Guys, I'm going to put the instructions for the conversion as I did it on my 55 .
Start here. 605 box and stock 55 box Now dismantle the 605 box. If it has a pitman arm on it, remove it first. Next there is a snap ring in the top of the 605 that holds the round top cap on. Look around the top edge and you will find a small hole near the line fittings. Take a pin punch or an awl and push it into the little hole. This will push the snap ring in so that you can get a screw driver under the snap ring and gently pry it out. Also remove the lock nut from the adjusting screw/bolt. This is left handed threads so be sure to turn it clockwise to remove. Now screw the adjusting bolt down into the box until the top comes off. NOTE: It is a good idea to soak the adjusting screw/bolt with WD40 (or the like) for a day or so before you start
605 Sector and 55,56,57 Sector First, if there is a pitman arm on the tri-five box, remove it. Next remove the three bolts holding the top on the 55-56-57 steering box. Remove the adjusting screw/bolt lock nut(turn counter clockwise)and screw the adjusting screw/bolt down into the top. When the top is off, look and be sure the sector is in a position so as to allow it to come out from the top of the box. It may be off center and if so, the sector will catch on the inside lip of the box. This can be moved by rotating the worm shaft. This is the difference in the two sector shafts. You have to cut the outside two teeth off the 55 sector shaft and remove 1/8" from the top of the 55 sector.
Modified 605 Sector
Stock Sector before end change
See next photo for modification.
Modified Sector Look closely and you can see that 1/8" was removed from the top of the 55 sector(right end). This is so the 605 top cap clears the end of the sector.>>>After this is done, you need to remove the adjusting bolt from the 605 sector shaft. There is a inside spanner nut that screws down inside the top of the 605 sector. Remove this and the adjusting bolt. When the bolt comes out, there will be a very thin washer on the bolt. Discard the thin washer.The adjusting bolt is all you need from the 605 sector, so you can put it aside. Place the washer from the original 55 adjusting bolt on the 605 adjusting bolt and slide it in the slot on the modified 55 sector shaft (shown below). Then just reassemble the 605 using the 55 sector.
55 Box lower section Next you will need to cut 4 5/8th" from the bottom of the 55 steering box. Most other instructions say 4 1/2 inches. But it usually leaves about 1/8 to 3/16" between the upper and lower boxes. It is best to use a power hack saw, but it can be done with regular hack saw. You will use the 4 5/8ths" section as a pillar block on the frame. The next photo shows the 4 5/8ths" section after the cut. It hasn't been done yet, but you need to drill and then install a grease fitting in the center of the pillar block. The box below has the fitting installed. One additional thing needs to be done to the pillar block. Turn the upper portion of the tri-five steering box top side down on a flat surface and look in the hole where the sector shaft came through. You will see a bushing(bronze). Look through your socket set and fine a socket that fits snugly inside the old box section, but not through the bushing. Then you can tap the bushing out of the old tri-five box and then install it in the top of the pillar block.
605 Together Now that the modification is complete, it's a good time to check the 605 hose connections. The 605's have two different types of line fittings, o-ring or flare fittings. The flare fitting has a "flared seat" for the hose while the o-ring is a bit deeper and a flat seat for the o- ring. Also there are a few 605's with a metric thread. The type of fittings you have is important since it will determine which pump you use, a pump with a flare fitting, or an o-ring fitting, Unless of course you have the lines custom made.
A mounting bracket will need to be used. I will add pictures of the pieces. The main bracket welds to the top of the frame rail and against the side of the 605 using the original bolt holes in the 605. Then two gussets need to be welded to the new bracket and the top of the frame rail.
There are aftermarket bolt on brackets available from several suppliers, but they have been known to flex and in some cases crack causing the box to move. Below is a picture of the bracket I made and used on mine.
605 Steering Conversion in place. (This picture isn't of mine but rather a friends.)
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I just completed my 605 conversion but I haven't put the column back in the car. I used a similar set of instructions and I would suggest cutting the pillar block a little longer. I ended up with about a 1/4 gap between the 2 pieces. I suggest cutting it at least 4 5/8 to narrow the gap.
Also, I received a tip to weld a triangular brace or gusset from the new mounting plate to the frame. This is shown in the last photo above but it is not mentioned in the instructions. This should take some of the stress off of the lower piece.
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You can also run the plate from the bottom,"the old 55 portion of the gear housing" up to the top portion of the new 605 box. In other words you make the plate cover both sets of bolt holes, this will give you the stiffness you need without having to weld to the frame. I used a 1/4" plate, this will work fine with small block chevy engines as you have plenty of clearance. It is pretty tight when installing a big block chevy engine. I have seen a 454 in a 55 chevy with the 605 conversion and even with the headers designed for this set up it was close. That is the only way I would do the weld brace on the frame. There is plenty enough clearance on a SBC to use the 1/4" plate and connect both the top and bottom portion of the 605 and keep you from welding on the frame. I know there is an offset but this can be a pre-bent plate on a metal brake or fabricated welded plate to obtain the offset.
If you are not going to go with the original steering column I hear that the 1965 and 1966 Impala steering column makes for an easy installation to the 605 steering gear in a tri-five car. The new aftermarket columns are getting expensive and you can still find these units around, some even have tilt and telescoping features. I picked up a clean 66 impala steering column with the tilt and telescoping out a junked impala from an individual owner...cheap.