Question?? rear half axles

[Rookie Vette owner]

Working on the big block conversion and have been looking at what the increase will effect on the axles that came on the small block cars that should be increased for the big block swap? what about trailing arm assemblies and the center section? Thanks!

 

[GTR1999]

SB SHP and some automatic use the HD yokes, same as the BB cars. These are still 17 spline axles and are good to 550-600 hp but if you start building crazy power you may want to look at the 31 spline Tom's axles.
Use Spicer solid joints and 3" 1/2 shafts from a 75-79.
The diff weak points are dependant on the year,there is no internal difference in the units. I heard there was an early HD BB posi case but I haven't seen one. The 65-68 posi were weak as were the spiders. The later posi cases are better as are the 10-17 spiders but the clutches are junk.
I would rather not type this all again so if you go to this link it will everything you want to know about building a very strong diff short of a 12 bolt.

http://www.vettemod.com/forum/forumdisplay.php?f=24

 

[Rookie Vette owner]

Thanks for all the info GTR1999 and especially the link. There is a lot of good info there.

 

[GTR1999]

Glad to help, if you have any questions let me know.

 

[john]

Hi GTR1999

I have about 1/4 inch side play on my right side yoke..in and out play.
Where is that side play coming from.....worn bearing ?
I have a 71 LT-1 with stock 411 gears and about 35k miles on the car.
My driving is mostly country roads.
I was thinking about changing rear gear ratios for better MPG's....what gear ratio would you suggest?

 

[TimAT]

John, look at Gary's link above. It'll have everything you ever wanted to know about Vette rear axles. More than you wanted to know.

 

[GTR1999]

Hi GTR1999

I have about 1/4 inch side play on my right side yoke..in and out play.
Where is that side play coming from.....worn bearing ?
I have a 71 LT-1 with stock 411 gears and about 35k miles on the car.
My driving is mostly country roads.
I was thinking about changing rear gear ratios for better MPG's....what gear ratio would you suggest?

Hi John,
As Tim said the link will have more info on these then you'll find elsewhere.
In your case I suspect the play may be in part from the posi. These were sloppy at times and I've found endplay caused by clutch play to a good part of the problem. I did a 71 LT1 diff with over 90k miles and the yokes were still good. Once the posi was custom tuned I had the grind a few thousands off them in fact.
A stock 411 uses a 4 series case which is the problem here. Going to a 355 370-373 gear will require a case change as well. It certainly can be done but the cost just jumped an extra $350 for a case.
What ratio would be good depends if you still have the M-21 4 spd. If so then I wouldn't push it beyond 355.

 

[JohnZ]

I have a 71 LT-1 with stock 411 gears and about 35k miles on the car. I was thinking about changing rear gear ratios for better MPG's....what gear ratio would you suggest?

I have a stock/unrestored '69 Z/28 that was built with a 4.10 axle and the close-ratio M-21 4-speed, and 3800 rpm @ 70mph in highway trip driving finally got to me, and I swapped out the 4.10 to a 3.55 (with a new Series 3 Posi unit). I wouldn't do it again, as the 2.20:1 1st gear and 3.55 axle are not a good match in town traffic driving, although 3300 @ 70mph is an improvement on the highway. If you have an M-21 in your LT-1 I wouldn't go any lower than a 3.70.

You'll also need a Series 3 Posi unit, and by the time you're done with the conver$ion, it'll take you forever to recover the cost with the minor fuel savings you'll see.

 

[TWINRAY]

I have a stock/unrestored '69 Z/28 that was built with a 4.10 axle and the close-ratio M-21 4-speed, and 3800 rpm @ 70mph in highway trip driving finally got to me, and I swapped out the 4.10 to a 3.55 (with a new Series 3 Posi unit). I wouldn't do it again, as the 2.20:1 1st gear and 3.55 axle are not a good match in town traffic driving, although 3300 @ 70mph is an improvement on the highway. If you have an M-21 in your LT-1 I wouldn't go any lower than a 3.70.

You'll also need a Series 3 Posi unit, and by the time you're done with the conver$ion, it'll take you forever to recover the cost with the minor fuel savings you'll see.

I agree with going no higher than 3.70 with a 2.20 1st gear. I think from the factory with the CR box, it was either 3.70 or 4.11. I made a big mistake once with a '68 GTO and a CR box, going from a 3.90 to a 3.36 rear. Lost the whole performance with the car. I think the real solution is a 6 speed with a change in the rear gear. My '67 has a 4.11 with the CR box - totally useless on the highway, I keep it under 60hnoes.

 

[john]

By reading all the post replys , it looks like the best way will be a 6 speed or keep her 60mph or less.....sure is fun to drive at any speed.

Thanks all.
John