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F.4.6) Blown Gasket


Despite carrying a whole load of spares and emergency equipment, I ended up needing a tow when the gasket between the inlet casting and reed block gave out. This meant no compression and without compression 2-stroke engines just don't work - the engine was just pumping air and fuel through the gap.


The Offending Edge.

A contributing factor to the leak was a partially stripped thread in one of the bosses in the casting. This meant that when the bolt was torqued up it wasn't actually fully tight but had reached the torque as a result of a knackered thread. 


Boss With Partially Stripped Thread.

After removing the power head* the casting was sent to Sussex Engine Supplies for re-facing on both mating surfaces. It was turned around in a couple of hours.


Re-faced Casting.
On the casting's return I wound-in a 6 mm helicoil into the boss with the stripped thread; due to the finer tolerances and the materials used helicoil inserts are generally stronger than the original hole. A 6 mm plug tap was used to clear the other holes of sealant, dirt, etc.,
Helicoil Insert
Having cleaned the mating faces with a razor and de-greaser some Wynn's Black Gasket Maker was used to make a gasket approx 2 mm thick (after it had cured and been tightened for the second time).

With various other gaskets renewed and the powerhead re-installed, it started on the first turn of the key.

* This is a variation of the phrase loved by the authors of workshop manuals "With the engine and gearbox removed as in Chapter 1..."
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