changing differential oil - The 1947 - Present Chevrolet & GMC Truck Message Board Network

dropped72shortbed · 03-24-2002, 03:15 AM

first time doing this, main reason im pulling the cover is just to see what gears i really have. previous owner says 4.11s, but hes been wrong on other stuff. it does rev pretty high on the freeway, i think the truck tops out under 100mph. so maybe they are 4.11s

anways what are the steps? just pull the cover and let drain? then do i need a new gasket for the cover, what weight oil? when im done just bolt the cover back on and then refill the oil from a fill hole up top? any idea where the hole is? im know is kinda hard if i cant tell you want rearend it is, but ill take pics while im working monday and show em to the board. im sure i can figure it out myself.

sorry, i must have been slacking off in high school auto shop cause i never did this chore.

sam's 72 · 03-24-2002, 06:01 AM

Get a pan, that you can drain oil into, about a gallion. put it under the rearend, pull the bolt, break the pan at the bottom with a screw driver, hold the pan in place at the top, break the bottom let the oil drain, then remove pan, I usually take a rag and rake under the ring gear on the diferetial, to get all the oil out from under the ring gear. just let it drain. Clean up the pan, I would use a gasket if I were you, I put RTV on the pan side, then set the gasket in place, the rtv will hold it in place. Then while this is drying check out your gear ratio. Put the pan back in place, once all the bolt are in and tight, on the side of the rearend ( driver side, I think) Look close, and you will see a plug, it has a square head inset down in it, a 1/2 drive ratchet will fit into the square, break it loose and remove. ( you will probabley need an extention on the ratchet) This is the fill port, fill it up with a good grade 90 wt. I used pennz oil when I did mine, just make sure it is gear oil, 90 wt or better, it will take either 2 1/2 guarts or 3 1/2 I can't remember, just fill it up till it starts running out the port you are filling at, then screw the plug back in, and your done! Good luck with it and use jack stand, dont ever trust the jack to hold up the truck while your under it.
Sam

*Fast68Chevy* · 03-24-2002, 09:38 AM

i bet they are 3.73, but if for some oddball reason they are 4,11 then i want to know more about the truck.

------------------
-71/C-10(thx teeitup and crews!)350 4+4/CH465(thx 71Chev, GMCPaul, and CPNE)longfleet bed(thnx meathead95)!still building it-
-'84 Z28(Z28) 1G1AP87GXEL100000 with H.O. 305/4bbl/190net/hp(L69) 3.73 rear(GT4/YC4) 700R4(MD8-MXO) & other goodies

-79 G20 pass van, 350/th350/10bolt,
paint is overrated
Randy- Lincoln, IL

r68c30@hotmail.com
© RSS Enterprises 2000

blue68man · 03-24-2002, 10:04 AM

how often should change the oil in the rearend?

67Blacksheep · 03-24-2002, 10:21 AM

You should have no trouble with draining the fluids. It's figuring out the gear ration that I would have problems with !! I was wondering if the rear case had any marks that would identify the gear. Fast68 says they were all 3:08 or 3:73 in stock form. I just got a used 1/2 rear and have no idea what it has, but I know it's stock. Whatever, it has it will be easier on the motor than the Eaton gears. ( Fast says most likely 4:47) Talk about ringing out the small block!!

------------------
67 Chevy C20 LB Stepside,
350 4-bolt with goodies.

Longhorn Man · 03-24-2002, 10:25 AM

Just count the teeth and divide between the two. (I can't remember if you divide the ringbby the pinion, or the pinion by the ring...either way, you'll do it once, or do it twice)
As far as the oil chainge, I used straight RTV on mine, no one had the gasket in stock, it's still holding up. (15 months later) Be sure to clean the gasket surfases real will. Being that the bottom portion is always in oil and all.(as opposed to say a valve cover that just gat's splashed with oil)

------------------
'69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man
I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it.
Need to pick up the one ton Longhorn with a PONTIAC 350/350soon.
Thanks every one!

Check out The Longhorn Webite.
If you need a pic posted, E-mail me at longhornmail@yahoo.com
Andy, in Columbus Ohio

71K-20 · 03-24-2002, 10:26 AM

A good practice if this is you daily driver or if for any reason you can't leave the rig out of service, make sure the fill plug will come out before you break your cover loose. A friend of mine learned this in Africa,he drove through lots of deep water and when he got to the village he popped his covers. He then discovered the rear fill plug was seized and he had no power tools.

