Figure out gears

[HotRodScottsdale]

So I got a 84 silverado k2500 with a 12 bolt semi float 8 lug rear end and a 10 bolt front. Problem is that the rear gears were destroyed. The guy had the truck sitting and was not 100% sure but thinks it's 3.73. I have never done gears and can't afford a shop. Is there any way to find out of it is 3.73 or not. I don't wanna buy gears and find out they are wrong

[74CustomK20]

Best way is to pull the cover and count the ring gears and pinion gears. That is a for sure way. Plus you can observe the diff and find out what is going on. Maybe the spider gears went bad and the r&p are fine? But pull the cover is my way.

[Jake Wade]

That should be a 14 bolt, not a 12 bolt. As mentioned, the best way is to count the teeth on the ring gear and pinion gear. Divide the ring gear by the pinon gear tooth count for your ratio. You want the same ratio front and rear.

[Alex V.]

IIRC dad's '86 K20 w/C6P has a semi-float 14 bolt - cover isn't shaped the same as my C3500's full-floater, but it's got 14 bolts. The tooth count should be stamped into the ring gear - since it's gonna have to come apart anyway might as well take the cover off and be sure.

[HotRodScottsdale]

Awesome thank you. I'm going to try and sneak into shop today and I will check inside for anything with gear numbers. If I can't find them I'll start counting any suggestions for places to order gears . I live in British Columbia and the less I have to pay shipping the better

[mongocanfly]

Even If the rears destroyed it would be best to check the front diff and see what it is before ordering gears...I bought a 67 4x4 once that had different ratios front and back..you couldn't tell it in the mud but if you hit pavement you immediately knew something wasn't right..

[wilkin250r]

Warning: Gears are NOT simple. They look simple, because it's only a few components, but they are actually very difficult to set up. They need to be very very precise, often putting in shims that are thinner than a piece of paper, and require specialized measurement devices to check clearances.

I don't want to be the jerk, but the simple fact that you're asking questions about ratios tells me that you're not ready to rebuild this differential yourself.

[nonstop]

Originally Posted by wilkin250r

Warning: Gears are NOT simple. They look simple, because it's only a few components, but they are actually very difficult to set up. They need to be very very precise, often putting in shims that are thinner than a piece of paper, and require specialized measurement devices to check clearances.

I don't want to be the jerk, but the simple fact that you're asking questions about ratios tells me that you're not ready to rebuild this differential yourself.

Follow that advice! Might be better searching Craigslist or a wrecking yard for a semi or full float 14 bolt to simply bolt in.

[akart]

Find out what you have ratio wise and go to a salvage yard or where ever. Pull some covers and if it looks good and not too much play. Should be like 100 bucks for a whole rear end. And swap them out.

[74CustomK20]

Yes. Gear ratios are precise, not the usual "meh" good enough. I'm the type of guy to just replace the axle itself. I can do brakes no problem but I don't have the equipment to rebuild an axle.

[wilkin250r]

Granted, finding a good used axle may be difficult and time consuming.

But learning to rebuild an axle by reading tutorials and watching videos will eat six hours at least, then another six in wrench time when you actually start rebuilding your own (less with practice, but this is your first), $250 for gears, plus another $200 for a rebuild kit, and that's assuming everything else is in perfect shape, and add nearly $200 for specialty tools like an inch-lb torque wrench, micrometer and magnetic base, or more expensive if you go with a case-spreader, plus you'll need a hydraulic press for the pinion bearing.

So rebuilding your axle looks to be a minimum 12hrs, and $700, but can easily run higher depending on how much equipment you have access to, or damage inside the axle itself.

Or you can spend a few hours hunting down a good used axle for $200.

(If you think this is an exaggeration, just ask around)

[HotRodScottsdale]

Originally Posted by wilkin250r

Granted, finding a good used axle may be difficult and time consuming.

But learning to rebuild an axle by reading tutorials and watching videos will eat six hours at least, then another six in wrench time when you actually start rebuilding your own (less with practice, but this is your first), $250 for gears, plus another $200 for a rebuild kit, and that's assuming everything else is in perfect shape, and add nearly $200 for specialty tools like an inch-lb torque wrench, micrometer and magnetic base, or more expensive if you go with a case-spreader, plus you'll need a hydraulic press for the pinion bearing.

So rebuilding your axle looks to be a minimum 12hrs, and $700, but can easily run higher depending on how much equipment you have access to, or damage inside the axle itself.

Or you can spend a few hours hunting down a good used axle for $200.

(If you think this is an exaggeration, just ask around)

Wow good advise maybe I will just hunt down a used axle

[74CustomK20]

That's the opposite of an exaggeration. That was an understatement from wilkin.

[hatzie]

Originally Posted by 74CustomK20

That's the opposite of an exaggeration. That was an understatement from wilkin.

Having done two axles I agree with him. Not simple or easy and the tooling is not cheap either. I will probably do it again but I need my head examined...

