Davis diesel head improvement

Yea it is the Contra piston
With a little bit of work it should work

Make sure you select the right O ring material otherwise it soon gets stiff and non-elastic under engine heat - thus sealing may be compromised- as I experienced with some common silicone rings put on COX cylinder necks to enhance goo control in exhaust mufflers.

Silicon and petroleum based fuels don't mix
Normal rubber O rings should work okey

As far as I know, most European diesels never ran an o-ring to seal the contra-piston. The piston was just a very tight machine-fit. It looks like the o-ring is an American idea, as I've only seen them in OK Cub diesels and the RJL conversion heads. Certainly of the dozen diesels I have, none use an o-ring. Australian, British, Russian, Chinese, Indian. All have a machined-fit contra-piston. The only real disadvantage is that it's common for the contra-piston to stick, particularly if someone unwittingly screws the comp screw down too far onto part of the bore which has carbon build-up. It can be very hard to budge a stuck contra and many diesels have been damaged by the owners trying. Generally, the first time the engine fires it will shoot the conta back against the comp screw.

I have never seen a diesel with a O ring seal
well exept for my RJL head

On the small engines I think it is hard to get a good enough seal and there is only room for one o-ring. On the PeeWee my o-ring head would not match the DDD head with a brass washer. Forget about the Teflon, it doean't seal, or you will pinch right through it when tightening the head. Viton rings can take the diesel fuel, but the PeeWee seem to be a little too small and sensitive, to me. For the .049 engines you can also get a Norvel head, the use the traditional press-fit seal.

Please keep in mind that the Teflon disc is intended as a safety barrier; rupturing before engine damage occurs.

Cox International wrote:Please keep in mind that the Teflon disc is intended as a safety barrier; rupturing before engine damage occurs.

yea I know
but the also wear fast and needs constant replacing
if the new contra piston works it is a nice upgrade for the people who are used to diesels

The danger I found was that the teflon discs will always tend to leak a little. This will trick you into increasing the compression setting in order to start the engine. But once it starts the compression setting is simply too high, thus risking to cause damage to the engine...

The safest option is to have a very good seal, e.g. a brass disc. This seals much better and gives easy starting at a compression setting that is also fine for the running engine, thus much safer in practice compared to the leaky teflon discs...

I agree that is why i switched to aluminium
it wears as fast as the teflon but it seals a lot better
(cant get the brass sheets)

Aluminium seals well but it will fracture after a run or two. Brass works much better, you can get sheets from e.g. K&S Engineering or regular hobby shops. Aim for something around 0.10-0.15mm thick.

really a hobby shop
never thought to look there lol
thanks for the tip

I don't really know what to call it, we don't have any RC hobby shops that one can go to, like one could in the past, now there are only online stores for that...

But Craft/hobby stores often have sheet metal and metal wires in various materials.

You should be able to find brass sheet at a model railway supplier. Also, automotive parts stores sell brass "shim stock" in various thicknesses.

well it the part arrived today
and good news it works
the only thing is the groove for the oring should be a little bit bigger (from 1.5mm to 1.55mm)
also I just reread my main text
it is a R3 O ring not a R9
also I made a video but my camera doesnt like the dark
so it is bad (still gonna post it)
the pictures are the RJL Head and my new design

update time
I made a few video's today
and I found out my orings are crap
they have a seam were they break all the time (gonna order some good orings)
the dimple I drew isnt realy nessesairy
i reversed the head to test if a flat contra piston would work
and i hardly notice the diverence
the video will be edited and uploaded when i am back tonight

also I dont have a supplyer so I cant sell these contra pistons
maybe Bernie (cox international) could sell them as a complete head/ upgrade kit?

also the updated drawing will be uploaded tonight

If you have an airport with a maintenacne facility near by see if you can find MS-28775-XXX O rings. The XXX is for the size.
Bring one of your o rings to compare for size.

fit90 wrote:If you have an airport with a maintenacne facility near by see if you can find MS-28775-XXX O rings. The XXX is for the size.
Bring one of your o rings to compare for size.

or just look at the chemical resistance sheet and order some on ebay
for the tests i will use what i have and when its done i will use the ones i have ordered

the new design

and a video


I am looking forward to your replies

update
the specialy selected orings are arived
the v1 head cant fit them
I am so glad I kept this in mind with V2
now to wait for a v2 prototype

Good news I received a few v2 prototypes
I am gonna run a few tests when home
And after that the results will be posted

Are you having these made somewhere?

Wouldn't it be easier to just get an RJL head instead?

RJL heads are expencive and hard to get for me
it is cheaper to design my own

update
the test engine ran
not great but that was becaus i was running a spi piston cylinder combo
lots and lots of power
my orings are still to fat but they do fit with a bit of force
so v3 needs a deeper groove and then it should be good

also my crank broke apeerently i forgot to instal a hardened shaft in the test engine
so for now succes

I shall order a new crank and do some long term tests
are people here interested in the spare contra pistons i have?