Gear ratio from pinion gear stamp? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mosel70 · 07-28-2018, 06:05 PM

I have a'70 C10 stepside. Appears the pinion gear stamp is NF GM86, hard to see in photo. I have photos of the other stamps on the differential exterior if that might help. Looking to swap trans from TH400 to 700R4 and need to know the gear ratio. Truck new had a M20 manual 4-speed. Thanks!

**Stocker** · 07-28-2018, 06:20 PM

It's already opened up, just count the teeth on the ring & on the pinion. Divide the ring count by the pinion count, and bingo there's the answer.

mosel70 · 07-28-2018, 06:35 PM

well, that was when I opened it last summer. dummy me, didn't think to count.

Bigdav160 · 07-28-2018, 06:40 PM

You're are looking for the numbers stamped into the ring gear not the differential case casting number.

**Stocker** · 07-28-2018, 06:49 PM

Jack up one tire and count the number of revolutions the driveshaft makes when turning the tire one full revolution.

Mike C · 07-29-2018, 08:39 AM

Quote:

Originally Posted by Stocker

Jack up one tire and count the number of revolutions the driveshaft makes when turning the tire one full revolution.

Twice. With an open diff you rotate the tire twice with one jacked up. This is because the 2:1 side gear to spider ratio is also in play. With a limited slip you jack up both tires and rotate one revolution to get ratio.

Even better, jack up rear of truck and with help from a couple of friends rotate BOTH tires at the same speed one rotation and count driveshaft turns.

hjewell2 · 07-29-2018, 11:54 AM

Quote:

Originally Posted by mosel70

I have a'70 C10 stepside. Appears the pinion gear stamp is NF GM86, hard to see in photo. I have photos of the other stamps on the differential exterior if that might help. Looking to swap trans from TH400 to 700R4 and need to know the gear ratio. Truck new had a M20 manual 4-speed. Thanks!

3.08 gear was very common in 1970 1/2 ton . without checking the numbers stamped on the ring gear its only a guess.

07-28-2018, 06:05 PM	#1
mosel70 Registered User Join Date: Jul 2016 Location: Las Vegas, NV Posts: 116	Gear ratio from pinion gear stamp? I have a'70 C10 stepside. Appears the pinion gear stamp is NF GM86, hard to see in photo. I have photos of the other stamps on the differential exterior if that might help. Looking to swap trans from TH400 to 700R4 and need to know the gear ratio. Truck new had a M20 manual 4-speed. Thanks! Attached Images

07-28-2018, 06:20 PM	#2
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,728	Re: Gear ratio from pinion gear stamp? It's already opened up, just count the teeth on the ring & on the pinion. Divide the ring count by the pinion count, and bingo there's the answer. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

07-28-2018, 06:35 PM	#3
mosel70 Registered User Join Date: Jul 2016 Location: Las Vegas, NV Posts: 116	Re: Gear ratio from pinion gear stamp? well, that was when I opened it last summer. dummy me, didn't think to count.

07-28-2018, 06:40 PM	#4
Bigdav160 Registered User Join Date: Apr 2011 Location: Klein Texas Posts: 3,852	Re: Gear ratio from pinion gear stamp? You're are looking for the numbers stamped into the ring gear not the differential case casting number. __________________ My Classics: '72 K20 Suburban + '65 Dodge Town Wagon '72 Corvette Roadster +'67 Corvette Roadster '73 Z-28 Camaro '63 Ford SWB Uni Pickup '50 Ford Coupe

07-28-2018, 06:49 PM	#5
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,728	Re: Gear ratio from pinion gear stamp? Jack up one tire and count the number of revolutions the driveshaft makes when turning the tire one full revolution. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay



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