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Thread: Modern-day Miura

  1. #161
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    Quote Originally Posted by C5GTO View Post
    I

    I started with partially threaded ½” stainless steel bolts. I used a lathe to drill holes into the center of the bolt head and down the shaft. The drilled hole was then threaded for 3/8”-24 threads using a hand tap. I then used a vertical mill to machine the bolt heads down to 1/8” thick so they would clear the rotor. The bolt shaft was cut off with a hack-saw and remaining shaft ground to final length on a grinding wheel. The picture above shows at various stages of machining where rearmost one is completed tee-nut with a 3/8" bolt in it. Like I said lots of machining and time to make each one and 8 tee-nuts were needed.




    It's an interesting approach to the problem. I have two observations about it nevertheless:

    1. How were you able to torque the caliper mounting bolts given that you only had a 1/8" gap to hold the tee-nut from spinning?

    2. I also worry about the amount of torque you can place on the tee nuts before they fail. Having hollowed out a 1/2" stainless bolt with a 3/8" inner diameter hole would leave only 1/16" thickness where the shank meets the head of the tee nut. Since the 3/8" stainless bolt has a cross section of about .11 sq-in and is designed for about 22 lbft of torque, the 1/2" tee nut would have 0.2 sq-in of cross section minus 0.11 drilled out equals 0.09 sq-in cross section. That's about a 20% reduction in area over the 3/8" bolt, which means the caliper mounting bolt should really only be torqued to 20% less than its design load, or 17 to 18 lbft. As long as you believe that's enough to hold the caliper on, it should be alright. Much more than that and you risk pulling the head off the tee nut.

    My 2 cents.

  2. #162
    Senior Member C5GTO's Avatar
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    Arrow

    It's an interesting approach to the problem. I have two observations about it nevertheless:

    1. How were you able to torque the caliper mounting bolts given that you only had a 1/8" gap to hold the tee-nut from spinning?

    2. I also worry about the amount of torque you can place on the tee nuts before they fail. Having hollowed out a 1/2" stainless bolt with a 3/8" inner diameter hole would leave only 1/16" thickness where the shank meets the head of the tee nut. Since the 3/8" stainless bolt has a cross section of about .11 sq-in and is designed for about 22 lbft of torque, the 1/2" tee nut would have 0.2 sq-in of cross section minus 0.11 drilled out equals 0.09 sq-in cross section. That's about a 20% reduction in area over the 3/8" bolt, which means the caliper mounting bolt should really only be torqued to 20% less than its design load, or 17 to 18 lbft. As long as you believe that's enough to hold the caliper on, it should be alright. Much more than that and you risk pulling the head off the tee nut.

    My 2 cents.




    Bloozberry: interesting observation and questions. As to question 1, I haven't torqued the caliper bolts yet but I plan to use a thin wrench to hold the tee-nut from spinning. I'll also glue them in with red Loctite prior to torquing.

    As to question 2, you've put a lot more engineering into forming your question than I than I did for my tee-nut solution. I believe the real critical question is, "how much torque is really needed when fastening these caliper bolts?" The initial direction that was passed on to me from Charley (but I believe was provided to Charley by Wilwood) was to drill/thread the aluminum upright and screw the caliper bolts directly into the aluminum. If that's an acceptable approach, then the tee-nuts I've made will surely be stronger than that. When I look at similar Wilwood aluminum caliper brackets themselves, they use what looks like a press-in steel threaded insert that just has a small knurled surface to keep it from spinning when torqued. It's hard for me to believe that insert will take much torque before spinning itself.

    I looked through the information provided by Wilwood in the boxes and don't see any torque specs for the caliper bolts. In looking at instructions for a Wilwood brake kit that uses a similar caliper with a 3/8 - 24 bolt to secure it, the instructions say to torque to 30 ft lbs, use red 271 loctite, and then wire the bolts in place. Well wiring the bolts in place does sound like a good safety minded thing to do. I'm using 3/8 - 24 bolts which show a 30 ft lb torque spec. Maybe I'll need to do some torque testing on the tee-nuts to verify they can take the full 30 ft lbs.

    Hmmm...thanks for bringing up the questions. I always want to make sure I don't have a safety gremlin hiding out somewhere.
    Last edited by C5GTO; 08-10-2019 at 04:39 PM.

  3. #163
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    Post Locating and Fabricating Flip Front Clip Hinge Point

    I decided to start building out the sub-structures that attach and hold body panels to the chassis. I’m starting with the one piece, flip-up front clip first. The front clip is hinged in the front and thus tilts forward to open. When opened, the bodywork that forms the top of the nose comes very close to touching the ground. Because of this, determining the hinge point location is critical.

