As some of you are aware, I've been messing around with this off and on for some time now. I was dissatisfied with the proportions of my first efforts and have come up with a revised pattern. This is a bunch of separate parts, still unfinished, tacked together with white glue:
1. Block/heads/oil pan
2. Rear gear case
3. Transmission bellhousing
4. Valve covers left and right
5. Turbocharger.

There is a lot left to figure out yet; you'll note that there are no intake or exhaust manifolds fabricated yet, no starter, no oil cooler, no engine mounts, no main pulley, and nothing of the accessories in front of the gear case. I could use any good photos of these components if they are out there. I'm thnking that radiator fans might be assembled onto the radiator assembly (seen in an earlier thread) and cast as a unit. Maybe. And the firewall with oil coolers and other bits also needs to be done yet. Also, anybody who has professional casting experience please get in touch. It would be really helpful to know what kind of things can be done as one lump and what needs to be broken out separately; there are things that seem to be possible with vacuum systems that I couldn't dream of successfully casting at home. I've reached a bit of an impasse with respect to engineering this little puzzle, so all ideas are welcome. If this thing ever comes to market, there will still be a LOT of scratch work that a modeler will have to do on their own. Stuff like the exhaust plumbing will vary between hull types and time period, for example. Let me know what you think.

I feel the Sherman fever coming around again Greg, it looks smashing, please keep up the good work...

Regards Thomas

Greg,

When you're done, I'll buy a few, like maybe six!

As for the resin casting, I did a little bit of this with Bill Miley, very early in the MP Models days, before the injection-molded plastic stuff.

Even with the vaccum equipment and so on, the biggest issues are still going to be there.

1. Resin generates heat as it cures. Heat will cause shrinkage, which will vary by thickness. Thicker pieces will generate more heat and so on. Sink marks are the result. Consequently, you want to limit the size of a single piece. For instance, the cylinder heads have the cast-in letters, so you want to limit thickness there to eliminate distortion. The main engine block could be a single piece, with any sink holes being fillable, to a point, or you could do it two parts.
2. Molding limitations are not as severe as with plastic. You still want to limit the amount of potential tearing of the rubber, as parts are removed. Again, the size of the pieces and the number, length and direction of any protrusion on the parts are important considerations.
3. You can do single piece molds with parts that have smooth backs. Generally, two-part molds are little trickier. You need to split the part at some logical point to reduce the tearing aspect. You also will need a vent opposite where you pour. Before pouring the second half of the mold, you need a barrier layer of an inert material that will not react to the rubber. Once both sides cure, you no longer need the barrier.
4. We did hand-pours for the resin, and we purged air bubbles with a smooth rounded dental burnisher, the kind they use to polish fillings. It's ok for short runs.

If you do get into some mechanical means, as in spin casters and vacuum pumps, don't use RTV (room-temperature vulcanizing) rubber, as it will distort under pressure.

You may want to contact Rob Irvin at Formations.

Don't forget, i'll buy at least a half-dozen.

Tom

Tom what would be the best brand of resin to use? Seems like some shrink more than others.

Thanks for the encouragement, Tom!
I'm familiar with the usual pitfalls of casting; I've done a little bit myself but nothing on this scale. One piece and two piece; softer and harder RTV compounds. Expensive, and lots of rejected parts. I've used Vaseline as the barrier on the two-part molds and it has generally worked out OK.

The block/pan/head assembly is a solid lump, so that would be the part most prone to shrinkage I suppose. But it is also a fairly regular shape. The valve covers with the raised lettering (thanks to Woody ar Archer Transfers!) are pretty small pieces, only .150" thick for the main part and a little more for the intake trunk. The bellhousing/gear case has a flat back, so it could be done as-is. The matching accessory drive cover on the other side will have to be a separate part.

It seems that the intake and exhaust maifolds will need to be kept separate, as adding more stuff to the block core will just add to the resin heat issues. Likewise, the front engine mount bracket and main fan pulley will also need to be kept separate and some alignment mark provided. This is going to be rather complicated, especially once I get down to making things like the air balance tube between the intake manifolds. If I was just going to stick the engine in something, I might use really thick solder for that vaguely U-shaped part but that won't hold it's shape to be molded--especially if I send all this to somebody like Rob. So lots of challengenges yet, and I'm still missing really good photos of the accessories behind the turbocharger. And I doubt that the wire-frame thing that supports the wiring and such is something that I can tackle as a castable part. Too fragile.

I'm engineering this using a DML M4A4 hull, but a Tasca could be used as well. I've fabricated the sidewall extensions for the engine bay, which DML needs but Tasca doesn't.

I am in for a few of these!
Try also the "new" guy at TWS.
ciao
m

I can't help with the resin brand. Been out of it way too long. Sorry.

Tom

OK, these show the work so far mocked up in the engine bay. Everything is tacked in with PVA glue if at all, so don't be concerned that things might not be lined up just right. I think that the proportions look about right. The filler cap on the funny U-shaped tank above the gear case fouls the engine deck; the opening has to be drilled out. Either my part is a little too tall, or the thickness of the DML deck is a little overscale. Some compromise might be in order here. Obviously there is a lot yet to fabricate, and that's getting interesting as this thing now has to stay as you see it: tacked together with PVA glue (really weak bonds) in order to properly shape and fit the things to come, like intake and exhaust manifolds, thermostat housings and piping, and all the clutter on top. I'm still looking for really clear photos of the accessory area in front of the gear case and behind the turbo, if anyone has something in their stash. Long way to go, but I think I will get there. I won't be surprised if there are three dozen parts in total when this is all built up, because so many things will have to be molded separately. The instruction sheet will be real fun, too...