contra piston question?
09-29-2011 | 07:18 AM
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From: Helena,
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contra piston question?
I have an itch to make a diesel head for a como 90, supertiger 61, or thunder tiger 46. I don’t know which one will be first but just want to try diesels out. If I get the 90 to run good, I will probably put it in my Corsair. I have looked at quite a few photos and plans for different heads and have recently seen the article in fly rc magazine. All the designs I have seen thus far have an o-ring seal on the contra piston. I have not come across a picture of the parts to the davis diesel head yet but they don’t seem to have any replacement o’rings on their site, so I assume they don’t have them in their design. Does the diesel conversion head need an o-ring seal or will a well fitted contra piston work just as well?
09-29-2011 | 08:01 AM
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From: London, UNITED KINGDOM
RE: contra piston question?
Well Norvel diesel heads do not use an O ring or a countergasket.<div>MECOA heads use an O ring.</div><div>Davis 1/2a heads is teflon countergaskets that are prone to bursting but that is 1/2a a stuff.</div>
09-29-2011 | 10:38 AM
#3
RE: contra piston question?
Normally a well fitted contra-piston doesn't need a O-ring to help seal it. You can see that with many of the diesel engines that have been made over the years. But if you use a Viton O-ring it wouldn't really hurt anything either.
The 1/2a engines were using O-rings on some of the different brand heads, mainly to help cut down on the time it takes to manufacture the heads in order to sell them for a better price.
The 1/2a engines were using O-rings on some of the different brand heads, mainly to help cut down on the time it takes to manufacture the heads in order to sell them for a better price.
09-29-2011 | 12:01 PM
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From: London, UNITED KINGDOM
RE: contra piston question?
yes and the fit would have to better in a 1/2a engine so an O ring makes it easier to seal without high precision so on a larger engine it would make less of a difference.
09-29-2011 | 02:16 PM
#5
RE: contra piston question?
I have made several conversion heads and they all worked very well. I simply made a blank head with the contra piston the same size as the original combustion chamber. I used a viton o-ring to seal it and a fine thread cap screw for adjustment. Davis fuel was the easiest to come by and it was a concentrate that I mixed with scented lamp oil. The smell was still so strong I gave up. In my opinion the fuel (kero, lamp oil etc) was worse to clean and keep on the outside of the model. Even after running all the fuel out and plugging the exhaust and carb the smell inside the van and garage were just too much. Good luck with the conversions.
Dennis
Dennis
09-29-2011 | 02:44 PM
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From: SydneyNew South wales, AUSTRALIA
RE: contra piston question?
ORIGINAL: flybyjohn
I have an itch to make a diesel head for a como 90, supertiger 61, or thunder tiger 46. I don’t know which one will be first but just want to try diesels out. If I get the 90 to run good, I will probably put it in my Corsair. I have looked at quite a few photos and plans for different heads and have recently seen the article in fly rc magazine. All the designs I have seen thus far have an o-ring seal on the contra piston. I have not come across a picture of the parts to the davis diesel head yet but they don’t seem to have any replacement o’rings on their site, so I assume they don’t have them in their design. Does the diesel conversion head need an o-ring seal or will a well fitted contra piston work just as well?
I have an itch to make a diesel head for a como 90, supertiger 61, or thunder tiger 46. I don’t know which one will be first but just want to try diesels out. If I get the 90 to run good, I will probably put it in my Corsair. I have looked at quite a few photos and plans for different heads and have recently seen the article in fly rc magazine. All the designs I have seen thus far have an o-ring seal on the contra piston. I have not come across a picture of the parts to the davis diesel head yet but they don’t seem to have any replacement o’rings on their site, so I assume they don’t have them in their design. Does the diesel conversion head need an o-ring seal or will a well fitted contra piston work just as well?
But you have another option open to you besides lapping a contra to fit or using viton O rings, and its the simple use of high temperature silicon tubing show here.
The flier who designed this head stated that he got a years worth of hard comp flying before replacing the tube, and replacement is very cheap and quickly done.
09-29-2011 | 05:53 PM
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From: Helena,
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RE: contra piston question?
I came across that design with the silicone tubing, but do not know where to get the larg silicone tubing from. I figure an hour or two on the lathe I should be able to get a real good fit with the conra but I never do bid my time well.
09-29-2011 | 09:49 PM
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From: SydneyNew South wales, AUSTRALIA
RE: contra piston question?
If you want to use a fully machined contra then consider this -
http://modelenginenews.org/weaver/We...tml#DCO_CONTRA
http://modelenginenews.org/weaver/We...tml#DCO_CONTRA
09-30-2011 | 05:33 AM
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From: Helena,
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RE: contra piston question?
Thanks Recycled Flyer, I did read that artical too. I want a concavical chamber instead of a flat top, but was going to incorporate the thin walled top part of that design into my design.