------------------
69'GMC 3/4ton 350 4sp
71'GMC 3/4ton 350 4sp
71'K-20 383 TH-350
Orcas Island, Washington

Longhorn Man · 03-24-2002, 10:38 AM

OMG..that would suck! Good tip too.

------------------
'69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man
I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it.
Need to pick up the one ton Longhorn with a PONTIAC 350/350soon.
Thanks every one!

Check out The Longhorn Webite.
If you need a pic posted, E-mail me at longhornmail@yahoo.com
Andy, in Columbus Ohio

ebfabman · 03-24-2002, 11:38 AM

The most fool proof way to find a rear end gear ratio when you have the rear cover off is to count the # of teeth on the ring gear, then count the # of teeth on the pinion gear. Divide the # of ring gear teeth by the # of pinion gear teeth.

Example; 37 teeth on ring gear/9 teeth on pinion gear = 4.11

Another way if you have a buddy that can count (not your dog...lol) is to mark a spot on the drive shaft or U joint yoke at the rear end and make a mark on the ring gear. Have someone turn the drive shaft, counting turns while the other person counts the # of turns on the ring gear. IMPORTANT- the counting by both parties must not be done out loud. When the person counting the ring gear turns gets to 10, tell the other person to stop. Note his # of drive shaft turns. Then move the decimal one place to the left.

The ratio is how many turns of the pinion gear to 1 turn of the ring gear.

Using the above example; 10.0 turns of the ring gear, 41.1 turns of the pinion gear. moving the decimal one place to the left gives a ratio of 4.11 to 1, or a rear end rear of 4.11

This method works good when its hard to see/count the pinion gear teeth. This can also be done the other way around, someone turns the wheel 10 times while someone counts the # of drive shaft turns. Just don't be fooled by a slipping differiential.

[This message has been edited by ebfabman (edited March 24, 2002).]

Longhorn Man · 03-24-2002, 11:41 AM

Even though the second way you posted wil work, I find it to be in acurate sometimes, and normally advise them to pull the cover. And like I always say, when was the last time the oil was chainged? The rear diff oil is the most neglected fluid in any rear wheel drive vehicle. It normally don't get replaced untill a seal goes bad and it all leaks out.

------------------
'69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man
I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it.
Need to pick up the one ton Longhorn with a PONTIAC 350/350soon.
Thanks every one!

Check out The Longhorn Webite.
If you need a pic posted, E-mail me at longhornmail@yahoo.com
Andy, in Columbus Ohio

dropped72shortbed · 03-24-2002, 12:04 PM

fast, the truck is a 72 1/2ton swb, used to be a 6 banger from the vin but now its got a v8 in it. im guessing it was a real base model, has the lower model guages, no a/c, no power brakes. at least its got power steering but that was probably added.

who ever put the v8 in also added hei, headers, intake and edel brock carb so chances are the guy might have changed the rear end as well

71-4x4-bb482 · 03-24-2002, 12:20 PM

The 2 numbers that you devide are steel stamped right into the outer edge of the ring gear rotate the ring and wipe it off as you go , they will be about a quarter inch tall, then just devide the larger by the smaller

ebfabman · 03-24-2002, 12:56 PM

This often times not the case. Many times the #'s stamped on the gears don't correspond with the # of teeth on the gears. Or many times there are no stampings at all. This is especially the case with some aftermarket gear sets.

To be sure while the cover is off, its best to just go ahead and count the teeth.

