Another Radial - this time 18 Cylinders

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Wow, more amazing craftsmanship & attention to detail. The finish line is in sight!

What effect do you think the big difference in thermal conductivity & expansion between the aluminum heads & stainless steel washers will have? Bottom line the plug runs cooler? Is that a good thing or a bad thing? You think there's any danger of the epoxy cracking & leaking down the road due to the differing expansion rates?

Definitely not trying to be critical; it just worries me. I'm no engineer though so ignore this if I'm being overly paranoid.
 
Those are all really good questions, and I have some of the same concerns. JB Weld says their product can take 550F-600F intermittently (10 minutes), and 500F continuously; and I'm hoping they mean it can also handle the accompanying expansion. My application, though, will include a clamping force of around 1000 lbs putting the epoxy in 360 psi compression when the plug is tightened, and I can see both good and bad points associated with that. I'm curing the epoxy under about 100 psi compression. The epoxy isn't inside the combustion chamber, and so I don't believe it will ever see even 300F. The prop on these big radials does a great job in keeping the heads cool. When I measured the head temperatures on my nine cylinder I don't believe I saw anything over 200F. The exhaust gas temperature might hit 300F-350F, and I'm a bit more concerned about the 450F solder I used on the intake/exhaust flange. Of course, besides the temperature measurements I made, my reasoning there was that if that intake flange ever reaches 450F I'm not going to get any fuel into the engine anyway. As far as affecting the plug temperature goes, there might be an effect, but it's the temperature of the center electrode that's most important and there is a pretty high thermal resistance between it and the shell of the plug. Also, JB Weld gets its 4000 psi tensile strength from powdered metal filler, and so its thermal resistance is a bit lower than other epoxies.
Instead of the epoxy I could use an automotive paper gasket under the disk, but that sounded like a lot of hassle when taking the lower plugs in and out as is constantly done on a radial. I'm in the process of curing only two more heads, and so it isn't too late to back away from the epoxy if others have had a bad experience doing something similar.
 
Have you tried making your own annealed copper washers?

Charles,
As a matter of fact I did make up several and tried them. I was concerned about running copper against the aluminum head and the corrosion I might get as a result. Because of the oil and fuel that would inevitably end up between them I figured Murphy would find a way to use the mixture as an electrolyte; and with the heat involved, galvanic corrosion seemed like a real possibility. So I switched to aluminum. I made up a few dozen aluminum washers and annealed them similarly to my head gaskets. But these had to be .070" thick to properly set the plug depth which, as it turned out was too thick to conform and flow into the surface defect. I started thinking about using two separate washers - one thick and one thin - but since that doubled the number of seals I had to deal with, and so I started down this epoxied disk road. It's not too late to re-consider, though. -Terry
 
Charles,
I wanted to add one other thing to my reply to your question. I initially tried a thin .005" soft aluminum washer under the compression gasket of the plug. This was before I discovered my gouge. My reasoning was that if there was some defect in the surface of my head causing the leak, the soft aluminum would flow into it and seal it. What I saw was that the contact patch of the plugs' compression gasket pressed down into the washer and caused its outer periphery to lift and there was no effect on the leak. In hindsight, the gouge probably extended out past where the aluminum washer lifted, and the gouge was too deep to be filled this way. This was when I went to the thicker washer, and ran into yet a different limitation. -Terry
 
I think the lay of the cutting marks is the problem here. If it was circular leakage should not be a problem. The radial lay allows a direct path across the sealing surface, even a soft O-ring won't seal properly. I would guess a plug seat milled using circular interpolation would be acceptable if a bullnose endmill was used along with proper feed rate. I use bullnose (also called corner radius) end mills almost exclusively and surface finish in aluminum is almost mirror smooth. You can be certain that in production plug holes and seats are finished with step drill/counterbore type tools.

I don't understand the comment about the gouge. I don't see it in the image.

Copper is used for RC glow plug washers. It's extremely rare to see corrosion that could be attributed to the copper washer. Besides, if galvanic corrosion was a concern do you worry about the copper bearing alloys in the rod bushes, valve cages, etc?

Thank you for providing the in depth details of this excellent build.

