How much can you lap out an iron mold?

[Patrick L]

I recently purchased a Lyman 429421 double cavity mold for my .44 Special. Unfortunately bullets seem to be dropping undersize and out of round. Granted I realize the first casting session can be a bit rough, maybe the mold will settle down a little once it breaks in. The bullets seem to be measuring about .428-.429 parallel to the seam line and .431-432 perpendicular to it.

My M24-3 has fairly generous throats. All will take a .432 pin gauge, most take the .433. And I don't think simply beagling will do it since, as I said, the smaller diameter is when measuring parallel to the seam.

Can I lap it larger with a few cast boolits and some compound, or does that only debur? I know the aluminum molds can be enlarged some, but I'm not sure about iron.

I would return it, but I suspect Lyman will just tell me the mold is in spec or to change my alloy to get larger bullets.

Any advice?

[cabezaverde]

I have a similar Lyman.

Depending on how many you plan to use, you can bump the bullets and then size them. Not a good high volume approach.

[docone31]

To lap iron,
Cast a bunch of boolitts with a nut instead of the sprue plate. Make the alloy 2% zinc also. Get the largest ones, smear coarse lapping compound on them. Slowly close the mold on the castings while turning with a wrench. You will go through several, the nut/sprue will turn off.
In one way, that acts as a safety. You will not be able to use excessive torque.
As you come to the size you want, go to finer and finer compound. You will get the feel as you do this.
On your final polish, Use one casting and go back and forth from one cavity to another. You want to go slightly larger than your target diameter for the final sizeing. From there, you can size down to the desired diameter.
It will take longer than aluminum molds, but you will also have more control over the size. As long as you swap castings to control the cuttings, you will have even cavities. The issue is to not have two different size castings. You will have no control over firing that way.
It won't take much to make them round, and larger. Go slowly, and take care.

[Dale53]

A friend and fellow forum member here, recently got a Lyman four cavity 429421 that was undersize, also. He called Lyman and they told him to send it back and they would make it right. He got pretty good turn-around and has received his new mould. He has not yet had an opportunity to cast with it, so I cannot, yet, report on the success of the exchange.

Personally, I would call Lyman and send it back. That will accomplish TWO things:
1 - You'll get a proper mould
2 - Let them know that undersize moulds are unacceptable (unless you
return this mould, the "powers that be" will have no way of knowing
that poor products are being shipped).

FWIW
Dale53

[462]

Patrick L,

Boy, do I know what you are going through. I have a 429421 that I use for a Model 24 and a 624, both with cylinder throats of .432". A combination of lapping and beagling increased the dropped size from .430" to .434". I had to lap a .431" sizer to .433", too.

Instead of using a nut, as docone31 mentioned, I drilled pilot holes into the boolits' bases, installed small screws, cut off their heads, chucked it all to a cordless drill, and using fine lapping compound, went at it. You won't be able to get the screw perfectly centered, so just let the drill and mould wobble round -- it will come naturally.

Lap each cavity at a slow speed for a bit, completely clean the mould, cast up another set of boolits, let them cool overnight, and repeat the process till you arrived at the size you need. It probably sounds more complicated than what it is. Remember, though, go slow.

If you use a lubri-sizer, you will need to lap the sizing die, too. There, again, go slow, cleaning it and running boolits through it till you're where you want to be.

If you want, send me a PM and we can discuss this via e-mail.

[jdgabbard]

You can go to the CastPics site, and look under the tab "Mold Referrences" and it will have one that says Mold Lapping.

[Patrick L]

Well, I emailed Lyman. Theys said to return the mold, along with a few casts, and they will check it out. I guess I'll go that route before I try doing anything to the mold.

[HeavyMetal]

Patrick L:
The only way a vendor knows his product has a problem is when it is returned for warranty work!

I think your doing the right thing for now, lets see if Lyman gets this fixed.

If not then you can take things into your own hands at a later time.