I have a 2 questions with no answer.
Can I just put a screw in the head to convert a glow to a diesel?
Can a four stroke be converted to diesel?
thanks

Yes to question one and no to question two. Bob Davis was doing research to make a fourstroke Diesel out of the Overhead Drum Valve Webras but Webra quit making them.

I've also wondered whether there is anything special to diesel heads or not.

How does using a screw in place of the glow plug compare to using Davis conversions or purpose-built diesels? Does anyone have enough experience with each to compare these different options?

Diesel heads have what is called a contra piston in them that is moved up or down with the screw thus changing the combustion chamber volume and therefore the compression pressure. On most European Diesels you just increase the compression pressure until the engine starts, let it warm up a few seconds, run it wide open and adjust the compression and the mixture for best running and fly. With the Davis Diesel conversions you use the same procedure except once the engine is set up you won't have to adjust it again unless there is a drastic change in the ambient temperature. With the Euro style Diesel you have to do the compression and mixture routine each time you start the engine.

Thanks
I don't fly planes, I run trucks but I wanted to convert an engine for my truck.
Do you think it will give it more power thank using glow?
thanks

Hobbsy, there is quite a difference between a European diesel and The davis conversion that has been copied by RJL and the late Irvine Diesels.
A classic diesel has the contra piston fitted into the top of the cylinder, this is one of the reasons for the slow warm up and fiddeling with the contra piston. There is quite a lot of mass to heat up to allow the timing to advance. The davis system is an advancement in that the head get all the heat and warms up quickly, the contra piston being in the head doesn't detract or impede heat transfer, in fact it makes for a cooler running diesel.

Thanks Dennis, I sure said that wrong

Any Dieselphytes have the specs for an Irvine .40 Diesel, I was comparing mine to an old series Irvine .40 glow and they do not have the same bore and stroke. The Diesel appears to have a 1.5MM smaller bore but a 1MM longer stroke than the glow. The crank in the Diesel is stamped 46. Also the Diesel head has the contra piston in it like a Davis head does but there the similarity ends.

I pulled a new Irvine Diesel down to check the papers in it. Sadly the specs for the motor are nowhere to be found. No bore or stroke is given anywhere and I'm too lazy to take it apart and measure. Sorry. Dennis

Same here Dennis, there is no mention of the specs in the paperwork. At the Just Engines site the new red .40 and .46 Irvines have the same stroke but I don't think the previous series did. This one is a jewell to run. I use RedMax in it and a 12x5 Graupner.