Over the decades, Minis were produced with several styles of timing chains as requirements for performance or durability with bigger engines and heavier valve springs dictated. The early small-bore engines used a single row chain and sprockets that were "tensioned" by two rubber disks affixed to the larger camshaft gear.
These placed axial loads on the chain, acting as a damper and kept the chain from slapping around inside the timing cover. As high performance engines with big cams came along, double row chains and sprockets provided the durability and accuracy for high RPM operation in typical hot-rod fashion. Finally with the move to A+ engines in the early 1980s, a very effective single row chain and sprocket set coupled with a spring loaded tensioner provided very reliable cam timing for the service life of the car.
Fast forward to today! You are building a high performance 1275 or bigger engine for the street, and have studied your options, settling on a stout "duplex" timing set. When installed however, you find that the chain just has too much play for comfort. Why bother setting the cam timing when you have 2 or 3 degrees of slop with a NEW timing set! Now what can you do?
The nature of the design of the A-series block means a crankshaft line-bore (if needed during a rebuild) will raise the crankshaft incrementally in the block, bringing it just a little closer to the cam. This results in the sloppy timing chain fit we want to avoid. Luckily, there are a few good solutions readily available- the late tensioner arrangement has proven reliable and is very cost effective.
As an option there is a high performance lightweight steel double chain row set that is available with incrementally shorter chains to make up for a line bore ranging from .002 to .010 inches and keep it tight AND allows for plus or minus six degrees of cam timing adjustment by selecting different keyways on the crank gear.