Thursday 28 May 2020

Chain Tensioner

The last piece of the jigsaw is the chain tensioner.  This simple blade tensioner was originally developed by John Weller (who also played a significant role in the early days of AC and, indeed, AC patented the chain tensioner). In addition to early OHC AJS motorcycles, this design continued to be used on post-war AJS 7R and Matchless G50 racing engines and was adopted by Jaguar for the very successful XK 120/140 and Mk 7.

You can see the crumbling remains of the K7 tensioner blade and spring below.

There are actually 3 springs, 2 pins and one sliding blade support that make up this system. The 3 springs are, of course, the blade itself that bears against the chain and the tension spring that is stretched between the 2 pins at either end, but also a thin sheet steel spring that retains the sliding blade support in its slot but allows its lateral movement. All of these parts (new ones!) are shown below.

By an amazing piece of luck, when I phoned the company that I have always used for making springs, namely Alberta Springs in Colchester, Graham, who owns the business and is springmaker-in-chief, told me that not only had they had made these before, but even had drawings! Sure enough, a couple of weeks later a new set of 3 perfect springs turned up in the post. Since the time when these springs for the K7 were made, Alberta have also made further sets of these chain tensioner springs for the subsequent AJcette and V-twin projects ….but more of these at a later date.

The picture below shows the new installed complete drive system for the overhead camshaft.

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