    I needed to know the front bodywork outline in order to measure backward to the hinge point. So with measurements taken from station buck, I used a piece of welding rod and masking tape to lay the outline. It also shows where the horizontal chassis tubes are too long and need to be cut off so they won’t protrude through the bodywork.



    Through some research and experimentation, I concluded the hinge point needed to be vertically about an inch lower than the front wheel spindle and horizontally about 10 inches behind the most forward part of the front bumper. I located this spot and marked it with an X on some masking tape. By putting in some angled chassis bracing members, it would give a strong hinge point and complete the chassis bracing in that area.



    I plan to use ½” bolts for the front clip hinges themselves. So I inserted some aluminum inside the rectangular tube to make it solid at the hinge point and drilled ½” holes. To ensure the holes on both sides aligned, a piece ½” tube was used to fixture them in place for tack welding.



    After tacking the cross brace tubes in place, the horizontal chassis tubes were trimmed to length.



    I’m going to hold off on final welding the cross brace tubes in place until I’ve built out more of the front clip sub-structure and can physically verify the hinge point placement.
    Joel Heinke
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  4. #164
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    Quote Originally Posted by C5GTO View Post
    I believe the real critical question is, "how much torque is really needed when fastening these caliper bolts?"
    That is indeed the million dollar question. It's one that can be calculated, or it can be estimated through a comparison of similar designs (as you've done using the Wilwood examples).

    Quote Originally Posted by C5GTO View Post
    In looking at instructions for a Wilwood brake kit that uses a similar caliper with a 3/8 - 24 bolt to secure it, the instructions say to torque to 30 ft lbs... I'm using 3/8 - 24 bolts which show a 30 ft lb torque spec.
    Are you sure you're not looking at the torque specs for carbon steel? The most common stainless steel bolt alloys (8-18 and 316) specify 21.6 and 22.6 lbft respectively.

    Quote Originally Posted by C5GTO View Post
    Maybe I'll need to do some torque testing on the tee-nuts to verify they can take the full 30 ft lbs.
    That's probably a good idea.

  5. #165
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    Post Flip Front Clip Hinges

    Next up on the build agenda was to make the flip-up front clip hinges. I wanted to have both vertical and horizontal adjustment in the hinges so it took some thinking to design that in. I started with a piece of 2 ¼” x 2 ¼” x ¼” angle in 6061 aluminum, 6” in length as the main part of hinge. This was then cut down so a 1 ½” wide rectangular tube for sub-structure base could bolt up to it.



    I put slots in the angle part of hinge where 5/16” bolts pass through for horizontal adjustment and vertical adjustment if via shim. For fabrication purposes, there’s a ¼” thick shim in the hinge for now. I’ll likely put some lightening holes in the hinge to shed some of its weight. The sub-structure members will be welded to the rectangular tube bolted to the hinge top.
    Joel Heinke
    Be original; don't be afraid of being bold!

  6. #166
    Senior Member C5GTO's Avatar
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    Post Door frame alterations

    I switched tracks a bit to working on the doors. Before I can continue building the front clip sub-structure I need to know the size and location measurements for the vents located on top of hood. I need these measurements as I plan to run sub-structure members around the vent edges and thus provide mount points in the middle of the front clip skin. It’s going to be a couple of weeks before I can get the needed vent measurements so I went to the next item on my build list which is the doors.



    The doors relate to the front clip in that they are adjacent panels but on the Miura there is also a common rocker panel, silver in the rendering above, that extends from the front wheel opening under the door all the way to the rear wheel opening. So it will be important to know the exact rocker panel height and thus where the top edge of this rocker panel will be under the front clip. The door bottom is the main constraint on rocker panel height and the door bottom is constrained by door hinge location and side window lift location.

    As a reminder, I’m using the door frames, hinges, and side windows from a 1991 C4 Corvette. The Corvette doors are taller by 5 ½” than what I need for the Miura. So I need to remove the bottom 5 ½” from the donor doors and bring the bottom, rear corner forward by 6 ½”. The two main impediments to doing this is that the electric window lift mechanism needs to be shortened and the bottom hinge moved upward. Here’s a door with the new size marked in masking tape and a mock-up of the shortened window lift.



    So now I needed to move the bottom door hinge upward by 1 ¾” as it is located too low in the donor door. The main limitation for a new hinge location is the wire harness routing to the door via a rubber boot. By relocating the bottom hinge to snug up under the oval boot hole, it will be real close to providing space for the 5 ½” door chop. Even if I can’t get the full 5 ½”, I decided it was better to have the rocker top edge a fraction of an inch lower than to completely re-do the door pillars.

