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Thread: Help with gear transmission advice

  1. #1
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    Default Help with gear transmission advice

    I am new to this board and have searched other posts for advice. I just purchased a 1984 GMC 3500 4dr. dually with 48,000 verified miles. It came with a huge camper and other electronic devices I'm unfamiliar with. I have removed the camper but have trouble with the transmission and gearing. I can't find anyone including the dealer to tell me anything about the transmission and rear end. It has a 6.2L with a Banks Turbo. The transmission is a 4 speed manual. It has a compound low in the upper left position with 1st gear in the lower left, 2nd upper right and 3rd lower right. I have been searching the internet and think it may be something referred to as a SM465. I have no way of confirming this as I can't find anyone to decipher the VIN. I have been told that the rear end will have a small tag of some sort attached to a bolt that will have some sort of encripted number identifying the gear ratio. I can't find it. While driving this truck back from South Dakota where I bought it I felt like I was going to blow up the engine at 65 MPH. The tachometer showed about 3200 RPM's. At 70 MPH it is about 3500 RPM's. I shift out of 1st into 2nd at 15 MPH. Out of 2nd into 3rd at 25 MPH. Can anyone help me with this delima. I have been told that if I change the rear gear ratio I will benifit by about 500 RPM's. I've also been told that I need to change the transmission to a NV4500 5 speed. these are not inexspensive modifications for me and will take some time. From what I read here my RPM's at cruising speed need to be near the 1800 mark.

  2. #2
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    edmonton
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    Default Vin

    Post your Vin . Someone may be able to tell you what it was built with.
    86C 6.2l gmc
    "This to shall pass"
    "Life is to short for a Tim Horton Line up"

  3. #3

    Default same with me

    I just posted about the same thing but my new rig has an auto tranny, im afraid to drive it over 55mph because the engine is screaming. It is very likely we have 4:10 gearing, and 3500rpm seems real high for a diesel, most I know red line at 2800, these may be diffferent but I dont know.
    your truck sounds like a truck I almost got, If you ever think of selling in the futre let me know.
    Did your truck have an owners manual? there may be some engine data there, if so will you post it, Ive been searching the net for days and no tech info.
    thanks and good luck,
    mike.

  4. #4
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    Apr 2001
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    Arrow

    Both of you have direct (1:1, no OD) for high gear. Unless you are running really small tires, your rear gear ratio could be 4.56:1, especially if they are CUCV's. Check the rear gear ratio yourself, and don't trust any markings or RPO's you find. The gears may have been changed at some time.

    If you have an open diff or loose limited slip, raise one wheel and rotate it exactly 1 rotation. Count the turns of the drive shaft (tranny out of gear or P, E-brake off). The driveshaft will turn exactly 1/2 the actual ratio. A 4.10 will a little more than 2 turns, and a 4.56 will turn 2-1/4 turns. If you have a locker or tight limited slip, you will have to raise both wheels (they will turn at the same rate, in the same direction), and the shaft turns will exactly match the ratio.

    Another option is to remove the rear end cover and count the gear teeth on the ring an pinion. Divide the ring gear teeth by the pinion teeth for the ratio.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  5. #5
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    Default

    Frank,

    From you description, The tranny may be the SM-465. I have on in my 84 C20. Good tranny, generally sounds like it is chewing on rocks when in neutral and the clutch engaged. Chevy put that tranny in everything up to the C-60's. Visual from under truck, 2 PTO plates, and a total length of about 12 inches.

    Given that you have a c-30, rear gear most likely will be a 4:10 . Like DmaxMav said, you could have a 4:56 back there, I think it was an option in those trucks. Rear axle is probably a Dana-70. Cover has 10 bolts and is a full floater.

    HammerWerf
    1999 k2500 Suburban Pyro, Tranny Temp, Boost, K&N Air Filter, +3.5 qt tranny pan. 190000 miles
    1984 C20 249k miles DSG gear, SM-465, 3.73
    1983 C30 CC dualie. 308K miles on Chassis, TH400, 4-45 A/C Looking for a TurboCharger

  6. #6

    Default

    Ok, I might be wrong here. But chevy used the SM465 tranny for everything from big to small for years! The 1/2 ton trucks got them, and few C60s got them also. It's the grand daddy of all standard transmissions, and will take more than you can imagine to break it. It does not have OD, but otherwise has great gear ratios. Top of the line tranny right there.

