OS FS70 Surpass Valve Spring Replacement.

My OS FS70 Surpass Mk1 stopped suddenly and cost me a Wot4.  It had a broken inlet valve spring.  Here, for any-one interested, is how I fixed it.

Inlet valve is nearest to us. See the gap??

It’s not a difficult task but the parts are very small, like the split colletts that hold the spring on…

Undo the cap head screw and remove the valve gear…

Unscrew the five cap head bolts holding the head and remove…

Remove the carb,  inlet manifold and exhaust pipe.

Lift out the valve pushrods, they are identical so just put them safe…

Use whatever method you can to hold the valve in place while removing the split colletts and spring retainer..

Broken inlet spring and the OK exhaust spring. Also the colletts and spring retainer with a screwdriver tip to give it scale.

Gave it a de-coke while it was apart.

No damage to the piston, and it’s nice and clean.

Re-fit the head taking care to seat the push rod tubes correctly. They have a rubber O-ring at each end.

Replace and tighten the five cap-head screws holding the head. Tighten diagonally, like wheel-nuts on your car. Do them bit-by-bit and do not over-tighten.

Replace the pushrods into their tubes. Turn over the crankshaft to make sure they have seated correctly.

Replace the valve gear. Check the gap when both valves are closed, when the piston is at TDC on the ignition stroke. The OS manual says the valve gap should be between 0.04 and 0.1 mm, not 0.4 mm.

At this point I normally add a few drops of after-run oil to make sure everything is slippery for the first start.

All back together ready for the first run in a test stand. Always test an engine in a stand before committing it to a model.

There, not so bad, was it.  Any questions please ask.

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4 Responses to OS FS70 Surpass Valve Spring Replacement.

  1. Graham says:

    That is a good write up – I am thinking of doing similar work on mine, but the OS manual says the valve gap should be between 0.04 and 0.1 mm, not 0.4 mm.

  2. Neil Watson says:

    How about explaining installation of the valve spring retainer and (especially!!) the two tiny collects. General explinations I have read in the past simply say to”press down the retainer and spring and install the collets”!!! IT AIN’T THAT SIMPLE!! Is there a special tool that can be made to compress and hold the vlave spring and retainer while re-installing the tiny collect retianers? Help!!

    • Hi Neil
      It actually is that simple my friend. Use a 10 or 12mm wrench to compress the spring and then apply a little grease to the tip of a small screwdriver and “stick” the collet to the screwdriver, carefully place the collet halves into the top of the valve stem. Release the pressure on the spring. It’s always good to so this under a sheet of clear plastic so if it goes wrong the collets and spring retainers will not fly across the room, never to be seen again.
      Hope this helps. Next time I do one I will take pics and upload them here.

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