Greg
 
Greg,
Thanks for the comments. Gouge isn't a good term for the chamfered edge that I think is the problem. It's not real obvious, but in the image without the disk it extends radially outward a bit further than it should and ends up kissing the compression washer contact point on both plug styles.
You have a totally reasonable comment about the machining marks. I, too, wondered if they could have been the problem, but I could not feel anything with my finger or by dragging a toothpick across the surface. I have noticed, though, after after a plug has been tightened down against an aluminum head a few times that a mark is usually left behind by the compression washer that I can feel with my finger. If a plug's compression washer isn't capable of sealing against a mark at least that shallow, it would seem that once a head has seen one manufacturer's brand of plug it would forever be tied to that particular brand if not that exact same plug.
By the way, I'm currently setting up an experiment to test the heat cycling capability of JBWeld in this application. I've drilled and tapped an aluminum plate and JB Welded three disks to it that are held in place with some (now scrap) Rcxel plugs. I plan to heat cycle the plate a dozen times or so with a torch and see if the epoxy bond shows any sign of deteriorating. I'm also going to cut out some gaskets, just in case. - Terry
 
I don't understand why a soft washer would be "too thick to conform". Surely the thicker it is (within reason) the more easily it will deform to accommodate its mating surface, since the percentage deformation will be less?
 
It looks like you've found an inherent problem with tiny spark plugs and their threads being coarser in relation to their diameter. Too bad they aren't made with taper seating or a bigger flange & gasket to get outside of that thread start.

It would seem to me that if a 1/4-32 glow plug seals OK with a soft copper washer, that method should work here too. I forget, what size are your plugs?
 
The NGK CM-6 plugs have M10x1 threads. They are the most common plug for RC gas engines. I'm surprised about sealing concerns with the RCEXL plugs as many people use them and I don't think there have been any issues. These plugs are unique in that they are very small, nothing else really comes close, and there are no taper seat plugs this small. You cam find 10mm taper seat plugs, but they have much deepr reacha nd overall length.
 
Charles,
You're exactly right. It's been the width of my shop-made rings that prevents them from sealing. Their thickness is .070" but their radial width is .090". I chose this width when I made them to match the width of the compression washers on the commercial plugs I have. The difference is that mine are flat, and the commercial versions are not so they can apply more pressure. The NGK's have a narrow ridge, and the Rcxel plugs have a broad contoured face. If I machined the face of my washers to have a ridge that would probably solve the problem on that side. I would still have to do something about the seal on the plug side. I can't just make narrow washers with a smaller i.d. since the seal has to occur somewhat away from the threads, but yet the washer will need to be centered on them. Your question does give me an idea for another option, though.

Dickiebird,
The plugs I'm using are 10 mm CM6's. The major o.d. of the threaded body is about .390".

Last night, I completed the assembly and testing of he two worst-case looking heads onto which I JB-Welded the two stainless repair disks. The results were perfect at 0.2 psi/sec leak down which closely agreed with the Mity-Vac data. I'm not doing anymore repairs that way, though, until I see the results of my heat cycling tests on the JB Weld as well as some more thinking. One Rcxel plug sealed, but not as well, and two would not seal. I just don't understand my experience with the Rcxel plugs. I've done a Google search and can find no one else with any complaints about them. Maybe it's a lot problem with the batch I have, since they have all come from the same box. Terry
 
I just don't understand my experience with the Rcxel plugs. I've done a Google search and can find no one else with any complaints about them.
Most likely, nobody else tested them for leaking and just assumed they were sealing.
 
Most likely, nobody else tested them for leaking and just assumed they were sealing.

You could be right. After what I've seen the past several days I could envision a scenario where someone finishes up a new engine and has a really difficult time getting it to start while they listen to the hiss from a leaky plug they mistake for the sound of a healthy intake. Then, to add insult to injury they make a compression test using a gauge that actually does seal to the head, and they end up with a great compression test that sends them off looking for ignition and/or carburetor problems. A simple test putting a few drops of soapy water at the base of the plug of a new engine is going to become S.O.P. for me. - Terry
 