    The door hinges use 3 bolts that screw to the door pillars via a cage nut plate. The cage nut plate is located on top of a heavy L shaped reinforcement plate spot welded inside the door pillar. By drilling out the spot welds, I was able to save and reuse all the hardware.



    The bottom 3 square holes are the original hinge location and the top 3 square holes were cut out for the new hinge location. The reinforcement plate was rosette welded into the new position via the open holes and a sheet of steel placed behind the remaining holes and rosette welded to seal everything up.



    The door frame was easier as I was able to just drill some new holes through it for the new hinge location.



    Here it is with hinges and wiring harness boot back in place. It’s tight but should work fine. I can now start cutting away at the lower door frame.
    Joel Heinke
    Be original; don't be afraid of being bold!

  7. #167
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    This is simply awesome work! One of the best bios threads on the web, if not the best. Can’t wait to see the progress on cutting the bottom of the doors!
    Luke

  8. #168
    Senior Member C5GTO's Avatar
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    Quote Originally Posted by luke.jenner View Post
    This is simply awesome work! One of the best bios threads on the web, if not the best. Can’t wait to see the progress on cutting the bottom of the doors!
    Luke
    Luke: thanks for your enthusiastic input!
    Joel Heinke
    Be original; don't be afraid of being bold!

  9. #169
    Senior Member AdrianBurton's Avatar
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    Man, I cannot wait to see this car's progress!!!!

  10. #170
    Senior Member C5GTO's Avatar
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    Post Door frame transformation

    The next step in the door frame transformation was to bring the bottom rear corner inward by at least 6 ½”. C4 Corvette doors are composed of a spot welded sheet metal frame that’s bonded to and sandwiched between a plastic inner door panel and a composite door skin. I’ve already peeled away the outer door skin so only the inner panel and door frame remain. This close-up shows the Miura door skin edge outlined in masking tape over the door frame to indicate places requiring modification or removal. I don’t plan to use any of the plastic inner panel in the final Miura door so I will be cutting it away in pieces when access is needed to the metal door frame for modifications.



    I first drilled out the spot welds between side impact channel and rear of door frame. I then made a pie cut below the door latch mount point to straighten an angled section, sliced off about 6 inches of the plastic inner panel and bent the rear edge of the door frame forward. At this point everything inside the door is now in close quarters. I had to remove some gusset material inside the door frame rear edge to provide clearance for the side window to be lowered. After a few rounds of trial fitting the door and side window in place and measuring everything multiple times, I put in a few tack welds to hold the door frame together in the desired shape.



    The long end of the side impact channel will get cut off once I’m sure about the modifications and it’s ready for final welding. Part of the verification process is making sure the side window glass won’t be touching metal when it’s fully retracted and when being raised. I thought the angled corner shown in the following picture had insufficient clearance and wanted to do something about it.



    I’ve been told that tempered glass cannot be cut and side window glass is tempered. I could compromise by taking away some of the door rear edge angle and thus free up some space inside the door frame. Alternatively, if I could just remove about ¾” from that corner of the glass I could get the needed clearance and keep the Miura signature door rear edge swooping angle.

    My son does lapidary (stone cutting, shaping and polishing) for a hobby. He has some nice industrial diamond tooling for shaping stones which turn out to be just about the same hardness as tempered glass. Specifically, he has a motorized grinder/polisher somewhat like a bench grinder but it has diamond encrusted wheels the coarsest being 80 grit. I figured what the heck; let’s give it a go, the worst that could happen is we’d end up with a bunch of shattered glass. Well it turns out you can fairly quickly grind away tempered glass with the right tooling.

    This same window corner was also making for a clearance challenge when the window is partially retracted with that corner in the door latch area.
    My first thought was to use a narrow “bear claw” latch to avoid the clearance issue. This would work but I’d much rather used the wider OEM Corvette door latch because it provides for external and internal door lock to latch interfaces. It turns out that with the window corner ground away; the door glass now just clears the Corvette door latch. It’s tight enough that I think some thin nylon shielding might be needed to ensure no glass to metal contact but I think it will work.



    My final check was to mockup the door rear edge in cardboard.



    It’s looking good! I need to have at least a ½” flange on the door frame in order to clamp an aluminum door skin to it. I plan to encase the door frame in aluminum sheet and the flange will be part of that. So here’s my final check to see if there’s adequate space for the flange. It looks like it.



    More to come on door frame transformation…
    Joel Heinke
    Be original; don't be afraid of being bold!

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