. Maybe someone can help me figure out the ratio anyway. I jacked up the rear and put a chalk mark on the tire and driveshaft. This is not a posi rear-turn one tire and the other tire turns opposite. When I turn the tire one full revolution, the driveshaft turns just a tad over 2 turns. What the heck is that??? Thanks, Bob __________________ Bob 63 Impala 283/PG/3.36 (under restoration) 66 Impala 327/PG/3.36 66 Chevelle SS 409/M22/3.55 69 Chevelle 307/PG/3.08 (future restoration) 69 C10 short stepside 454/TH350/3.73 69 Custom 30 tow truck 350/4 spd/4.10 (resto done, CRUISIN TIME!) 71 Cheyenne 20 fleetside 350/4 spd/4.10 71 Chevelle 6/PG/3.08 72 Chevelle 454/M21/4.10

08-22-2003, 09:59 AM	#5
71GMC_3/4T Weapons Of Construction Join Date: Apr 2000 Location: Colorado, USA Posts: 1,095	That number in the pic is only for the casting. The number you are looking for is stamped on the top of the tube, somewhere between the punkin and the brake plate. Takes a little scraping sometimes, but it's there. Sometimes there's even a little tag that has the ratio under one of the bolts on the cover plate. Sounds like you have a 4.11. If you were to turn the one tire two exact turns, your pinion would rotate the same if one tire turned one full turn. This applies to conventional rearends. __________________ 1971 GMC 2500, 402/TH400 4.10 Daily Driver Lafayette, CO

08-22-2003, 11:50 AM	#6
bobs409 1969 Custom 30 wrecker Join Date: Sep 2001 Location: Pottsville, Pa Posts: 2,611	I just checked and there is nothing stamped on the tube. I have the box off so I can see very clearly. I scraped and wire brushed the passenger side top and front and even a bit of the back. No tag either. I will be cleaning it in more detail soon so maybe I will find it yet. I have a Chiltons book that says the 69-73 has a serial number stamped on the bottom flange of the differential carrier housing and on the front of the right axle tube for 1974 and later. Does that sound correct? Will have to try turning the tire twice yet too. Thanks for the help so far... __________________ Bob 63 Impala 283/PG/3.36 (under restoration) 66 Impala 327/PG/3.36 66 Chevelle SS 409/M22/3.55 69 Chevelle 307/PG/3.08 (future restoration) 69 C10 short stepside 454/TH350/3.73 69 Custom 30 tow truck 350/4 spd/4.10 (resto done, CRUISIN TIME!) 71 Cheyenne 20 fleetside 350/4 spd/4.10 71 Chevelle 6/PG/3.08 72 Chevelle 454/M21/4.10

08-22-2003, 09:39 AM	#4
SLS Finish the'71..or bust! Join Date: Jun 2003 Location: Nashville, TN. Posts: 441	On NON-posi units the tire needs to be rotated TWICE instead of once...with one tire blocked (or left on the ground).

08-22-2003, 12:24 PM	#7
Classic Heartbeat Project Junkie! Fishing Poor!! Join Date: Dec 1999 Location: Olympia,Wa. 98512 Posts: 10,770	Those are not casting numbers. They are the numbers that are supposed to identify the rear end. They are hand stamped letters that are in the correct spot. They just don't make sence. There must be something missing. According to my GM Master Parts Catalog, all of the "70" and "71" rear ends started with a "T" and "72" they started with an "R". For all three years the letter codes were 3 letters long and prior to "70" they were 2 letters long. WES www.ClassicHeartbeat.com __________________ On-line catolog Classic Heartbeat Pickup Parts WEHEPP@comcast.net Call us Toll Free (888) 338-2502 Like Us on Facebook No appointment necessary. 72 BB Chevy Custom Deluxe / Custom Camper 67 I-6 (Soon to be 5.7 LS1) Panel 68 BB Chevy Short Bed 72 6.0 LQ9 Short Bed Chevy 4X4 Olympia's fastest growing truck parts supplier. Olympia's home for Wayward Chevys

08-22-2003, 12:28 PM	#8
Classic Heartbeat Project Junkie! Fishing Poor!! Join Date: Dec 1999 Location: Olympia,Wa. 98512 Posts: 10,770	If it were missing the "T" and the first 3 letters were TJB then it is a 4.10 non possi. That is the only way I can make sence of it. WES www.ClassicHeartbeat.com __________________ On-line catolog Classic Heartbeat Pickup Parts WEHEPP@comcast.net Call us Toll Free (888) 338-2502 Like Us on Facebook No appointment necessary. 72 BB Chevy Custom Deluxe / Custom Camper 67 I-6 (Soon to be 5.7 LS1) Panel 68 BB Chevy Short Bed 72 6.0 LQ9 Short Bed Chevy 4X4 Olympia's fastest growing truck parts supplier. Olympia's home for Wayward Chevys

08-22-2003, 04:56 PM	#9
bobs409 1969 Custom 30 wrecker Join Date: Sep 2001 Location: Pottsville, Pa Posts: 2,611	That sounds like a possibility then Wes. Thanks! If I turn the tire two revolutions, it most likely will turn the driveshaft over 4 times so sounds about right and it is non posi. (will try it yet) I've noticed mention of a 4.11 and now 4.10. Isn't that the same thing? __________________ Bob 63 Impala 283/PG/3.36 (under restoration) 66 Impala 327/PG/3.36 66 Chevelle SS 409/M22/3.55 69 Chevelle 307/PG/3.08 (future restoration) 69 C10 short stepside 454/TH350/3.73 69 Custom 30 tow truck 350/4 spd/4.10 (resto done, CRUISIN TIME!) 71 Cheyenne 20 fleetside 350/4 spd/4.10 71 Chevelle 6/PG/3.08 72 Chevelle 454/M21/4.10



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