Steel Epoxy versus Aluminum Epoxy for Crankcase Filling/Porting

[JC-SuperJet]

So in a recent thread discussion about epoxy filling, some Members used steel epoxy (JB Weld) while some recommended using aluminum epoxy like Devcon.

Seems like having like/similar metals would make sense, but does this make thermal expansion sense? As in the epoxy filling/patch expanding and contracting in harmony with the aluminum casting...

Lets start with the fact that the Aluminum coefficient of thermal expansion (CTE) is about 24 x 10 to the -6 power per degree; I will just use 24 to keep things simple. So we want the epoxy filling/patch to have a CTE as close to 24 as possible, that way it expands and contracts in synch with the Aluminum casting as much as possible thereby reducing cracking and flaking.

Steel has a CTE of about 12 and Epoxy Resin has a CTE of about 50. So the CTE of the Steel Epoxy Resin mixture will end up with a CTE somewhere lower than 50, but higher than 12, because the metal will help to lower the CTE and make it more stable.

Then, Aluminum plus Epoxy will end up with a CTE somewhere between 24 and 50. And Steel plus Epoxy will be somewhere between 12 and 50.

Assuming the same amount of metal is added to each epoxy mixture, which mixture will have the lowest CTE closest to the Aluminum Casting?

The Steel Epoxy mixture will have the lower CTE. So JB Weld Steel will perform better than Devcon because it has a lower CTE, thus more stability and strength. And JB Weld is so much cheaper.

What say you?

 

[WFO Speedracer]

I say my head hurts right now, going to find Aspirin .

 

[Jr.]

Cases dont get hot enough for your theories to matter, All heat generated is high up on the Cyl.
Epoxy failures happen due to poor prep of the voids that is being filled. Cast alum will act like a sponge and become oil impregnated.
If cases are not thoroughly cleaned, and etched. They will have a shortened lifespan.
All Top builders use Devcon 10710

P

 

[kevbo]

Look for an aluminum filled epoxy rated up to 250F. Most aviation epoxy will work with some good to 350F. Blast with aluminum oxide for best adhesion.

 

[waxhead]

I use jb weld. as longg as you sand blast before to rough up the surface it works really well, if you have the time you can read through here and see it https://www.waxracingproducts.com/blogs/news/1100-waveraider-performance-modification

 

[WFO Speedracer]

When this thread is done it needs to be merged with the other thread in the tech section

 

[WFO Speedracer]

I am going to say I have seen some seriously crazy stuff done with JB Weld, one of the craziest was on a carb for a racing go cart, the guy took tubes of JB Weld mixed it up and built up the outside of the carb up with it , then they bored the Aluminum out of the inside and installed a bigger throttle plate , it looked like ass but it smoked everyone elses nice shiny high dollar carbs.

 

[JC-SuperJet]

"J-B Weld™ has a tensile strength of 5020 PSI and sets to a hard bond overnight. It can withstand temperatures up to 550ºF when fully cured."

versus

"DEVCON Aluminum Liquid (F-2) Adhesive Tensile Shear 2,700 psi and Temperature Resistance Wet: 120°F, Dry: 250°F"

JB Weld is twice as strong and can handle twice the heat at a very low price.

There is no Corrosion issue with the Steel particles in the JB Weld reacting with the Aluminum Case. Engine already have a Cast Iron Sleeve in contact with the Aluminum Cylinder Block.

Crankcases do get warm during sustained operation such as a Hot 12-Lap Moto around the Buoy Course. In the Texas Summers, I can't even keep my hand on my STOCK Engine Flywheel Cover.

Maybe that's why some Dirt Bikes have external Cooling Fans for the Cases.

Posters have said they favor Aluminum Epoxy but not WHY it's better than JB Weld...

 

[kevbo]

The aluminum is just the filler. Its a great material for this application. Cheap epoxy turns soft when the temps get up to 150F. Carefully consider the max temps. Sometimes an expensive higher temp epoxy is required.

 

[JC-SuperJet]

JB Weld would be wide open to expensive Liability Lawsuits if they misrepresented the Properties of their Epoxy Resins. I understand if it doesn't meet Aviation Standards.

I think the choice is really just one of convenience. Devcon Aluminum Liquid is easy to work with, as it pours well and easily fills voids and crevices.

 

[Pro-pulsion]

Yeah the bearing journals get hot from the crank bearings, if the bearings are getting worn out too then things get quite hot. I burnt my hand on the coupler of my old GP760 because the crank bearings were done and put that much heat out. The airspace where the fuel transfer is below the crank webs isn't very thick though and heat dissipates pretty efficiently there. Plus the fuel charge helps with that. If anything adding the epoxy will somewhat insulate the base and encourage a little bit of heat retention but it's still not going to be more than what the epoxy can withstand. You'll probably also have some water splashing around below the cases adding to the cooling so I wouldn't over think it. I would just go for what offers the higher heat resistance, as mentioned above it's the prep to keep it in place that matters most. I've seen some rather disgusting epoxy jobs lol...but they still held up. It took a little bit of work to pop them off because they didn't do a good prep job and the cases acted more like moulds then a bonding surface lol.

 

[bird]

You're trying to be scientific, yet you're using your hands touch as a measuring device?

I used normal JB weld, not the high temp, on the exhaust port of my 1976 15hp Johnson to match the tuner pipe from later years(mid 80s). It absolutely rips, haven't had any issues with it after days of fishing and river running in Canada. It keeps up with a 2003 15hp Mercury. That JB Weld is right at the exhaust port, where temps exceed 400 degrees easily.

 

[WFO Speedracer]

The whole idea of these forums is to collect and put the info out there , Scientific , seat of the pants , word of mouth or felt while your fingers were burnt on a couple doesn't really matter it all fits together so you can form a hypothesis , the more info you have the more of an informed decision you can make for yourself .

 

[waxhead]

If your crankcases are hot, that means there’s friction somewhere. They’re filled with atomised air rushing through, which naturally helps cool and balance temperature — so they should actually stay cold. Anyone remember carb icing? That same principle applies here. The “finger test” followed by an “oh licorice” definitely counts as a valid measurement tool in this situation, haha.

 

[Jcary85]

what I'm about to say is purely anecdotal - just my experience so far in the engine business...

I've fixed a bunch of cases from motors that I didn't build in the last year that used devcon. The devcon always appeared to have shrunk on every repair I've done - particularly evident in the area behind the stator. Devcon looks amazing - hell, sometimes its hard to tell where the aluminum begins and ends. it also levels out beautifully and pours nicely out of the box. JB weld is more difficult to work with as you have to get it out of the tubes, warm it so it flows (but not too warm or you'll kick start the exothermic reaction) and it doesn't level out as nicely, regardless of how warm it is. However, I believe, mostly from observing results post-cure (I've used both JB weld and devcon on cases btw), that the JB weld does has better adhesion.

All that said, time will tell - epoxy will eventually fail - its usually a question of how many years it will last.

Some other info: I follow a lot of 2 stroke stuff pretty closely these days outside of jet skis. It seems its a 50/50 shot if a builder is going to use devcon or JB weld but I rarely hear of anything else being used. I really think either is fine, with JB weld having a slight advantage. Its also considerably cheaper than devcon.

 

[Jcary85]

oh and just like wax said, a nice rough sandblast before epoxy is really the key to making it last (every set of cases I do is sand blasted and cleaned first)