Oh $hit!!

[BrettS]

I'd use a dime's worth of poxy and call it "good" for the next 3-4 lifetimes.

Or the oil in the magnesium causes the epoxy to fall off, or the oil weeps from the repair.
I tried fixing the case of a B&S engine with epoxy, while the epoxy staunched the flow the oil is still weeping.

 

[HansFranz]

If you degrease it right, it shouldn't leak.

I've seen marine V8s with cracked engine blocks (water jackets cracked from freezing) that were repaired with epoxy and they held the pressure and ran for years and years until the engines wore out...and cast iron is famous for getting crapola embedded in it...a lot of CI also has graphite in it, and I don't think anything will adhere to graphite for very long...

(Not that a little bar oil weeping should bother anyone, since most saws leak oil like MOPARs...)

 

[BrettS]

If you degrease it right, it shouldn't leak.

I've seen marine V8s with cracked engine blocks (water jackets cracked from freezing) that were repaired with epoxy and they held the pressure and ran for years and years until the engines wore out...and cast iron is famous for getting crapola embedded in it...a lot of CI also has graphite in it, and I don't think anything will adhere to graphite for very long...

(Not that a little bar oil weeping should bother anyone, since most saws leak oil like MOPARs...)

Went the whole nine yards with it, still leaks....
My fathers boat, the engine in that has a crack in the water jacket in the block, I don't know how but it doesn't leak.

 

[Brufab]

Is the oil tank pressurized while in use?

 

[BrettS]

Is the oil tank pressurized while in use?

Vacuum I would have thought.

 

[Brufab]

My old saws use crankcase pressure to pressurize the oil tank for automatic oiling, I'm not sure how modern saws work.

 

[djg james]

We'll see tomorrow if the repair worked.

 

[Chevboy0167]

This stuff sounds like it would cost more than a used case half but still.......


https://www.epoxyworks.com/index.php/a-magnesium-crankcase-repair/
https://www.masterbond.com/applications/bonding-magnesium-substrates

 

[HansFranz]

We'll see tomorrow if the repair worked.

99 times out of 100, if it doesn't work, it will probably be due to incomplete degreasing.

One other point that I haven't mentioned is the importance of initial "wet out" of the interface between metal and epoxy. I don't think I've ever used JB Weld, but if its consistency is more like clay than like water, then you would probably help your case by starting out with a watery consistency epoxy right on the metal, then letting that start to set up, then, while the first layer is still soft enough to dig your fingernail into it, apply the thickened JB Weld on top of that. According to the guys at West System that I've talked to, pretty much all epoxies from all manufacturers are chemically compatible (this was a surprise to me), and if each resin-hardener combo is mixed properly, a layer of Acme Epoxy under a layer of Brand X epoxy should all bond together chemically if stacked up while still green. West says you can thin their epoxies by up to (IIRC) 15% by volume with acetone if you want to get a really thin initial layer for that all-important interface between metal and epoxy...

As an illustration of the importance of this interface, another thing the West guys once told me to do for a really critical bond to aluminum I was attempting on an aluminum boat once, was to wet the (oxidized, weathered) aluminum hull with unthickened epoxy, then use sandpaper to abrade the aluminum hull through the wet epoxy so that the epoxy would immediately contact fresh, unoxidized aluminum (aluminum oxidizes into aluminum oxide almost immediately upon contact with air) and then stack up my composite on top of that. (Otherwise your bond is really between epoxy and aluminum oxide and then between aluminum oxide and aluminum...and although the epoxy might bond to the aluminum oxide just fine, the bond between aluminum oxide and aluminum might not be as strong.) But I really don't think you need to go to that much mess and trouble here...(I didn't bother with this extra step on my aluminum boat repair, and the repair is still fine 15 years later)...