*Fast68Chevy* · 03-24-2002, 01:05 PM

well 4,11 would be listed on glovebox door, i gaurantee you dont have 4,11
only one person on this forum that I know of has actual 4.11 gears in their 12 bolt axle
it is very rare and very optional

you have 3.73, you just gave me the info to prove it, the teeth count is stamped into the outside edge of the ring gear visible
i highly doubt anyone swapped the gears out, sounds like youll be the first one to do so now on it

almost everyone thinks and claims they have 4.11 in a 67-72 truck, that is until they pull the cover and find out its 3.07 or 3.73

good luck

------------------
-71/C-10(thx teeitup and crews!)350 4+4/CH465(thx 71Chev, GMCPaul, and CPNE)longfleet bed(thnx meathead95)!still building it-
-'84 Z28(Z28) 1G1AP87GXEL100000 with H.O. 305/4bbl/190net/hp(L69) 3.73 rear(GT4/YC4) 700R4(MD8-MXO) & other goodies

-79 G20 pass van, 350/th350/10bolt,
paint is overrated
Randy- Lincoln, IL

r68c30@hotmail.com
© RSS Enterprises 2000

[This message has been edited by Fast68Chevy (edited March 24, 2002).]

dropped72shortbed · 03-24-2002, 05:35 PM

whould the six cylinder have been given diff gears? the vin is CSS so it was originally a 6 and now its got a 350 in it.

i figured it might not be the original rear end set up since its got a posi. would a six cylinder truck have come with a posi?

[This message has been edited by dropped72shortbed (edited March 24, 2002).]

Longhorn Man · 03-24-2002, 06:07 PM

Yes, a 6 could have posi.

------------------
'69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man
I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it.
Need to pick up the one ton Longhorn with a PONTIAC 350/350soon.
Thanks every one!

Check out The Longhorn Webite.
If you need a pic posted, E-mail me at longhornmail@yahoo.com
Andy, in Columbus Ohio

03-24-2002, 03:15 AM	#1
dropped72shortbed Registered User Join Date: Nov 2001 Posts: 303	changing differential oil first time doing this, main reason im pulling the cover is just to see what gears i really have. previous owner says 4.11s, but hes been wrong on other stuff. it does rev pretty high on the freeway, i think the truck tops out under 100mph. so maybe they are 4.11s anways what are the steps? just pull the cover and let drain? then do i need a new gasket for the cover, what weight oil? when im done just bolt the cover back on and then refill the oil from a fill hole up top? any idea where the hole is? im know is kinda hard if i cant tell you want rearend it is, but ill take pics while im working monday and show em to the board. im sure i can figure it out myself. sorry, i must have been slacking off in high school auto shop cause i never did this chore. __________________ Blake A. - Huntington Beach CA 72 c10 swb 5/5 drop, 350/350, partialy shaved, soon to be 5/7 on 20s.

03-24-2002, 06:01 AM	#2
sam's 72 Senior Member Join Date: Nov 2001 Location: Pasadena, Texas, USA Posts: 1,005	Get a pan, that you can drain oil into, about a gallion. put it under the rearend, pull the bolt, break the pan at the bottom with a screw driver, hold the pan in place at the top, break the bottom let the oil drain, then remove pan, I usually take a rag and rake under the ring gear on the diferetial, to get all the oil out from under the ring gear. just let it drain. Clean up the pan, I would use a gasket if I were you, I put RTV on the pan side, then set the gasket in place, the rtv will hold it in place. Then while this is drying check out your gear ratio. Put the pan back in place, once all the bolt are in and tight, on the side of the rearend ( driver side, I think) Look close, and you will see a plug, it has a square head inset down in it, a 1/2 drive ratchet will fit into the square, break it loose and remove. ( you will probabley need an extention on the ratchet) This is the fill port, fill it up with a good grade 90 wt. I used pennz oil when I did mine, just make sure it is gear oil, 90 wt or better, it will take either 2 1/2 guarts or 3 1/2 I can't remember, just fill it up till it starts running out the port you are filling at, then screw the plug back in, and your done! Good luck with it and use jack stand, dont ever trust the jack to hold up the truck while your under it. Sam __________________ 72 LWB chevy truck. Pasadena, Texas

03-24-2002, 10:04 AM	#4
blue68man Registered User Join Date: Feb 2002 Location: camden,ar,usa Posts: 68	how often should change the oil in the rearend? __________________ Mike Cummings proud owner of a blue 68 lwb 250 straight 6, 3 on the floor.

