Captain Obvious

Nice work. Looking quickly at the model, I've got a couple questions, etc. First, the small pilot holes for the mustache bar bolts might be trouble. I think those may be be too small and too fragile of a sand tower and will just snap off when they pull the master out of the sand. I would just make that surface flush, maybe leave a small locating dimple or something, but I'm not sure you're going to have good success casting those pilot holes. And... You're already going to have to drill all the mounting holes around the perimeter, so it's already going to be in the mill. Why not just locate those holes when you do everything else? Why do you need a pilot? Second, I can't tell from the master part if you've got draft on all the necessary surfaces. I'm assuming the foundry will look into that in detail. Last, hard to tell just from pics, but it looks like you might have an interference between the oil hole (fill hole?) and the internal cast rib. Maybe the fill plug hole misses that internal rib, maybe not? Can't quite tell.

Gotcha. @Derek I like yours better. I think everyone should run one. I just wish I could afford it!

Yes, if you overfill the carbs, it will spill over inside the domes when you put the piston plunger back in. That's totally normal and I'm assuming that's where the oil is coming from. It's seeping out of the carbs around the nozzle and eventually ending up on the mixture adjustment nuts. The only thing I'm wondering about is... How long ago did you overfill the carbs? Was it yesterday and the knobs are oily, or was it five hundred miles ago and you've been driving it for six months and the knobs are oily?

You are correct that overfilling the carbs with more oil than necessary won't really cause a problem. It'll just run down into the carb throat and be pulled into the engine when you start the motor. But 500 miles later, that should have happened already and should have burned off by then. In other words... If you put oil in the carbs 500 miles ago, but the mixture nuts are wet today... I'm skeptical that it's oil from the carbs that is making the mixture nuts wet. I'm thinking that maybe it's partially evaporated gasoline mixed with a little blowback from the intake manifold mixed with a little road dust? There was a thread many moons ago where we talking about oil leaking out of the damper tubes. Seemed improbable, but possible, with the flat top carbs, but seemed impossible with the round tops? I can't dig it up right now, but maybe someone can find that old thread?

@Derek Any input from you? Is this your head?

That's actually a very good thought. I've been unconcerned about getting the finished part out of the sand without draft, but getting the master part out of the sand before casting IS a concern. Any "vertical" surfaces where the master will have to slide against the sand should be avoided if possible. And the more sliding, the more chance you'll pull some sand off the wall and mess up the mold cavity. Like I said... I'm no casting expert! None of that would apply to an investment casting, but for sand made using a master packed in and then removed, it certainly could be. So you need to look at the master and picture how it will be pulled out of the sand. Picture the parting line and plan the draft accordingly. The parting line is going to be right at the mounting surface, and your current design has several areas where you could probably benefit from some draft. For example, the fins around the bottom perimeter are tapered, but not in the direction that produces casting draft. Those fins are going to slide a couple inches scraping against the sand walls as the master is removed from the sand. Any surface feature that is perpendicular to the parting line is suspect.

I was wondering what those bosses were for. I couldn't find any internal feature that would explain the necessity for those on the outside. I got it now. So you were talking about tapering the fins so the part would release from the mold. In molding speak, they call that "draft", as in... "Are you sure you have enough draft on those surfaces so the part can be removed from the mold?" I'm no mold expert, but it's my understanding that you only need draft if you are splitting the mold open into reusable pieces such as opening a die after die-casting. You aren't doing that with sand or investment casting. With sand and investment casting, you remove the mold in little pieces and don't need to salvage any of the original mold. And because of that destructive (to the mold) process, draft is not required. Now, it might help for other reasons like better feature definition or mold filling without air pockets or porosity, but I don't think you specifically need draft like you would with a die-casting. As to the specific question... "How thin can I make the fins?" I would ask the foundry for guidance on that. And about the holes... I'm assuming you will be drilling all those in post-casting. You aren't going to to try to have any through holes cast right into the part, right? And about accuracy of "as cast" surfaces, I'm assuming you'll be machining any critical dimensions into the finished part. If there's a sealing surface or bolt hole, etc, you'll be finish machining that and not counting of that coming out of the mold with enough accuracy. The thicker the material, the greater the shrinkage. Lastly, have you talked to the foundry about the locations of gates and vents? In my experience, things to consider... Locations for gates and vents for proper fill and prevention of trapped air bubbles. Uneven or too rapid cooling. Sink marks on thicker sections or where different cross section thickness intersect. Porosity in the final part and how deep you need to machine precision surfaces. Warping or twisting of large parts. Sorry for the novel.

That's what I was asking. "Investment casting" is another name for lost wax.

I would expect one of the primary variables would be surface area. However, you should know... Thermo was my least enjoyed class of anything I ever had. Ever. And (like most people), I learn better when I like what I'm learning. @Jeff Berk, What casting method are they going to use? Are you going to supply them with a wax model so they can do an investment casting? Making a 3-D model that they will press into a sand casting? What?? Honestly, I can't get over the belief that someone is willing to do a one-off casting of that thing for $40!!! Unless you are supplying the consumable model and assuming all the risk of whether it works or not, I'm completely floored by that cost!!