Hi Terry,
I know that you are using commercial plugs but I have found that we as engine builders try to replicate plug proportions when making them, for that matter even Rimfire plugs have a small seat area relative to the thread O.D. I have had similar problems to what you have described. I guess the cure would be to increase the body diameter to make sure that the seating are was well out of the way of the thread chamfer or thread start. I use copper washers that I punch out of a sheet of copper then anneal them. Even at that I get the occasional leak. The plugs are so small that torquing them down is not an option lest the body break away from the threads. I'm tempted to try a tapered seat on my next engine build.
gbritnell
 
Hi Terry,
I use .020
gbritnell
 
So, after verifying my leak-fix on three assemblies, I was ready to apply it to the rest of my heads. JB Weld's temperature spec of 500F continuous and up to 600F intermittent sounded plenty adequate for my needs. Then, Dickiebird asked if I had considered a possible issue with temperature cycling the epoxy. After some more thought and web research, I could imagine JB Weld's loosely worded high temperature spec might not actually include an ability to be temperature cycled. And, I ran across lots of stories of users' repairs gone bad that were blamed on temperature cycling.
I decided to run my own test before repairing any more of my heads. I made a test bed by drilling and tapping a 3/8" aluminum plate for five spark plugs. Stainless washers were epoxied into three of the positions and temporarily secured with spark plugs tightened with a clamping force of about 100 psi while they cured just as was done on the first three head assemblies. For comparison, I also threaded in two additional plugs with stainless washers sealed against the plate using gaskets I made from commonly available 1/64th inch thick brown automotive gasket material. In fact, it's the same material I'm using for my exhaust flange gaskets. I decided, at the last minute to include these gaskets in my testing as another potential solution. After considering some of the other readers' comments; and, depending upon my test results, I might find myself machining a batch of ridged compression washers. Finally, I sprayed portions of the plate with a Rustoleum 500F high heat paint that I also want to test. The black painted surfaces will also provide nice targets for my non-contact thermometer.
As a control, I JB Welded a similarly cured sixth washer to a separate plate that will not be heated. This control might be used later to quantitatively compare the forces needed to push a cycled vs. non-cycled epoxied joint to failure.
After a 15 hour cure and with the plugs torqued to 75 in.-lbs., I used a torch to heat the back side the plate for 2-3 minutes until the target test temperature was reached and then held for about 30 seconds on the washer side of the plate. After 45 minutes or so when the plate had cooled back to room temperature, I repeated the cycle using the same test temperature. After the fifth cycle I un-torqued and re-torqued all the plugs a half dozen times each before removing them and attempting to dislodge the three epoxied washers from the plate by pressing hard against their edges with a wood block. After the tenth heat cycle, I repeated the torquing tests and then I also baked the entire plate of re-torqued plugs in an oven for three hours at the test temperature. The purpose of this constant temperature bake was to accelerate the aging of the resin. I performed the same ten cycle heat, torque/un-torque, and aging bake for test temperatures of 300F, 350F, and 400F.
I really don't expect the JB-Weld to even come close to 300F on my radial because of the huge finned heads and massive prop wash. The reason for the higher test temperatures is to accelerate the failure mechanisms created by temperature cycling the epoxy and to force the failures to occur as early as possible during testing. Also, it's an opportunity to learn more about JB Weld for a possible future application.
At the end of the 300F tests, I began having doubts about a gasket solution. Both gaskets had become stuck fast to their washers which was a good. But, the side of the gasket against the smooth aluminum plate did not stick; and the material had become very hard and its surface glazed from the heat. These gaskets would probably continue to seal until the plugs were removed the first time. But, it would be difficult to re-install them with the washers in their original orientations; and the gasket material no longer seemed to have the compliance needed to re-seal the surfaces against combustion pressures. I've dealt with this on old automobile engines. Some of their irregularly shaped gaskets, if not damaged during disassembly, can sometimes be returned to their original positions and re-used but only in low pressure sealing applications.
After successfully passing the 400F tests (total accumulated 30 temperature cycles, 36 torque/un-torque cycles, and 9 hours total bake time) I decided it was safe to continue on with the JB-Weld repair of my heads.
As one of the photos shows, the gaskets had by this time completely deteriorated. Since none of the epoxy bonds had yet failed, I continued on with the 450F test. At a 450F head temperature the Loctite 620 sealing my valve cages would decompose and the soft solder used on the exhaust flange would melt, and so further testing was just more for my curiosity.
I modified the 450F test slightly by leaving out one of the spark plugs to see if the clamping pressure had been playing a major role in maintaining the bonds on either side of the washer. I also stopped the gasket testing since there wasn't anything left to test. After five 450F heat cycles, all three washers were still holding tight; and so I baked the test plate containing two torqued spark plugs and one open washer at 500F for three hours. At the conclusion of that test the washers were still being held in place by the JB Weld, but using the same moderate force I was able to dislodge all three of them from the plate. Inspection of the washers showed it was the bond between the epoxy and stainless that released.
In any event, I came away from this little diversion with a healthy respect for JB Weld. One important manufacturer's application tip I ran across in my research is that cleaning the surfaces to be bonded with acetone or lacquer thinner is recommended, but use the use of alcohol is specifically discouraged. I previously assumed that JB Weld became hard and brittle when cured. But after recently mixing several batches of the stuff and monitoring the leftover I discovered that there is always a small degree of compliance left in the material that probably accounts for its ability to be temperature cycled. - Terry