03-24-2002, 10:21 AM	#5
67Blacksheep Registered User Join Date: Mar 2002 Location: Ohio Posts: 135	You should have no trouble with draining the fluids. It's figuring out the gear ration that I would have problems with !! I was wondering if the rear case had any marks that would identify the gear. Fast68 says they were all 3:08 or 3:73 in stock form. I just got a used 1/2 rear and have no idea what it has, but I know it's stock. Whatever, it has it will be easier on the motor than the Eaton gears. ( Fast says most likely 4:47) Talk about ringing out the small block!! ------------------ 67 Chevy C20 LB Stepside, 350 4-bolt with goodies. __________________ '94 GMC Seirra Ext. Cab '67 Formula Firebird '89 GSX-R 750

03-24-2002, 10:26 AM	#7
71K-20 Junior Member Join Date: Jul 2001 Location: Orcas, WA. U.S.A. Posts: 44	A good practice if this is you daily driver or if for any reason you can't leave the rig out of service, make sure the fill plug will come out before you break your cover loose. A friend of mine learned this in Africa,he drove through lots of deep water and when he got to the village he popped his covers. He then discovered the rear fill plug was seized and he had no power tools. ------------------ 69'GMC 3/4ton 350 4sp 71'GMC 3/4ton 350 4sp 71'K-20 383 TH-350 Orcas Island, Washington __________________ 69'GMC 3/4ton 350 4sp 71'GMC 3/4ton 350 4sp 71'K-20 383 TH-350 Orcas Island, Washington

03-24-2002, 09:38 AM	#3
Fast68Chevy Account Suspended Join Date: May 2000 Location: IL Posts: 0	i bet they are 3.73, but if for some oddball reason they are 4,11 then i want to know more about the truck. ------------------ -71/C-10(thx teeitup and crews!)350 4+4/CH465(thx 71Chev, GMCPaul, and CPNE)longfleet bed(thnx meathead95)!still building it- -'84 Z28(Z28) 1G1AP87GXEL100000 with H.O. 305/4bbl/190net/hp(L69) 3.73 rear(GT4/YC4) 700R4(MD8-MXO) & other goodies -79 G20 pass van, 350/th350/10bolt, paint is overrated Randy- Lincoln, IL r68c30@hotmail.com © RSS Enterprises 2000

03-24-2002, 10:25 AM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Just count the teeth and divide between the two. (I can't remember if you divide the ringbby the pinion, or the pinion by the ring...either way, you'll do it once, or do it twice) As far as the oil chainge, I used straight RTV on mine, no one had the gasket in stock, it's still holding up. (15 months later) Be sure to clean the gasket surfases real will. Being that the bottom portion is always in oil and all.(as opposed to say a valve cover that just gat's splashed with oil) ------------------ '69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it. Need to pick up the one ton Longhorn with a PONTIAC 350/350soon. Thanks every one! Check out The Longhorn Webite. If you need a pic posted, E-mail me at longhornmail@yahoo.com Andy, in Columbus Ohio

03-24-2002, 10:38 AM	#8
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	OMG..that would suck! Good tip too. ------------------ '69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it. Need to pick up the one ton Longhorn with a PONTIAC 350/350soon. Thanks every one! Check out The Longhorn Webite. If you need a pic posted, E-mail me at longhornmail@yahoo.com Andy, in Columbus Ohio

03-24-2002, 11:38 AM	#9
ebfabman Registered User Join Date: Nov 2001 Location: Concord, NC Posts: 2,489	The most fool proof way to find a rear end gear ratio when you have the rear cover off is to count the # of teeth on the ring gear, then count the # of teeth on the pinion gear. Divide the # of ring gear teeth by the # of pinion gear teeth. Example; 37 teeth on ring gear/9 teeth on pinion gear = 4.11 Another way if you have a buddy that can count (not your dog...lol) is to mark a spot on the drive shaft or U joint yoke at the rear end and make a mark on the ring gear. Have someone turn the drive shaft, counting turns while the other person counts the # of turns on the ring gear. IMPORTANT- the counting by both parties must not be done out loud. When the person counting the ring gear turns gets to 10, tell the other person to stop. Note his # of drive shaft turns. Then move the decimal one place to the left. The ratio is how many turns of the pinion gear to 1 turn of the ring gear. Using the above example; 10.0 turns of the ring gear, 41.1 turns of the pinion gear. moving the decimal one place to the left gives a ratio of 4.11 to 1, or a rear end rear of 4.11 This method works good when its hard to see/count the pinion gear teeth. This can also be done the other way around, someone turns the wheel 10 times while someone counts the # of drive shaft turns. Just don't be fooled by a slipping differiential. [This message has been edited by ebfabman (edited March 24, 2002).]

