Mavi91

I've just begun experimenting with making my own diesel heads/contra pistons. I'm looking for viton O-rings to use on the contra pistons. I can find any number of suppliers in North America who sell in lots of 50 or 100 in a given size. . .but that's a little over the top when you only need a few. I suppose I could substitute Buna-N o-rings, but I can't imagine that they would hold up very long on a contra-piston.

Any advice?

Jim Thomerson

I use the black O-rings from the local hardware store and they last just fine.

[email protected]

I purchased 10 viton O rings from Marco last year online.

Dzlstunter

I located some at a hydralic equipment place which sells, services and rents contruction equipment. Good luck, Dzl

chevy43

I bought mine from Mcmaster-carr....

DarZeelon

AJC - Andrew Coholic - 'ajcoholic' in this forum (and a person that does engine reviews for this web site) uses these Vitonâ„¢ O-rings in his Diesel conversion heads.

I believe he can disclose his source, or supply them himself...

wvarn1957

My Feedback: (3)

Viton "O" rings plus a lot of other "good" stuff for modelers

http://www.smallparts.com/

ajcoholic

My Feedback: (10)

Viton O rings are available through most decent industrial supply stores. I live in a small town of 6500 people and even the supplier here has them.

AJC

PS I have LOT of them in stock in about 5 sizes I use in my heads. If anyone cant find them PM me and I will sell you some cheap (I think I pay about 30 to 60 cents per ring depending upon size). But I generally use much heavier rings - in anything .40 and up I use a pair of 1/8" thickness rings which of course requires a slightly deeper contra, but the sealing properties and longevity seem to be much improved.

AJC

Mavi91

I always thought that double o-rings would be a good idea if for no other reason to keep the contra perfectly lined up in the bore. How do you space them? I realize the actual dimensions would depend on the depth of the contra. On the one contra I've made so far, I put the ring about 1-1/2 ring thicknesses above the bottom. I'd be tempted to put the second ring about the same distance down from the top.

Come to think of it, I just guessed as to how deep the ring groove should be. I sized it so that the ring stood proud just enough that there was room for it to squish flat into the groove when pressed into the bore. The contra worked & has stood up to nearly 3 flying seasons of use. But I'd like to know if what I did is good practice.

George

ajcoholic

My Feedback: (10)

Generally (after much experimentation and testing with numerous methods) I have settled on two rings side by side in a wide groove. Since on anything larger than say a .25 size engine I use 1/8 thick rings, two of them side by side already spaces the rings wide enough to have a very stable contra in the bore. If I were using thinner rings like I used to I would leave a ring thickness in between grooves.

As for depth? I find the rings slightly oversize - so I cut the groove 1/8 deep and find the ring needs to be compressed for the contra to be instered into the cavity (I usually bore the contra cavity to a 1/2 thou to a thou larger than the contra diameter.

AJC

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AJ, very interesting info. I have a couple Drone BB diesels with Aerodyne variable compression heads. I have had a couple O ring failures single 1/16" thick and 9/16" OD. The contra piston is about 1/4 inch high with the grove about 1/16" from the top. Would you suggest going to two 1/16" O rings or maybe one 1/8" O ring? Jack Hiner

ajcoholic

My Feedback: (10)

On my smaller heads I use two 1/16 thickness rings side by side as well, in a single groove. A single ring will work but allows the contra to slightly rock (depending upon how much clearance of course you have) whereas two do not.

I just think two rings are better than one in all cases except the fact the height of the contra cannot be as small. But in my own heads, even with twin O rings the conversion heads are physically no taller than the glow heads they replace.



AJC

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Thanks AJ. One more question what material do you use for the contra piston? Mild steel? Jack

ajcoholic

My Feedback: (10)

Nope, I use 6061-T6 (the most common "aircraft" aluminum alloy I think) for the heads and contra's.

AJC

Mavi91

I've had good luck so far using aluminum billets I cast in my backyard from old lawnmower engine pistons & rods. This material seems to machine well. I did run into a slight porosity problem with one of the larger forms I made when I didn't get the melt hot enough, although not bad enough to prohibit an attempt to make a head. I ended up messing up the drilling of the head bolt holes, but that was my fault--I'm sure the metal would have been OK.

George

ajcoholic

My Feedback: (10)

George,
I also do castings (mainly antique engine parts) and have wanted to try and machine a diesel conversion head from a casting instead of bar stock. The fins would be so much easier to just cast instead of nmachine.

AJC

Mavi91

AJ--

I've been using the lost foam technique to get my basic billets. I tried making a custom header this way but wasn't able to get the thin cross sections to fill. I was tempted to get some investment plaster & use the lost wax technique (I used to make cast jewelry) but that would mean I'd have to make a burn out furnace. I've cast things like small aspen leaves and insects (legs & all) using the burn out + centrifuge so I know things like head fins would be easy. I haven't cast aluminum this way, however, & wonder if there would be problems with shrinkage--silver & gold hold their dimensions very well & you can cast them with high accuracy.

Have you been making your parts by sand casting?

George

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AJ, thanks for the info on contra pistons and O rings. Jack Hiner

ajcoholic

My Feedback: (10)

Yes, I use petrobond sand (oil bonded). I can do some pretty fine stuff relatively speaking - engine heads are generally no problem.

I have made a lot of heads, maybe a few hundred. I have learned a lot from the experience and am willing to share what I know with anyone else.

AJC

Mavi91

AJ-

OK, here's something I'd like to know: how do you line up the head bolt holes when making a conversion head? As I noted above, I made a head for an OS .15 fp without a problem but then messed up the head bolt holes. I figured for my next attempt that I'd make a stud threaded 10-32 on one end (to match the contra screw on the diesel head) and 1/4-32 on the other end to match the plug threads on the original glow head. That would allow me to clamp the two heads together and then use the holes on the glow head as a drilling guide (on the drill press).

Thanks,

George

ajcoholic

My Feedback: (10)

George,
I use a rotory table set up on my milling machine. There is really no other way to get the holes perfect. But of course that requires the use of a mill/drill and rotory table.

AJC

Mavi91

AJ-

Well, looks like I'm stuck with my proposed approach as my milling equipment consists of an attachment I made for my mini-lathe--no indexing capability. But, I don't think I need absolutely exact hole location since the engines I want to convert have a head design that spigots into the sleeve. So it seems like if I get a very close fit head-to-sleeve and drill the holes in the head a little oversize, I might get away with it.

After over 50 years of adjusting needle valves, I finally managed to stick my left index finger knuckle into the prop this past weekend, so I have to wait a bit to heal before I can get back to my lathe. I'll report the results when I do

Thanks,

George

AndyW

My Feedback: (1)

George,

What I do is to use the original head as a guide as you have done but with a difference that might help. Both heads will have the bit that fits into the cylinder. Just turn down a sleeve that both will fit into snugly. Then rig this up square in your drill press and drill away. Perfectly spaced holes with no oversize required.

Mavi91

Andy,

Thanks for the suggestion--I think that will be easier for me than my proposed method. Now if my hand will hurry up and heal!

George