Magnesium vs Aluminum 915 for RSR Clone

[tom1394racing]

I am ready to rebuild the transmission for my RSR clone. I have lined up a ZF Limited slip and plan to have my '73 transmission rebuilt using the stock 7:31 ring and pinion with stock 1st and 2nd gears and 22/29, 25/27, and 27/25 for 3rd, 4th and 5th gears. The motor will be '78 3.0 ltr built to make aroud 230-250 HP. The car will be a combo street/track car that may become a vintage race car at some point.

I have been advised that I should forget the mag case 915 and get a later model aluminum case transmissin with 8:31 ring and pinion and build the transmission gear set around the later model 915.

I've already got the gear set and limited slip for the mag case tranny and was planning to have it rebuilt this week.

Will the later model tranny really make a difference for my application?

[GeorgeK]

The Mg case is plenty strong enough. What will make a difference is the reinforced aluminum side cover.

[Jack Olsen]

The mag case is strong enough, with the usual upgrades (like the side plate George mentions). But if you're going to track the car with a limited slip, I'd install a cooler. Those clutches generate a lot of heat in track use.

An aluminum-case 915 might leak a little less.

[rattlsnak]

Im going with a mag case 8:31. Yes, I found one, they only made them for @ 6 months in '75!

[rcwaldo]

Trouble with the mag case is keeping the pinion head bearing bore, round and true. They tend to oval and move around after time and use. IIRC, some people have remachined and pressed in a steel sleeve to fix this.

Other drawbacks with mag is the thermal expansion delta of .001" per inch MORE than steel, at 200F.

Mag is quite a bit lighter than aluminum, though.. FWIW

Chris

[wevoid]

Originally posted by rcwaldo
Trouble with the mag case is keeping the pinion head bearing bore, round and true. They tend to oval and move around after time and use. IIRC, some people have remachined and pressed in a steel sleeve to fix this.

Other drawbacks with mag is the thermal expansion delta of .001" per inch MORE than steel, at 200F.

Mag is quite a bit lighter than aluminum, though.. FWIW

Chris

It is true that keeping the outer race of the pinion roller bearing anchored in the case is a reliability issue with the 915.

However, the mag cases are superior in this respect as there is an iron insert resident in the casting, meaning that the pinion roller bearing is fitted into an iron bore - with lower expansion than the surrounding mag, or the aluminum bore in the aluminum cases (that do not have the iron insert)

Both mag and alu versions suffer in this respect, the aluminum version significantly more so.

The bare cases have a 14 lb difference in weight, a mag case with the late model side cover in aluminum still enjoys a 12 lb advantage over a late model aluminum case.

Regards

Hayden

[tom1394racing]

Hayden

I assume you would still recommend the aluminum reinforced side cover for my mag case as an enhancement to the steel bore.

[wevoid]

Tom,

That is correct.
Not so much as an enhancement to the steel bore, but to eliminate the flexibility of the Mag side cover.

Regards

Hayden

[Plavan]

Hayden,
Do you know the weight difference between a Mag and Aluminum 901 tranny?

[wevoid]

Chad,

Sorry, I dont have any 901's here.
My guess based purely on the weight variation of the 915 cases with same shape, different alloy, would be about 9 or 10 lb.

Regards

Hayden