The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 1967 - 1972 Chevrolet & GMC Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 07-28-2018, 06:05 PM   #1
mosel70
Registered User
 
Join Date: Jul 2016
Location: Las Vegas, NV
Posts: 92
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
  
mosel70 is offline   Reply With Quote
Old 07-28-2018, 06:20 PM   #2
Stocker
20' Daredevil (Ret)
 
Stocker's Avatar
 
Join Date: May 2003
Location: Jefferson State
Posts: 10,543
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
Stocker is offline   Reply With Quote
Old 07-28-2018, 06:35 PM   #3
mosel70
Registered User
 
Join Date: Jul 2016
Location: Las Vegas, NV
Posts: 92
Re: Gear ratio from pinion gear stamp?

well, that was when I opened it last summer. dummy me, didn't think to count.
mosel70 is offline   Reply With Quote
Old 07-28-2018, 06:40 PM   #4
Bigdav160
Registered User
 
Bigdav160's Avatar
 
Join Date: Apr 2011
Location: Klein Texas
Posts: 3,121
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
Bigdav160 is offline   Reply With Quote
Old 07-28-2018, 06:49 PM   #5
Stocker
20' Daredevil (Ret)
 
Stocker's Avatar
 
Join Date: May 2003
Location: Jefferson State
Posts: 10,543
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
Stocker is offline   Reply With Quote
Old 07-29-2018, 08:39 AM   #6
Mike C
Registered User
 
Mike C's Avatar
 
Join Date: Dec 1999
Location: Austin, TX, USA
Posts: 6,664
Re: Gear ratio from pinion gear stamp?

Quote:
Originally Posted by Stocker View Post
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.
__________________
44 Willys MB
52 M38A1
64 Corvette Coupe
68 Camaro 'vert LT1 & TH700
69 Z/28 355 12.6's @110
69 Chevy Short Step 4 1/2"/7" drop
72 Jimmy 4WD 4spd 4" & 35's
02 GMC 2500HD 4x4 Duramax
Mike C is offline   Reply With Quote
Old 07-29-2018, 11:54 AM   #7
hjewell2
Registered User
 
hjewell2's Avatar
 
Join Date: May 2011
Location: mich
Posts: 492
Re: Gear ratio from pinion gear stamp?

Quote:
Originally Posted by mosel70 View Post
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.

Last edited by hjewell2; 07-29-2018 at 11:54 AM. Reason: mis spell
hjewell2 is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 03:09 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2018, vBulletin Solutions Inc.
Copyright 1997-2013 67-72chevytrucks.com