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Hi Terry,
While thoroughly enjoying your build the thing I like most about it is your in-depth experimenting with different materials and processes. Although I have the machining part down I can't tell you how much I have learned from this thread.
Thanks so much,
gbritnell
 
...In any event, I came away from this little diversion with a healthy respect for JB Weld. Terry

Well, those tests were very enlightening. Thanks for the effort & documenting real world results. I would have placed a small wager the washers would have dropped off in a sticky goo. But I'm glad you found a potential solution & now have the confidence & knowledge to proceed.

I made a few carbon fiber composite tuned pipes for RC engines back in the day, playing with different shapes & baffle configurations. The resins have changed over the years but it was basically this kind of 600F stuff until cost & hazmat shipping made it silly expensive for me.
http://www.cotronics.com/vo/cotr/ea_ultratemp.htm

I recall (and experienced) an important pre-requisite of these HT resins successfully achieving their spec T is following the temp ramp-up & soak schedule to the letter. Nominally it was about 40-60% of ultimate temp, but it varied by formulation. That's why I'm amazed that an ambient cure formulation like JB achieved those temps. However I also think JB-like filled adhesives are a different animal than laminating resins used in composite parts, specifically resin concentration. A typical layup, even vacuum bagged, is hard to achieve much better than 40/60 resin/matrix proportion. But possibly they can pack more metal/ceramic filler in a glue/putty form. The way I visualize it is, you can have 10,000 deg Kryptonite powder, but if its suspended in a matrix of 400 degF resin binder, basically things will start to flow at that minimum, critical temp. Typically I think resins expand a bit, so when joints are in compression & then cool, maybe its not as bad as tension or flexural?

Anyway, blah-blah (composites gives me a nerd-on). I guess the only thing I could offer is:
a) try a moderately elevated temp pre-cure soak of the cured resin & see if that improves JB results even more
b) If you are feeling the need for a bit more safety factor, I suspect there are other industrial strength versions out there, for ex
http://www.cotronics.com/vo/cotr/rm_adhesive.htm
c) give it the representative 'oily' test of engine conditions. Some epoxies can start to degrade in the presence of light ends like gasoline, oils etc.
d) I have not heard that particular no-alcohol rule before, but I'd agree. For epoxies I really prefer acetone anyway. It cleans oils & waxes aggressively & flashes almost instantly. Same for LT although I cant stand the smell anymore. Alcohols come in too many variants, % grades by water content, some I suspect with extra additives packaged as 'rubbing' alcohol. I could see that being undesirable.

But yeah, for an adhesive you can get at the hardware store, JB has moved up my personal glue ranking! :)
 
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Peter,
Thanks for the Cotronics website. I looked through it pretty closely, and they have a lot of really neat products. Their RK454, in fact, sounds a lot like JB Weld. Some of their repair products are the kind that can get you out of those corners that we sometimes paint ourselves into.
A lot of epoxies are exothermic, and so trying to add heat during their cure cycle can be counter-productive. The JB Weld people don't recommend adding heat until after the first six hours of room temperature curing time. I've dealt with some high temperature ceramic paints, though, that had to be cured with a high temp cycle before they could be taken to their full service temperature. -Terry
 

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