You are correct that the higher sitting piston would pull the needle out of the jet tube further, but it's certainly not the same as raising the engine speed with the idle screw. Goes like this... Raising the engine speed with the idle screw increases the airflow through the carb (because you have opened up the throttle butterfly). That increased air flow will cause the suction piston to rise a small amount to keep the air velocity across the venturi constant. That's the whole theory of these CV carbs. In contrast, if you lift the piston using a protrusion on the underside of the piston (like the little nubbies you have discovered), you will actually DECREASE the air velocity across the venturi, and the result is a leaner mixture. The part I really don't understand is why didn't they use a same diameter insert as the original non-metallic bumper. I mean, I can see someone thinking their bumpers are all smooshed out and ineffective and wanting to replace them, but the part I don't get is why they thought they needed to drill out the piston to accommodate the replacement. Why didn't they reduce the diameter of the insert to fit the piston. Why permanently modify the piston???

Actually it would do just the opposite. It would lean out the mixture just like using the little lifter rod on the side of the carb body to push the piston up. Not sure how much with that amount of lift, but leaner none-the-less. That aside... The concept is pretty much stock. I'm sure you are familiar with the little non-metallic chunk (presumably Phenolic) pressed into the bottom of the suction piston. That little button acted as a bumper stop in a crude attempt to prevent metal to metal clunk when the piston bottomed out on the inside of the carb body. Are you sure that non-stock piece inserted is in fact metal? Is it magnetic? Hard to tell from the pics, but I wouldn't be surprised to find that it's a hard nylon or Delrin material. I'm thinking someone tried to replace there old smooshed out original bumper stop.

Glad to help. But out of curiosity... I believe more modern materials are better in pretty much every way than that old brown stuff. Why would you not use current age glass reinforced board material? I mean, I get originality, but in an area like that where it is completely invisible it seems unnecessary. You can get the new glass FR-4 in black color solder mask. Would that be "good enough"?

A little web digging turned up some search terms that might be helpful. FR-2 (predecessor to what we use today which I believe is FR-4) And a couple brand names: Pertinax Paxolin

With apologies to Gary Larson

Whatever works. If you want to send one of the R160 shims, I can poke it with the tester. I'm sure 3.5mm would be thick enough. One would assume that the new 160 shims are similar to your old original, but stranger things have happened.

Agreed. Those look pretty much dead soft. (Annealed.) If you dare, you could hit the edge of your original factory shim with the file the same way and see how it compares? I think you could do that and not affect performance. That could give you something to compare against as far as what hardness you are looking for?

Well based on the application (king pin shims), I would assume they are relatively hard and would be suitable for the differential application. You can get a quick and dirty idea of how hard they are using a small hand file to see it it will cut those new shims. The corner of a small file will a) cut like butter, b) completely skate across the shim without cutting, or c) somewhere in between. You want "in between". Another thing you could try is if you fold one of those shims in half... Does it fold easy and stay pretty much completely folded? Does it snap into two piece? Or does it fold but springs back a bunch when you let it go. I think you want the third option. I know this is all just subjective words, but from a distance, that's all I got. And I may have mentioned this already, but I can run a Rockwell hardness test on parts if necessary to determine the Rockwell rating. Although I'm not sure it will be accurate on something as thin as .005.

Yeah, I'm not surprised about the minimum. At this point, I could probably be convinced to cut one part if you want to cover costs, etc. Send me a PM if you're interested.

What about @Zup? Any word?

I've heard people talk about a "diff clunk", right? Hahaha!! And I totally get you about the cold temps. I used to be able to work on cars in the cold. Not so much anymore. My hands just can't take it.

Maybe it's just me, but I suspect the first thing your machine shop is going to do is convert your drawing to English system. With that in mind, I'd just do it for them. I know any machinist ought to be able to do the conversion, but I'd just do it for them. Also, is the pinion shaft diameter smaller than 35mm? If so, you're good. But if the shaft is right at 35mm OD, then a 35mm ID spacer isn't going to fit over it. You'll need to open up the ID a couple thousandths just to get it to fit and not bind when you try to slide it on the shaft. As for material, I think 1045 in the annealed state would be a reasonable choice. It should certainly be harder than the 1008 stuff.

The only possible use for an amplifier like that in 72 would have been to kill the application of the throttle opener device as you came to a stop. But in order to do that, your throttle opener would have to be the defeat-able style design, and I don't think that even started until the flat top carbs. Does your throttle opener control valve even have a wire going to it?

Wow! That must have made quite the noise!

And what about @Zup? Anybody heard a peep. Hey FrogSquisher. You still read the forum??

My thoughts? I agree. Having not been through this exercise myself, I'm not sure of all the effects of changing the pinion position like that. Maybe you'll get lucky and end up exactly where you need to be. And if not, you'll have a better handle on the effect(s) of the change.