    Now, for rear gear ratio.. That's a little trickier.

    Now, in the early years, chevy put a tag on the glovebox with the gear ratio, right there, rear simple to read, no prob. Just open the glove box, and look for a sticker with all the trucks options.

    Now, in the later years chevy changed that tag over to codes that you can no longer decipher with ease.

    If you have the second style tag you have another option for checking gear ratio.

    Jack up one rear tire, mark the tire with soapstone on the bottom, then mark the driveshaft directly towards you. Turn the tire one complete turn counting how many times the driveshaft goes around. Then simply multiply that times 2, and you have your gear ratio.. Simple enough?
    Standard
    If the driveshaft goes around 1 and 3/4 times, you probably have 3.73s
    if the driveshaft goes around 2 times, you have 4.10s
    if the driveshaft goes around 2 and 1/4 times, you have 4.56s.

    I do this all the time, as I'm trying to find a good set of 3.73s for my future dump truck which will be powered by a 6.2 diesel.

    Most 1ton trucks will run 4.10s, or 4.56s. My buddies 1ton 3+3 dually 454 auto runs 3.73s! Which is pretty amazing for that big of a truck.

    Good luck,
    Len

  7. #7

    Default

    Oh yeah, and swapping to an NV4500 is a VERY expensive venture. Expect the price tag to run 2k unless you have a lot of friends in good places.

    And for the rear axle. If you have wide width duallies, you'll have a D70, super strong, and really nice.

    If you have a Narrow width dually, you'll have a Corp 14b. Which is also a super strong rear axle. If you don't have duallies at all, you'd still have the corp 14b.

    Len

  8. #8
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    Jan 2007
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    Muskogee Oklahoma
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    Default

    Quote Originally Posted by moody
    Post your Vin . Someone may be able to tell you what it was built with.
    Moody,
    "Post your Vin . Someone may be able to tell you what it was built with."

    Great Idea. There seems to be a wealth of knowledge here that is absent at my local GMC dealer.

    Thanks Moody.

    1GTHC33J9ES525960

  9. #9
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    Default

    schoitmike,
    "I

  10. #10
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    Default

    DmaxMaverick,
    "Both of you have direct (1:1, no OD) for high gear. Unless you are running really small tires, your rear gear ratio could be 4.56:1, especially if they are CUCV's. "

    What is a CUCV?

    "Check the rear gear ratio yourself, and don't trust any markings or RPO's you find."

    What is an RPO?

    "If you have an open diff or loose limited slip, raise one wheel and rotate it exactly 1 rotation. Count the turns of the drive shaft (tranny out of gear or P, E-brake off). The driveshaft will turn exactly 1/2 the actual ratio. A 4.10 will a little more than 2 turns, and a 4.56 will turn 2-1/4 turns. If you have a locker or tight limited slip, you will have to raise both wheels (they will turn at the same rate, in the same direction), and the shaft turns will exactly match the ratio. Another option is to remove the rear end cover and count the gear teeth on the ring and pinion. Divide the ring gear teeth by the pinion teeth for the ratio."

    Thanks, I have never had that explained before. I

  11. #11
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    Muskogee Oklahoma
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    Default

    HammerWerf,
    "From you description, The tranny may be the SM-465. I have on in my 84 C20. Good tranny, generally sounds like it is chewing on rocks when in neutral and the clutch engaged."

    Thanks HammerWerf, now I know that it

  12. #12
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    Jan 2007
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    Muskogee Oklahoma
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    Default

    Thejunkman2005,
    "great gear ratios. "

    Do you think I could benefit by going to a 3:42 rear end?

    "Top of the line tranny right there. Now, for rear gear ratio.. That's a little trickier. Now, in the early years, chevy put a tag on the glovebox with the gear ratio, right there, rear simple to read, no prob. Just open the glove box, and look for a sticker with all the trucks options."

    I checked there and could not find anything with that information on it. I also checked the drivers door and radiator housing. Drivers door was tire pressures and GVWR nothing on the radiator housing.

