Originally Posted by
Lou Crane
The Teflon disk is not a gasket, as such. (It is however 'softer" - more compliant - than the stock copper Cox head gasket.) Without a 'seal' across the full diameter, there is NO sealed chamber allowing combustion. The loose slug above the disk WILL leak through the adjuster screw and through any burnt-through segment around the periphery.
And, yes, I have tried to start such DDD converted reed Cox engines with a burnt disk. I will not try to run a diesel with the comp screwed down to piston contact with the head. I could not find a comp adjuster position that fostered combustion with a 'failed' disk.
Perhaps you have found such a setting?
But of coarse the Teflon disc has to act as a gasket in order to seal the combustion chamber. It needs to seal well all around its periphery, that's ecactly my point. The Teflon material is not an adequate gasket material, a soft metal is much better for that. The piston and screw above the disc does not seal anything, it merely supports the teflon disc.
I have never been able to burn out a disc, instead they leak around their periphery. If you bring the piston to TDC and hold it there, yuo will see this as a rather rapid loss of compression, much like a worn piston/cylinder would behave.
By replacing the Teflon disc with a metal one, you can in contrast get a fully adequate seal around the periphery and a much slower loss of compression when held still at TDC. The compression setting on the head screw will now be lower for starting the engine, compared to the Teflon, hence also more gentle running for the engine.
Around 30 years ago there where also other conversion heads for Cox engines, with better performance but at a higher cost. There was for instance a Swedish product which consisted of a classical iron contra piston running in a steel head bore, much like in a classical diesel, that outperformed the DDD ones.
So 30 years later I see no reason to continue to fiddle with those silly Teflon disc that never did the job well. Most engineers would get this without even trying to run an engine with it....