Hi Ladies and Gents, Richard Bill Wood here, AKA Platenboy/Coronaboy

I am not a typewriter engineer, just a collector who hated the rasping zip of his SM4 carriage when he discovered the 'silencer spring' was broken.  Here is my repair blog.

This job is not for the faint hearted, it takes a lot of patience and time, but the results are worth it.

There may be other ways to replace the spring, but this is how I did it not being able to successfully complete the repair from below.

Please use reasonable caution and care when attempting this repair as I do not accept responsibility for any injury that may result from your repair to yourself or your machine.  It's entirely at your own risk.

I tried to make notes whilst doing this repair, but I apologise if I have omitted any part of the repair, hopefully, with these tips and instructions you can get through the repair.

The groove where the silencer slips onto, viewed top down, platen and paper pan removed.

A SM4 Silencer Spring in good order

Post script; installing parts is the reverse, please ensure you replace the metal washer against the 4 coiled spring and the one against the platen collar.  Also, when you replace the platen, make sure the corrections table (sprung loaded), is NOT tucked under the platen.  It's so easy to forget this and screw everything back into place then realise it's stuck under the platen. Also, remember to replace the guard which protects the escapement if you replaced or repaired the secondary spring.

Good luck!