Since the 1955 Buick 3.42 ring & pinion set didn't fit (too large a pinion) into a 46 third member, I was extremely disappointed and frustrated. I had already drilled the ring gear rivets out from the original carrier, and enlarged the holes to accept 3/8" bolts, so I can't use the original 4.10 ring gear anymore either.
So, this whole gear chase started when everybody told me that a straight 8 won't accept any transmission with an OPEN driveshaft. Moreover, the trans to engine bolt pattern is too weird to make an adapter. When everybody offer something as a commonly accepted fact, I doubt it. So why not keep the standard Buick bellhousing and clutch, and mate another tranny onto it?So I did some research, and found out that a later GM 4-speed would be a relatively easy swap!
A Saginaw , a Borg Warner T-10 (1957-63), as well as a 63-70 Muncie M20, M21 or M22 would even have the same 10 spline 1-1/8" input shaft as Buick does. However the 1975 and later Super T-10's, 71-74 Muncies, and T-5's have a 26-spline input shaft, thus you cannot use your original clutch plate (10.5" in Roadmaster - don't know about Super/Special). However you can use any GM 10.4" clutch plate w/ 26 splines. Even the shaft length is the same 6.66", and the Buick throwout bearing and pilot bearing will work too. If you want durability, stick with a Super T-10 or a Muncie. The old T-10 and the Super-T10 look the same, but are totally different from inside. If you select a T-5, be sure to get a WC (World Class) T-5. The non-WC T-5, found in third-gen F-bodies, is a weak design.
I found a Super T-10 from a 1979 Trans-Am. Any ST-10 will have a wider ratio than any Muncie, thus preferred.
I have now the ability to use any rear axle I want, and opted for a third gen F-body rear, that is, a 1989 Firebird unit with highway-friendly 2.73 gears and posi.Update 2005: Bendtsen's sells an adapter that accepts any GM automatic trans, and comes with a mini-starter.
Update 2009: If you want a 5-speed, get a Tremec TKO 500. Youl'll get multiple shifter locations and it will hold up 500 lb-ft of torque. The TKO 600 will hold up 600 lb-ft. You can get a kit from Classic Motorsports Group that includes all you need. The swap is similar to the one outlined here.
| Use the existing bearing
counterbore to
center the bellhousing in the mill, then machine a larger hole
for
the 4-speed front bearing retainer, like here. In Buick, the
bellhousing
itself acted as the retainer. I used a CNC mill to make a snug fit.
The later GM 3-speed or
4-speed bolt
pattern is a bit different from that of Buick. You need to file a
little
material from the upper bolt holes at the trans. For the lower 2 holes,
you need to weld some material onto the bellhousing to widen the
flange, then machine it level with the surface, drill and tap new
holes.
The bolts here show the new holes. |
 |
| Looks like it came from the
factory? The ST-10 comes with 32-spline output shaft, and takes a TH-400 yoke. While at it, I also replaced both front and rear seals, and swapped new speedo gears. Only there is no gear combination for an ST-10 with 2.73 gear and 28" tall tires. So I had to take the inner spline from the original 7-tooth drive gear, and mate it to an outer half of a 10-tooth automatic trans drive gear. A press fit, secured with Loctite. I'll use a 20-tooth driven gear.
I thought I would have trouble mating the speedo cable to the trans, but bless good old GM - the trans and gauge ends are same from the 30's till today! |
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