03-24-2002, 11:41 AM	#10
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Even though the second way you posted wil work, I find it to be in acurate sometimes, and normally advise them to pull the cover. And like I always say, when was the last time the oil was chainged? The rear diff oil is the most neglected fluid in any rear wheel drive vehicle. It normally don't get replaced untill a seal goes bad and it all leaks out. ------------------ '69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it. Need to pick up the one ton Longhorn with a PONTIAC 350/350soon. Thanks every one! Check out The Longhorn Webite. If you need a pic posted, E-mail me at longhornmail@yahoo.com Andy, in Columbus Ohio

03-24-2002, 12:04 PM	#11
dropped72shortbed Registered User Join Date: Nov 2001 Posts: 303	fast, the truck is a 72 1/2ton swb, used to be a 6 banger from the vin but now its got a v8 in it. im guessing it was a real base model, has the lower model guages, no a/c, no power brakes. at least its got power steering but that was probably added. who ever put the v8 in also added hei, headers, intake and edel brock carb so chances are the guy might have changed the rear end as well __________________ Blake A. - Huntington Beach CA 72 c10 swb 5/5 drop, 350/350, partialy shaved, soon to be 5/7 on 20s.

03-24-2002, 12:20 PM	#12
71-4x4-bb482 Registered User Join Date: Mar 2002 Location: donnellson,Ia. USA Posts: 25	The 2 numbers that you devide are steel stamped right into the outer edge of the ring gear rotate the ring and wipe it off as you go , they will be about a quarter inch tall, then just devide the larger by the smaller

03-24-2002, 12:56 PM	#13
ebfabman Registered User Join Date: Nov 2001 Location: Concord, NC Posts: 2,489	This often times not the case. Many times the #'s stamped on the gears don't correspond with the # of teeth on the gears. Or many times there are no stampings at all. This is especially the case with some aftermarket gear sets. To be sure while the cover is off, its best to just go ahead and count the teeth.

03-24-2002, 01:05 PM	#14
Fast68Chevy Account Suspended Join Date: May 2000 Location: IL Posts: 0	well 4,11 would be listed on glovebox door, i gaurantee you dont have 4,11 only one person on this forum that I know of has actual 4.11 gears in their 12 bolt axle it is very rare and very optional you have 3.73, you just gave me the info to prove it, the teeth count is stamped into the outside edge of the ring gear visible i highly doubt anyone swapped the gears out, sounds like youll be the first one to do so now on it almost everyone thinks and claims they have 4.11 in a 67-72 truck, that is until they pull the cover and find out its 3.07 or 3.73 good luck ------------------ -71/C-10(thx teeitup and crews!)350 4+4/CH465(thx 71Chev, GMCPaul, and CPNE)longfleet bed(thnx meathead95)!still building it- -'84 Z28(Z28) 1G1AP87GXEL100000 with H.O. 305/4bbl/190net/hp(L69) 3.73 rear(GT4/YC4) 700R4(MD8-MXO) & other goodies -79 G20 pass van, 350/th350/10bolt, paint is overrated Randy- Lincoln, IL r68c30@hotmail.com © RSS Enterprises 2000 [This message has been edited by Fast68Chevy (edited March 24, 2002).]

03-24-2002, 05:35 PM	#15
dropped72shortbed Registered User Join Date: Nov 2001 Posts: 303	whould the six cylinder have been given diff gears? the vin is CSS so it was originally a 6 and now its got a 350 in it. i figured it might not be the original rear end set up since its got a posi. would a six cylinder truck have come with a posi? [This message has been edited by dropped72shortbed (edited March 24, 2002).] __________________ Blake A. - Huntington Beach CA 72 c10 swb 5/5 drop, 350/350, partialy shaved, soon to be 5/7 on 20s.



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03-24-2002, 06:07 PM	#16
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Yes, a 6 could have posi. ------------------ '69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it. Need to pick up the one ton Longhorn with a PONTIAC 350/350soon. Thanks every one! Check out The Longhorn Webite. If you need a pic posted, E-mail me at longhornmail@yahoo.com Andy, in Columbus Ohio

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