How to modify a Slab Roller

I got two old sprockets for free from a local bike shop. It doesn't matter what size the sprockets are, but they MUST have the same number of teeth. Mine are 18 tooth (about 3" in diameter).
The sprockets come with very large center holes. Washers are welded onto the sprocket to decrease the center hole diameter to 1/4 inch.

The sprockets are attached to the cross-shaped black plastic adjustment knobs that came with my Northstar slab roller. There are a couple of ways to do this. I used a short 1/4" screw. I drilled and tapped a 1/4" hole in the center of each knob. I then drilled and tapped a second, smaller, off-center hole so that I could secure each sprocket with an additional screw. This prevents slippage when turning the sprocket/knobs.

An alternative, and perhaps easier and more elegant way to attach the sprockets to the knobs would be to just discard the plastic knobs, and weld the sprockets directly to the height adjustment screws, or to nuts of the appropriate size.

The bike shop also gave me a couple of free used chains. You will probably need to piece two together to get the length needed.

LEFT:
Besides a welder, these are the only special tools I needed.
At top right is a chain link extractor, used for breaking and connecting bike chains.
Also shown is a cheap Sears tap and drill set, used for making the threaded holes in the plastic knobs.

After attaching the sprockets to the knobs, it's time to attach the chain to the sprockets. When I did so, I found that the chain was very slack. But if I shortened the chain by a single full link, then it would be too short to use. The slack made the dual-adjustment not work very well. The chain kept trying to slip off of the sprockets.

Clearly, I needed to tighten the chain up somehow. I accomplished this by passing the chain through a short piece of pipe. This increases the travel path of the chain, and also increases the amount of chain wrap around the sprockets.
For the pipe, I used a piece of 3/4" EMT (Electro-Metallic Tubing or conduit -- get it at Home Depot), about 11" long. Depending on how slack your chain is, you may need a longer or shorter piece, or none. PVC conduit could be substituted for the EMT.
Once connected, you do not need to break the chain to pass it through the pipe. A doubled-over bike chain will pass through 3/4" EMT perfectly.

The dual-adjustment feature now works perfectly.

Mine works best with two hands, one pulling, and one pushing the chain.

Trust me, this is very easy to do. Once the two sides are synchronized, you will never again have to fumble around trying to make sure that you are not making wedge-shaped slabs.

I have been using this set-up for a few years, and my dual-adjusters are holding up very well, with no sign of slowing down. They have saved me countless, possibly hundreds of hours.