Heat transfer characteristics of JB weld

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Staff member
Dec 14, 2002
New Jersey
Hi, Per my post in the general OT forum, I just had a diff rebuilt, and in shipping it was dropped. This caused one of the cooling fins, that BMW puts on the diff cover (nothing special, no fancy cover, etc) to bend and break off. Its clear that the diff landed right on the fin, causing the breakage. Anyway, I was thinking that at least as an interrim solution, since all else appears fine, that I JB weld the fin back on. The metal BMW makes the cover from looks like a cheap base metal, based upon the grains seen at the breakage point. Its not aluminum or anything special. So my question is, how are the heat transfer characteristics of JB weld, and would it make sense for me to JB weld the fin onto the diff (leaving it bent, Im not going to try and fix that)? Would the JB weld make it more or less like original, or does this stuff inherently impede thermal transfer? It looks metallic. The fin is a single in terms of placement, only one fin in the particular area where it was located. It was also relatively long, likely to be able to move as much heat as possible from that spot. How annoying... But how will JB weld do? Thanks! JMH
Maybe you could JB weld most of the seam but drip some solder in a couple of spots? Will aluminum take brazing? (can do that with a propane torch) They have a "goo" for computers that goes between heat sinks and chips. Maybe put some goo in one corner and JB weld the rest. It's called "heat sink compound" or somesuch. You can verify how well this works with one of those infared laser pointed remote thermometers after a long drive.
JB Weld is primarily epoxy. 10% or less steel particles. See MSDSs: http://jbweld.net/techinfo/JB%20Weld-Epoxy%20Steel%20Resin-Overnight-48009.pdf http://jbweld.net/techinfo/JB%20Weld-Epoxy%20Steel%20Hardener-Overnight-48008.pdf And they also state it is an electrical insulator. So heat transfer is probably slim to none. So JBwelding it back on would be same as w/o it. I wouldn't even bother trying to fix it. You will never come close to overheating the fluid unless you are doing some major road racing or continuous HIGH speed driving.
JB has metal in it (or used to) but is basically a plastic material. The thermal xfer properties are likely non-existant, however I don't think you'll see much difference thermally from the repaired fin as it is mostly cosmetic anyway. I would patch it as best you can, make it look good and not worry about the thermals. If you really, really want to repair it thermally, apply thermal grease (a compound you can get at a place that sells computer components liek processors and heatsinks) and apply it to the mating surfaces (ONLY). Make sure the grease does not get on any surfaces that you will apply the JB to as NOTHING sticks to thermal grease. Once you have the fin placed, with the grease sandwiched in place, clean the area with a good solvent like lacquer thinner. Thermal grease is messy stuff and less works better than more. Then, apply the JB weld as a fillet along the outside of the break to bridge the parts. It won't look good but will conduct better than if JB is between the fin and base.
can you post a pic? aluminum can be brazed/soldered- http://durafix.com/ http://www.aluminumrepair.com/faqs.htm http://www.aladdin3in1.com/ A local welding supply should have one of these. I picked up a 1 pound package of the aladdin rods for $30. It's the same stuff they repair aluminum boat props with. Just need a good torch, preferably oxy-acet. to heat the piece so the rod will flow. This will also give you the heat transfer. The 1-pound package I got contained about 50 2-foot rods, at 3/16" diameter I believe. You would get stuck with a lot, you might be able to get just 1 piece from the store for a buck if they're nice. The stuff is supposed to do all types of cheap metal- zinc cast and pot metal, the kind of stuff when you break it looks like a 3 musketeers bar inside.
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