Hey all. My recently purchased 68 442 has a W27 rear-end (or at the very least a W27 Cover with a ridiculously short gear. 65 -70 MPH = apx. 3000 rpm.
So, I wanted to try to figure out exactly what was in there. Simplest method I know of is to jack up the rear, spin the tire and count the driveshaft revolutions. Simple, right? However, when I did this I discovered that for each revolution of the driveshaft, the wheel turns about 0.6 revolution. I was expecting about 4 revs of the wheel per driveshaft rev. The math on this one is driving me crazy. Am I doing something wrong? Anyone have any ideas what might be lurking behind that W27 cover?

 

Yeah, your math is backwards. A 3.90:1 ratio means that the driveshaft turns 3.9 times for each turn of the wheels.

 

Right - that's what I meant :-)

But still - what does that leave me with when the propeller shaft turns once, and the axle turns 0.6?

 

Well, for starters, turn the tire ten times, count the driveshaft rotations, and divide by ten to get the ratio. It's a lot easier to see the difference between 39 turns and 37 turns than between 3.9 and 3.9. Second, be sure BOTH tires are turning. If only one tire turns, you are getting a 2:1 reduction in the differential. Of course, any factory W-27 housing will have anti-spin, so this isn't an issue unless the posi unit is toast.

 

try it the other way and turn the driveshaft and count the turns of the tire maybe, i agree with the ten times count

 

Doesn't matter. The problem (since now that I re-read the original post, it's only a W-27 cover) is that the axle is non-posi and the math isn't accounting for the 2:1 reduction in the differential when only one wheel turns.

 

I'm convinced I've done something seriously flawed. A ratio of 1.66 doesn't make sense, particularly given the performance I am experiencing. I'm going to re-check.

 

The 2: 1 reduction has to be it. I only had one rear wheel raised - so there you go. No posi, and a newbie error. Also, suggestion to turn the wheel ten times makes sense to expand the sampling. Thanks everyone.

 

And 1.66 x 2 is 3.32, which is likely a 3.42:1 ratio.

 

And now you see why I suggest ten turns of the wheel. It's difficult to resolve 1.71 turns of the driveshaft (for 3.42:1) from 1.62 turns (for 3.23:1).

 

I'll bet it's a 3.42 based on the rpms, not too ridiculous unless you do a lot of high speed highway miles. 3000 rpm won't hurt anything but mpg. I'll also bet it's just the cover.

 

I'll vote for 3.08 ratio, what with the measurement error on "0.6 turn" of a wheel, x2.

What's the problem with pulling the cover, inspecting, change fluid, and READ THE NUMBERS stamped into the ring gear? Not only is it entirely certain, and takes about the same amount of time, but you have the benefit of new fluid and an inspection.

If you are really bent on turn counting, look up past discussions that explain all that with links to the 1940's GM video explanation of the differential. Which shows why you need two tire turns to get one ring gear turn. Whether you get two turns of one tire, or one turn on each of two tires [as with posi], or 1.5 of one and 0.5 of the other, is up to you.

U-Joints make counting driveshaft turns to +/- 1/8 turn super easy.
A mark on the wheel/ brake drum makes getting your 1 or 10 or 26 or whatever # of tire turns easy.

 

After re-checking - it's a 3.73

Octania, i was just looking for a quick and dirty way to come up with an answer, although I appreciate your perspective

 

Well, if it's a 3.73 ratio then the rear is either a Chevy 12 bolt or a 10 bolt, and not an O-Type 12 bolt that US assembled cars would have (Canadian cars had Chevy 12 bolts, or so I have read).

 

There are aftermarket W-27 covers available with the Chevy 12 bolt pattern.

 

Yes, they are available for the O Type 12 bolt, the Chevy 12 bolt, and the 8.5" 10 bolt. The 3.73 gears rule out the O Type, thereby leaving the Chevy 12 bolt and the 8.5" 10 bolt as possibilities.