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Old 10-14-2015, 03:40 PM
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Default C4 batwing to fit my C3 differential

Making slow progress on getting the C4 batwing modified to put on the '69. This batwing is about 7 pounds lighter than the C3 batwing currently on the car. Here's the (not yet finished) adapter ring (3/4" aluminum), and the batwing has been drilled out for the C3 pattern. Still need to weld up a couple of the original C4 bolt holes.

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I need to narrow the batwing 2 1/4 inches to clear the narrowed C4 suspension in the car. I grabbed a bunch of old steel remnants and welded up a temporary jig to hold the outer mounts in place while I cut the batwing and reposition the mounts inward 1 1/8 inch. Not seen in the picture, but I used the threaded holes at the outer mounts that would otherwise hold the dampening weights.

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Here's the outer mount moved inward 1 1/8 inch. I now need to clean up the cut and start welding the pieces back together.

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The adapter ring is 36 ounces (additional weight), and the stock C4 spring bracket is about a pound lighter than the steel adapter bracket I currently have on the car (to be able to use the '84 Z51 spring). I should be able to lose an additional pound or two by now being able to use the toe mounts cast into the C4 cover, rather than the steel adapter bracket I had to fabricate to attach toe links to a C3 cover. I might also be able to use shorter/lighter spring hanger bolts with this batwing (won't know until later). All in all I'm looking for about a seven pound net weight reduction off the car. The reduction is down low and way back unfortunately, but it's still mass that won't be there needing to be accelerated and decelerated.
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Old 10-14-2015, 05:05 PM
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How will you keep the heat (from welding) from burning the bushings?
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Old 10-14-2015, 07:47 PM
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How will you keep the heat (from welding) from burning the bushings?
Well, right now I'm just planning on welding small sections at a time, primarily to keep the heat induced expansion/distortion to a minimum. It looks like the stock bushing has a steel sleeve around it, which I hope will help keep the large bore round. And finally, most likely I'll pop in a poly bushing setup afterward. For what ever reason, I've got a spare poly bushing setup that's been gathering dust on the shelf for probably the last 20 years. Glancing at the rubber bushings it looks like it's going to be a PITA to extract them though.
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Old 10-14-2015, 09:20 PM
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You could put batwing on the adapter if you used a wider piece of al plate. But you'd need a mill to cut the webs. Much safer to use billet than weld (what looks like) a casting.
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Old 10-16-2015, 12:40 AM
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You could put batwing on the adapter if you used a wider piece of al plate. But you'd need a mill to cut the webs. Much safer to use billet than weld (what looks like) a casting.
I appreciate and share your concern for safety. I'm trying with all the modifications to not make my antique any less safe than it was when it was new (not a terribly high bar, I admit). I've been running a similarly narrowed, but C3 model, batwing in the car for the past six years with good results. The area I'll be welding is similar in width and thickness (similar square area) to the piece on the car, so I'm hoping for similar positive results.
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Old 10-16-2015, 05:43 AM
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Just holesaw out those rubber bushings then with a die grinder slit the metal outer sleeve and hammer them out. Dress the bore for a chamfered radius and hammer in some urethane bushings, That way you won't have to worry about the heat.
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Old 10-16-2015, 11:02 AM
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You could put batwing on the adapter if you used a wider piece of al plate. But you'd need a mill to cut the webs. Much safer to use billet than weld (what looks like) a casting.
I appreciate and share your concern for safety. I'm trying with all the modifications to not make my antique any less safe than it was when it was new (not a terribly high bar, I admit). I've been running a similarly narrowed, but C3 model, batwing in the car for the past six years with good results. The area I'll be welding is similar in width and thickness (similar square area) to the piece on the car, so I'm hoping for similar positive results.
Can't you mill 7 lbs off the old batwing? It must be pretty beefy to weigh more than the wider c4.

I was just commenting above about safety as it was a race car or weekend race car and driver. If the batwing breaks your spring probably digs in and it would be bad for you and competitors.

Oh never mind, I see the old one is welded too.

Last edited by rtj; 10-16-2015 at 11:07 AM..
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Old 10-16-2015, 06:13 PM
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after welding if you were concerned you could just box in the ends of the batwing with a couple of pieces of 1/8 sheet alu... should be plenty strong...
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Old 10-18-2015, 12:29 AM
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Originally Posted by 84rzv500r View Post
after welding if you were concerned you could just box in the ends of the batwing with a couple of pieces of 1/8 sheet alu... should be plenty strong...
We're pretty much on the same wavelength here. I was planning on adding some additional material bridging the weld area but I hadn't gotten far enough to figure out the configuration yet.
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Old 11-10-2015, 02:15 AM
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Default New bolt pattern bolt-hole question

I've got most of the batwing mods complete (about 90% done with the welding), but have a question regarding the (newly drilled) cover holes that I did to match the C3 cover pattern. Most of the new holes are on the flat part of the cover, and my experience with aluminum parts (and I suppose other material parts) is that there is a raised island of material surrounding the bolt holes. I can speculate what the purpose of those islands is, so I'm trying to duplicate those on the cover. I've thought of cutting out some aluminum washer-shaped pieces and welding them over the bolt holes, or (and I'm not sure this would actually work) seeing if I can place a short length (quarter inch) of 3/4-1" diameter copper tubing around the hole and then MIG some material thickness inside the copper ring (and hopefully get the ring off after each weld). I'm just looking for some input/advice on this task.

Thanks.
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