mag case 915 has 7:31?

[jhelgesen]

Do all mag case 915 trannies have a 7:31 ring and pinion? I have a mag case 915 that came with my 77 c3 targa that I think was a sporto to start, extra lines on oil tank are crimped off.

The numbers on the tranny are all ground off, so other than opening the tranny (which is already out of the car) is there any way to tell what r&p is in the car?

[JP911]

From what I've read here, 74 was the last year of the 7:31. I have a mag case 915 from a 76 and it has the later R&P.

[Steve@Rennsport]

Originally posted by jhelgesen
Do all mag case 915 trannies have a 7:31 ring and pinion? I have a mag case 915 that came with my 77 c3 targa that I think was a sporto to start, extra lines on oil tank are crimped off.

The numbers on the tranny are all ground off, so other than opening the tranny (which is already out of the car) is there any way to tell what r&p is in the car?

John:

Sportomatics had an 8:27 R&P and you can pull the side cover to remove the diff to count the ring gear (and pinion) teeth.

[SC-targa]

Why not just check the speeds in gears? With the 7:31 axle, first gear should run about 32 mph in first gear at 6,000 rpm. With the 8:31 it would be about 36.5 mph at 6,000 rpm.

Of course that would depend on an accurate speed reading.

Regards,

Jerry Kroeger

[jhelgesen]

The car was a sportmatic, converted to a 5 speed, 915 tranny. Can't drive the car, its currently in boxes, tranny is sitting in the storage building.

If it helps, it had the earlier 4 bolt cv flanges.

Guess I'll have to pull the cover.

[Bill Verburg]

Can you read the # on the bottom of the case?

[jhelgesen]

Sorry Bill, the tranny has been dragged across something before I got the car, can't see any of the numbers.

[Elombard]

Cant you spin the axle in 1st gear and count the revolutions of the main shaft? Not sure what the revolutions would be but I checked one like this on a datsun one time.

[2.7RACER]

This covers most of the transaxles.

Please note the 75/77 ratio in 4th gear is 1:1.
Put the tranny into 4th gear, carefully count the input shaft rotations for 1 output rotation.
Under 4 rotations = 8:31. 3.875
Over 4 rotations = 7:31. 4.428

[jhelgesen]

Okay, I'll see if I can figure out 4th gear with a vise grip on the shifter rod on the nose of the tranny.

Since the tranny didn't originally come with car best I can tell, is there a casting date or something that might tell me the aproximate age of the case?

[jhelgesen]

Cleaned the storage building and can play with the tranny now. I did find a '75 casting date on the housing, so I'm leaning toward the 8:31 ratio now. We'll see what I can find tonight.

[GBX]

bring back an old thread. Looking at a car with a 915/61 gearbox. Can anyone confirm if this is a mag box and also what R&P it would have?

Case reads as follows:

915/61 719 1974

[Walt Fricke]

Post 9 gives a simple way to tell the diff ratio. It isn't hard to tell the difference between aluminum and the mag alloy. Mag is pretty gray. Aluminum is, well, aluminum. Much shinier, especially if you buff a bit of it.

But the numbers will tell - I don't have my list handy, but I think the 9 means it is a 1979, and it is a 5 speed, so it would be an aluminum case.

[dannobee]

911/61 has an 8:31 R&P.

[Matt Monson]

915/61 were made in both mag and aluminum, but were all 8:31.