Question about expansion

[timelake]

Here's a query for this forum's community:

My project calls for restoring the threads of a hole in a housing made of diecast zinc - I intend to drill out the hole's damaged threads in the housing then inserting a threaded sleeve made of brass. Caveat: Once I drill out the damaged threads, there'll be only a wall thickness on two sides of no more than 1/8" - That's where a potential structural failure may occur if the expansion ratio of these two metals is very different.

Whatever the diameter of the drilled out hole in the diecast zinc housing will be, the insert's O.D. will be slightly larger - This, because I intend to freeze the brass insert, to reduce its diameter slightly before inserting it into the drilled out hole.

My hope is to let the insert expand at room temperature in the hole hopefully seizing the insert in place, with the expectation that the brass threads of the insert will outlast the original zinc-diecast threads of the housing.

I'll appreciate any and all input, critical or helpful, as to whether I'll have a structural failure once the insert is in place and begins to thaw at room temperature.

I will gladly clarify further if needed.
Many thanks!

 

[Charles Lamont]

Is the diecasting still sound, or is it so old that it is getting a bit crumbly?

In any case, I don't think I would use a shrink fit for a repair.

Other approaches to think about include taping the hole out larger and using a (bought or home-made) threaded insert, filling the hole with a repair epoxy such as JB-Weld and tapping a thread in the epoxy, using epoxy to fix an insert in place, using Loctite to fix an insert in place.

Depending on the service conditions of the part, you could get electolytic corrosion of the zinc in contact with brass. A steel, or stainless, insert would be better from this point of view.

 

[Krutch]

If you freeze, use dry ice and heat the die cast. Better if the insert has OD serrations or grooves to keep it from spinning with screw tightening pressure.
Epoxy is a viable solution. Will a helicoil work? You would have to figure if the dimensions fit your app. STI tap is oversize to allow for the coil thickness.
Not sure who sell them, but there are inserts for die cast, though most are for precasting and are intended to be placed in the mold first.

PS: if you freeze also freeze the tool you use to press in the insert. This will keep a warm tool from ruining the shrinkage of cooling. I do this when installing Harley stuff with interference fits.

 

[modelman1838]

I would go for Loctite 603 every time, never known it to fail.
Hugh