Find a used axle. If you have 6 lug wheels and don't have a large investment in tires and wheels... upgrade your LD 3/4 ton to HD 8 lug with a front (Dana 44 or 10 bolt) and a full floating rear off the same 75-87 K20. It'll be easier to mate the E-Brake cable from an 81-87. Whatever your axle year grab the entire E-Brake cable setup from your donor so you get all of the bits and bobs to make it fit.
You can sell your front axle to offset some or all of the cost of 16" tires and 8 lug wheels.
3/4 ton GM CK & RV Light trucks used 16" 8 lug on 6.5" bolt circle from 1967 to 2009. They will all bolt right up. You want to avoid split rims because most places won't service them and you don't want 16.5" tires because they are getting harder to find.

The only issue after that is matching the speedometer gear to the axle ratio and tire diameter.

[rich weyand]

Originally Posted by hatzie

The only issue after that is matching the speedometer gear to the axle ratio and tire diameter.

Minor nit.

K trucks usually use a speedometer ratio adapter mounted on the transfer case, because the transfer case had a limited number of gear options for the speedometer drive, unlike the transmissions. So you might have to change out the ratio adapter to a different ratio to match your tires and gears.

You can still find original ones around, though they aren't cheap, and you can also get aftermarket ones.

[nonstop]

The truck should already be 8 lug, so it should basically be plug and play if you can find a rear with matching gears. You might also luck out and find a rear with bigger brakes (depending on what ones you have now) so you might inadvertently get an upgrade at the same time!

[hatzie]

Originally Posted by nonstop

The truck should already be 8 lug, so it should basically be plug and play if you can find a rear with matching gears. You might also luck out and find a rear with bigger brakes (depending on what ones you have now) so you might inadvertently get an upgrade at the same time!

Weren't a chunk of the mid 80's Light Duty 3/4 tons with Semi-Floating 9.5" 14 bolts 6 lug?
I probably should've said "If you don't already have 8 lug wheels." but I assumed that was pretty easy to figure out.

Originally Posted by rich weyand

Minor nit.

K trucks usually use a speedometer ratio adapter mounted on the transfer case, because the transfer case had a limited number of gear options for the speedometer drive, unlike the transmissions. So you might have to change out the ratio adapter to a different ratio to match your tires and gears.

You can still find original ones around, though they aren't cheap, and you can also get aftermarket ones.

The NP208 uses the same driven gears as the TH350, TH400, & TH700R4. There's a pretty wide selection still available.
The drive gears are another story and the NP205 is pretty exciting too.

[nonstop]

Originally Posted by hatzie

Weren't a chunk of the mid 80's Light Duty 3/4 tons with Semi-Floating 9.5" 14 bolts 6 lug?
I probably should've said "If you don't already have 8 lug wheels." but I assumed that was pretty easy to figure out.

I though that they were all 8 lug - but I've definitely been wrong before - the wife liked to remind me! I know there were some in later trucks. And agreed - hopefully should be easy to figure out... if not, welllllll.....

[hatzie]

Originally Posted by nonstop

I though that they were all 8 lug - but I've definitely been wrong before - the wife liked to remind me! I know there were some in later trucks. And agreed - hopefully should be easy to figure out... if not, welllllll.....

Same here. I've seen both on the 3/4 ton GMT400 chassis trucks but the squares are older and longer ago and I honestly am not positive. I thought so but CRS creeps up on you.

[Tash4]

if it is k20 or k25 it will be 8 lug. Alex is right the gear ratio will be stamped on outside of ring gear. The front cover should have a tag between two bolts on the outside withe the ratio as well.

[Ski-me]

Most that I saw on the 14SF in the square bodies were all 8 lug. I obviously wanted 6 lug so I didn't need to change wheels, so it can be done.

I went to a junkyard and found a 8 lug, 14SF from a suburban with 4.10 gears. Took the backing plate off as well as the axle shafts.

Then I found a later model 6 lug, 14SF from a pickup and used those axle shafts, backing plates, brakes and drums. Mounted to the suburban axle and bought the whole thing from junkyard as a complete assembly. I then had a 1/2" spacer plate made and put between axle mount and backing plate to account for the (very slightly) longer axle shafts.

Then everything bolted up directly and I've been running it for years.

I even took the front axle from the suburban (with 4.10 gears), and removed all the way to the ball joints, and took my 1/2 ton shafts, spindle, brakes and calipers....all bolted right up and back to 6 lug.

To determine the ratio you have, pull the cover and there should be markings on the ring. There will be some numbers stamped on it so you can tell what gears you have.

3.08 gears - "40:13"
3.42 gears - "41:12"
3.73 gears - "41:11"
4.10 gears - "41:10"
4.11 gears - "37:9"

[Ski-me]

Note:
There are two widths in the later year 14SF, 6 lug axles. I used the narrower one. My final result is almost exactly the track width of the front (wider than the stock).

The stock 1/2 ton blazer rear axle is about 1-1/2" narrower on each side than the front axle.