    "Jack up one rear tire, mark the tire with soapstone on the bottom, then mark the driveshaft directly towards you. Turn the tire one complete turn counting how many times the driveshaft goes around. Then simply multiply that times 2, and you have your gear ratio.. Simple enough?
    Standard
    If the driveshaft goes around 1 and 3/4 times, you probably have 3.73s
    if the driveshaft goes around 2 times, you have 4.10s
    if the driveshaft goes around 2 and 1/4 times, you have 4.56s.
    I do this all the time, as I'm trying to find a good set of 3.73s for my future dump truck which will be powered by a 6.2 diesel. Most 1ton trucks will run 4.10s, or 4.56s. My buddies 1ton 3+3 dually 454 auto runs 3.73s! Which is pretty amazing for that big of a truck."

    Do you think that a 3:42 or 3:73 will work well with my 4 speed w/o OD?

    "If you have wide width duallies, you'll have a D70, super strong, and really nice."

    I have the wide dually with the regular truck bed
    Thanks Len
    Frank

  13. #13
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    Arrow

    [quote=Frank M. Hardcastle II]DmaxMaverick,
    "Both of you have direct (1:1, no OD) for high gear. Unless you are running really small tires, your rear gear ratio could be 4.56:1, especially if they are CUCV's. "

    What is a CUCV?

    "Check the rear gear ratio yourself, and don't trust any markings or RPO's you find."

    What is an RPO?

    "If you have an open diff or loose limited slip, raise one wheel and rotate it exactly 1 rotation. Count the turns of the drive shaft (tranny out of gear or P, E-brake off). The driveshaft will turn exactly 1/2 the actual ratio. A 4.10 will a little more than 2 turns, and a 4.56 will turn 2-1/4 turns. If you have a locker or tight limited slip, you will have to raise both wheels (they will turn at the same rate, in the same direction), and the shaft turns will exactly match the ratio. Another option is to remove the rear end cover and count the gear teeth on the ring and pinion. Divide the ring gear teeth by the pinion teeth for the ratio."

    Thanks, I have never had that explained before. I
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  14. #14
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    Aug 2003
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    Bakersfield, Ca
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    Default 6.2 Redline

    Frank,

    To answer you question about the redline. On these engines, it is around 3500 - 3600 rpm. Detroit Diesel likes to build high speed engines. Cummins redlines around 2800 rpm. Not sure about the Ford, but I don't think it is nearly as high as the GM.

    DmaxMav is right, these engines will run at the governer all day.

    HammerWerf
    1999 k2500 Suburban Pyro, Tranny Temp, Boost, K&N Air Filter, +3.5 qt tranny pan. 190000 miles
    1984 C20 249k miles DSG gear, SM-465, 3.73
    1983 C30 CC dualie. 308K miles on Chassis, TH400, 4-45 A/C Looking for a TurboCharger

  15. #15
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    Muskogee Oklahoma
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    Default

    Quote Originally Posted by HammerWerf
    Frank,

    To answer you question about the redline. On these engines, it is around 3500 - 3600 rpm. Detroit Diesel likes to build high speed engines. Cummins redlines around 2800 rpm. Not sure about the Ford, but I don't think it is nearly as high as the GM.

    DmaxMav is right, these engines will run at the governer all day.

    HammerWerf
    3500-3600. Looks like I've been at redline on more than one occassion.
    Governor? Do all 6.2L's have governors? I haven't hit it yet. If it does have one I'll feel much better about the RPM's I've seen. Perhaps too much damage hasn't occurred.

  16. #16
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    Granby, Missouri, USA
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    Default

    Quote Originally Posted by Frank M. Hardcastle II
    3500-3600. Looks like I've been at redline on more than one occassion.
    Governor? Do all 6.2L's have governors? I haven't hit it yet. If it does have one I'll feel much better about the RPM's I've seen. Perhaps too much damage hasn't occurred.
    Yes, all 6.2L's have governors. You can't damage the engine by running down the highway too fast. The only way to damage the engine from high rpm is by trying to hold the truck back by means of engine braking when coming down a mountain, but that would be hard to do.

