Rear end gear/chunk

willy3486 · Post by **willy3486** » March 9, 2007, 11:44 pm

Heres a question for you experts thats probably easy to answer. I don't know the proper term,I always heard it called the rear end chunk. But what I am asking about is the assembly that the drive shaft goes into and is in the rear end. Then the axles go into it. I will call it the rear end assembly. Well the question is this. How do you tell the gearing rating? I am assuming you jack it up and count how many turns the wheel goes around completely when you turn the driveshaft completely. I have one of these in my truck and a second one as a spare. What I want do is pull my camper. So do I let the driveshaft go around 1 time and count the revolutions of the wheel to get the ratio or do I rotate the wheel and count the driveshaft? I am thinking the assembly that the driveshaft spins more around for one revolution of the wheel would be the better one to use for pulling the camper. My current assembly I was told come out of a 4x4 70s truck. The only problem it trys to spin the wheels more than I like.

banjopicker66 · Post by **banjopicker66** » March 10, 2007, 8:41 am

The correct technical name is "punkin."

Also called third member, chunk, banjo.

Is there a tag on it, bolted to one of the front nuts?
The second line will tell you - if it is original, of course.

The other way is to jack up one side of the truck, and make a mark on the driveshaft and the tire in the air.
I suggest you mark one of the trunions on the driveshaft or pinion; it will help I think to find fractional rotations.
Rotate the tire one full turn, counting the number of times the driveshaft turns.
The driveshaft rotations will give you the ratio, e.g, 3.25:1

This will not work on a locking rear end. You will find it necessary to jack the entire rear axle to get this to work.

Finally, if you are not sure about the rear and it's condition, especially the rear axle bearings, now is a good time to remove the punkin, replace the bearings and the seals, and change the 40 year old gear oil. You can get the precise ratio much more easily as well.

64 f100 · Post by **64 f100** » March 10, 2007, 9:44 am

If the tag is still on the rearend, attached by one of the nuts holding the hog head ( another term for the differetial carrier assembly ). Yoy can read these as a general rule with a little cleaning under the truck with a flashlight. The numbers will be on the lower part of the tag right next to the nut. there will be a number 3 and either an open space or an L then the last two digits like 25. If the third member came out of a 4X4 then it could be anything from a 3.50 to a 3.89. Also, did they cahnge the whole rearend or just the third member ( hog head )? You may or may not find the whole rearend different, in a number of ways. May be harder to swap parts out. Axles, brakes etc.

Rich

jakdad · Post by **jakdad** » March 10, 2007, 10:04 am

The name you were searching for is "differential". It is inside the chunk and the axles slide into it. Chunk or third member is correct enough for us. The "banjo" is not a chunk style rear end. Ford used "banjo housings" back in the 20s through the late 40s. The real fun part about the banjo is that if you wern't paying attention, it could be installed upside down and your transmission would work completely backwards. Don't ask me how I know this....................

ICEMAN6166 · Post by **ICEMAN6166** » March 10, 2007, 12:13 pm

if you have an 8 lug rear end (dana 60 or 70), you can take the cover off and the number of teeth on both ring and pinion gear will be stamped into the ring gear, divide the larger # by the smaller to get the ratio. 41:10 would be 4.10.

FORDBOYpete · Post by **FORDBOYpete** » March 10, 2007, 7:28 pm

Jim,

I Bet I Know how you know that about the Halibrand rears and,

BTW I can spell "Halibrand" & I can also say "Columbia 2 spd" too.

All the other information is pretty much correct on the center sections name

.

FWIW only the front loading rear end assys such as the Hotchkiss type FoMoCo uses or the 8-1/2" Dana in the 70s MoPaR Musclecars, or Eaton, or Timken and any rear where gears in it bolt in from the front in a 3rd member or centersection.

The Dana rears are a Salisbury type. The centersection is permanently formed into the axle tubes. The housing does not remove at all. All in a Salisbury type. All Salisbury type assemblies are rear loading or the gears are loaded into the assembly from the rear, usually after a cover is removed.

And a Banjo Looks like a Banjo and not a like a centersection, 3rd member, or pumpkin probably why it's called a Banjo Huh

FBp

willy3486 · Post by **willy3486** » March 10, 2007, 7:51 pm

Thanks guys. This truck is 40 years old know and I was a teenager when I got it. So the original chuck is long since gone. The oriiginal seals ,gear oil and such have also been long. Remember being a teen guys? Spinning in all 3 gears

. This pumpkin is the 3rd in there now mainly for the tree reasons listed

. But you guys have made it clear as to tell what it is. One last question I will be towing a small camper,with a sway control, weight distrubuting hitchs,etc so It will be as easy as possible on the old gal. But I was wondering when I find the ratio which would I be better with? a higher ratio or a smaller ratio? If the one in now has a ratio of 3 to 1 and my spare has a ratio of 5 to 1 which should I use?

FORDBOYpete · Post by **FORDBOYpete** » March 10, 2007, 8:32 pm

Willy,
When I hear or see the Volunteer State I think of Hills & Mountains, and some really great trout streams, err wait I digress,. . . . . .

I would really like to know what you have for a power train?

If you have a 300 I-6 & a 4 gear Manual a 3.93:1 up to a 4.10:1 with 30inch rubber ought to work fairly well.

If you have an A/T and a different engine I'd need to have an idea B4 I put my foot in my mouth, huh?

I have a 460 with an A/T & 3.00:1 rear. If I could get a set of wheels under my Concrete Block house I'd pull it up Mt Rushmore, or like "Mudder Truckers" here say "It could climb the face of a Cow in high Range if ya get the Cow to stand still.

So it's about available torque and the best operating Rev Range for the engine we're talking about.

As far as having 70's centersection, it's possible to have a 70-71 or 72, but after that a lot of trucks came with 31 spline axles. You can't find 31 spline axles to fit narrow 72 & earlier rear end housing assys. 73 & up (later) are +3" wider than 72 & Back (earlier). In some instances there are axle bearing issues to deal with also.

If you can find a 28 spline Centersection with the ratio you need it would most likely work. Vans, Broncos, & 70-1-2 F-100 thru F-150s are where to look. If you get the year & actual rear axle code you can determine ratio and unit specifications from that number set.

FBp

willy3486 · Post by **willy3486** » March 11, 2007, 2:09 am

FBP the truck is mostly original EXCEPT for the rear end. It has the original 352 and the original 3 speed manual. As far as the rear end I haven't checked yet but the chunk was suppose to be from a 4x4 70s truck. But it spins more than I would expect for a 4x4 but thats my thinking. It could be due to the carb. By the way folks I have decieded to just go ahead and buy a rebuilt one. It has been rebuit by me twice and 2 other times since I have had it. Plus I want to think there may be other issues since the last rebuild did not help a bit.

didgeytrucker · Post by **didgeytrucker** » March 11, 2007, 11:38 am

CORRECTION:

If you jack up one wheel (and the rear end is not a "locking" rear end) you must spin it twice and count the driveshaft revolutions to get the ratio.

64 f100 · Post by **64 f100** » March 11, 2007, 9:47 pm

FBP there are some 31 spline axles of equal length that will work in our rearends. However kinda defeats the purpose, as the brake drums are wider on those axles and the axle only slides into the gear about 3/8ths inch on the drivers side, because the axle was moved out further to accomidate the wider brakes.

Rich