sq4 1939 cam tensioner

[Henning G]

Hello,
During renovation on my sq4 , I found this ”extra” cam chain tensioner, don’t believe it belongs in my 4G, but where does it belong?? Certainly well made and fits perfectly..

[Roger Gwynn]

That is a previous owner's modification or repair to damaged casting

[Henning G]

Thanks a lot Roger, the casting is fine, just thought it was from a later sq4..Unbeliveble what people put into their engines…

[roland robinson]

As the fibre rubbing strip is missing I wonder if a previous owner put that in to stop the spring tensioner from flattening out too much. Also intrigued by the four screws behind the magneto sprocket.

[paul.jameson]

I have a very similar item, in alloy, but held in place by a bolt with hole down the middle which screws into the end of the breather threads. A more elegant solution is to use the equivalent part fitted as standard on the Mk I engine.
The purpose of these devices is to prevent the chain riding over the sprocket teeth if the engine kicks back. You and I may not be able to replicate this in the workshop but I have had it happen on a simplex chain in a Mk II engine.

I assume the 4 screws behind the magneto sprocket hold some form of oil seal in place.

[Henning G]

I have of cause installed the fiber chain guide as original, and the 4 screws was made before I got the bike, and I have now made a holder for an oil seal, to prevent oil leak at the magneto, seem to work…