    You can hit the governor in these engines, but it sounds like you're about to push the pistons out of the heads! But it doesn't hurt anything, it just sounds bad. Next time you drive the truck, just push the throttle all the way down when in third gear, and it'll rev up and all of a sudden stop accelerating and hold a steady high rpm. Then you can shift into 4th. It's a funny feeling.

    When my engine was still n/a, I used to run up to the governor in each gear before shifting when towing large loads. Now with the turbo and other mods, it has enough torque that it likes to shift much sooner.

    Casey
    1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter

  17. #17
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    Default

    Frank,

    My expirence of " hitting the Govenor " was that it defueled the engine, and the engine acted as a brake. I went from smoke & howl to decelerating rapidly in a moment.

    the 3500 rpm is the factory set speed for the engine. There is a video on this site showing a gentleman in the Netherlands sled pulling with a Massy-Fergeson tractor that has a 6.2l turbocharged twin injector pump for an engine. He runs the engine up to about 5000 rpm in the pulls. really something to watch, especially when you see a full size volvo(?) front loader not be able to pull the sled as far.

    HammerWerf
    1999 k2500 Suburban Pyro, Tranny Temp, Boost, K&N Air Filter, +3.5 qt tranny pan. 190000 miles
    1984 C20 249k miles DSG gear, SM-465, 3.73
    1983 C30 CC dualie. 308K miles on Chassis, TH400, 4-45 A/C Looking for a TurboCharger

  18. #18
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    Muskogee Oklahoma
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    Default

    Quote Originally Posted by HammerWerf
    Frank,

    My expirence of " hitting the Govenor " was that it defueled the engine, and the engine acted as a brake. I went from smoke & howl to decelerating rapidly in a moment.

    the 3500 rpm is the factory set speed for the engine. There is a video on this site showing a gentleman in the Netherlands sled pulling with a Massy-Fergeson tractor that has a 6.2l turbocharged twin injector pump for an engine. He runs the engine up to about 5000 rpm in the pulls. really something to watch, especially when you see a full size volvo(?) front loader not be able to pull the sled as far.

    HammerWerf
    Is this engine really indestructable? I'm begining to think I may need to start a collection of 6.2L's and Banks Turbo Chargers. I'll need one for my K1200LTC motorcycle, one for my wife's Toyota Camry and one for the family G20 van. Hey, if Boss Hoss can put a 500 ci V-8 on a motorcycle why can't I put a diesel on one? The information avaible from the posters of this board is great. I didn't think I could find a group as helpful as BMWLT.net but after getting all this advice so quickly I find I was mistaken. You guys are great.
    Thanks

  19. #19
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    Default

    indestructable? No.

    A number of issues to be aware of.

    Prior to using the search function, Go to bottom of page and expand the range of dates it will search through.

    Keep the cooling system clean. Overheating is an enemy of any engine. Head gasket is vulnerable to overheating.

    The engine Dampener (harmonic balancer) needs to be monitored when the miles start getting high ( say >95,000). The rubber shear ring can harden, soften, bulge, or anything else it feels like. Cranks have been broken after this gets out of phase.

    The main web area ( crankshaft's home). This area is a known weak zone. Some blocks are more delicate than others. Cracks can penetrate into the coolant jacket. Several fixes are being tried at the moment. Jury still out on the fixes. AM General (Hummer) ownes the engine now, and has instituted multiple changes to improve the block.

    Cylinders 7 & 8 can be galled when failure involves high exhaust gas temperature. Heat management is very important in these engines.

    Look for a posting by ronniejoe. He has a 3 part article, in his signature line, that describes the failure and rebuild of the motor in his suburban.
    The articles that are available to the members are always full of info that helps keep thing running

    HammerWerf
    1999 k2500 Suburban Pyro, Tranny Temp, Boost, K&N Air Filter, +3.5 qt tranny pan. 190000 miles
    1984 C20 249k miles DSG gear, SM-465, 3.73
    1983 C30 CC dualie. 308K miles on Chassis, TH400, 4-45 A/C Looking for a TurboCharger

  20. #20
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    Default

    Well I found some numbers but don't know what they mean.
    On the differential casing.

    082
    70HD
    C37453

    Anyone have any idea what that means?

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