Last note and then I'll shut up: In almost all composites, the inclusion of a reinforcing fiber is very important. It can be fiberglass, carbon fiber, polyester fabric, cotton gauze or about a million other materials (there are a few exceptions that epoxy will not bond to, though -- such as Spectra fiber and Amsteel/Dyneema which are all forms of polyethylene -- a wax -- so check first). This reinforcing fiber does the same thing that rebar and screen does in poured concrete -- it makes it somewhat flexible, and greatly increases its tensile strength. So as a matter of course, if I were to attempt a similar repair on a chainsaw (or V8 or whatever, even if it were just using epoxy to provide a durable coating for the bed of my pickup truck -- again, the West guys were the ones who tipped me off to this) I would always include a reinforcing fiber. Once it's thoroughly wet-out with unthickened epoxy, nearly all fabrics become essentially transparent and invisible...in the case of fiberglass cloth, you basically can't see it anymore once you wet it out with epoxy, and all you see is what's underneath (many of the beautiful wood-strip kayaks you see have a layer of FG over the wood, but it just looks like a clear finish)...then, after it hardens up, you can sand and paint as normal and it won't look any different. Some guys apply a water-thin layer of thinned epoxy over the sanded FG composite after sanding to "seal it all up" -- especially in the case of wood-strip boats -- but it's not necessary in all cases...mainly cosmetic.

 

[BrettS]

According to the guys at West System that I've talked to,

They make some pretty good boat products and have been around for years so they know what they are talking about.

 

[HansFranz]

They make some pretty good boat products and have been around for years so they know what they are talking about.

Yeah, I've been using their products for over 25 years, but what I like best about them is their expertise. They do a LOT of engineering testing and R&D and report their findings in "Epoxy Works" magazine (available online) and every time I've called them with some harebrained question or idea, their tech support folks have been enormously helpful to me. (And not just to sell me their latest whiz-bang product...many of their products are just prepared formulations of fairly common materials that are marketed to yacht owners who have more money than time, expertise or materials, and what you're really paying for is convenience -- but you can often duplicate their products using materials you already have...and they'll tell you how...) I have no financial connection to them, but I can't say enough good about this outfit.

You can get a good guide to using epoxy here:

https://www.westsystem.com/instruction-manuals/user-manual-product-guide/
It's mainly tailored to boat building and repair, but it also explains a lot of the key concepts of using epoxies from any manufacturer in any application, so it's a good primer for anyone using epoxy.

 

[ericm979]

JB weld is just runny enough that if you're trying to build something that has some height to it and needs defined edges, you'll need to make a bit of a form. There's also a stiffer clay like version that comes as two bars of material instead of two tubes of liquid. I'd use the the original two tubes version for this patch. You want some flow into the crack.

When patching cracked engine cases, there's no need to make a composite using fibers- the patch is holding in the oil or keeping air out but not providing stiffness. It's only if you're patching a significant hole that you'd need fiber. And then the results get a lot less certain.

Failure of a simple crack repair like this would be due to poor preparation.

 

[HansFranz]

When patching cracked engine cases, there's no need to make a composite using fibers- the patch is holding in the oil or keeping air out but not providing stiffness. It's only if you're patching a significant hole that you'd need fiber.

Fiber just ties the patch together better, and gives the epoxy something to soak into...that's why, when I asked the West guys about how to coat the bed of my new truck with epoxy to help protect the paint, etc., they recommended I cut a 4' x 8' piece of some kind of fiber to help soak up and hold the epoxy and resist flaking off or being scraped off by stuff I drag into the bed. I was just going to roll/brush the epoxy on like paint. (They also recommended adding aluminum powder to the epoxy to block UV...epoxy doesn't hold up to UV and needs paint or some other UV blocker to stand up outdoors.)

Fiber isn't necessary, but it wouldn't hurt and might help. To each their own, but I always use it.

 

[ericm979]

We're talking a thin 1" diameter patch here, not a truck bed or a boat. Different application.

 

[djg james]

Update:

The patch seems to be holding. Cut a load of low grade Cherry the other day for BBQ wood and no problems. Thanks everyone.