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Discussion Starter · #1 · (Edited)
Hi all,
About a month ago I decided to fix the driver side door hinge's bushings and try to eliminate the door droop that has plagued the car since I bought it a few years ago. This thread shows some of the work I'm doing. (Hope it's not too redundant.)

I wanted to be able to remove the door by myself, within the very limited confines of my one-car garage. So first, I built a wooden rig to support and lift the door out of the way.



I then sneaked it into the door, undid the hinges, and pulled it away.


Note the bed posts and casters I use to adjust the height of the wooden frame.

Removing the door from a car is not for the faint of heart, but this frame helped things tremendously, as I did not have to be so worried about dropping the door (and made this a one-person job.)

The top hinge is an earlier-model GTV hinge, even though my car is a '74. Not sure how this happened. But the car is a driver anyways, so I won't worry too much about that now.


I went with the "cut the pin through the hinge" method, and used a big sledge to remove the small stubs left inside the hinge bottom and top sections. The check strap is in very poor shape, but the rest of the hinge is in pretty good shape.

More to come...
 

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Discussion Starter · #2 · (Edited)
Check strap - what to do?

So, the check strap needs to be fixed or replaced. I found these threads to be very useful:
http://www.alfabb.com/bb/forums/spider-1966-up/67483-door-limit-strap-installation.html
http://www.alfabb.com/bb/forums/spider-1966-up/175998-those-****-door-check-straps-problem-solved.html

I ordered one of replacement straps along with replacement strap rollers (which were missing from my door), and lo and behold, it's a plastic-coated piece.


At this point, the leading options I was considering for dealing with the check strap are:
1. Just use the new plastic/metal parts and go with it.
2. Use rubber/soft strap rollers - the softer rollers provide some give, and make the door work OK.
3. Make my own check strap.

Being an engineer (and very dense and stubborn), I went with option 3 (the most complicated one...) I don't expect to save any money from this, but I figured I've always wanted to learn how to cast urethane/silicone parts, and this is as good an excuse as any to try and learn (to paraphrase Benjyboards from one of the other threads, it's all about the challenge.)


First, I made a silicone mold of the new plastic-lined strap (the "model"). Here's the box for the mold, with the new piece.


Then I poured the silicone onto the mold. Scary at first, especially for someone who has never done this before:


The next day, with a lot of trepidation, I pulled the model out from the mold. Not bad for a first timer, if I may say so... :)


Next, I cleaned the old check strap, and test fit it into the mold:


So far, so good. I am waiting for some urethane to arrive from the east coast, and I'll be ready to pour the cast (I am using a 70A durometer urethane.) I am hoping this will provide enough give while being firm enough to hold the door open. I suspect I'll be doing this after T-giving, but I'll keep updating this thread as progress (or disaster) is made.
 

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Discussion Starter · #6 ·
Another update.

I finally got back home from the holidays. Hope everyone had a fun, safe time.

So, continuing the thread:

First, I sanded and degreased the metal insert, in order to help the urethane stick.


Next, I primed it with two coats of Devcon Flexane FL-10 primer. Not completely necessary, but I want to avoid any potential rework if possible.


While the primer dried (30-45 minutes or so), I applied some Mann-Ease 200 release compound to the silicon mold. Not completely necessary, either, but it helps keep the mold from wearing out. Not sure if I'll need to do this again, yet.




The metal insert then goes into the mold:



Then, I mixed about 75 grams of Smooth-On PMC-770 (54 grams of part A, 27 of part B) with some added black colorant (6 drops) and some UV protectant (<0.5 grams), and poured it into the mold:


24 hours later, the new reproduction piece comes out:


A few more pics of the original model (the aftermarket piece) and my reproduction:





Next up, I'll have to cut out the excess urethane, drill the hole for the retaining pin (on the A-pillar side of the strap,) and the piece will be ready.

The urethane does provide some give, as it is softer than the hard plastic in the aftermarket piece. I also notice that the original metal insert core is much stronger (stiff?), and hence, the whole reproduction piece is more resistant to bending than the aftermarket piece.

I'll need to reinstall the door before I can tell for sure how well the reproduction works. But so far, I feel very encouraged.

More to come....
 

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Discussion Starter · #8 · (Edited)
Here's a mostly-final update on my adventures so far.

After the part was cast, I trimmed away the excess with a utility knife and some rough sandpaper (60 grit.) The urethane reproduction is in the foreground.



There was a bit of shrinkage in the silicon mold, which made the reproduction about a millimeter shorter than the original model.



I also used some nylon bushings (from the local True Value Hardware store) as a liner for the strap at the a-pillar pin.



I also used nylon spacers as rollers for the check strap mechanism. The rollers' outside diameter turned out to be a bit too small, so they don't catch the strap as well as I was hoping. But they are still functional. I might have to revisit them, though.



I then reinstalled the check strap and the newly-bushed hinge (bronze bushings) onto the door. I had to ream the hinge bushings a bit in order for the hinge pin to fit. Using a sledge hammer to press the pin into the hinge worked OK, but you can see the paint scarring on the hinge (I'll have to get a hydraulic press at some point in my life.) You can also see the original color of the car (which I think I like better than the current red.)



Next, I installed the door using the wooden frame/rig I built to support the door. The frame made this into a one-person job, even within the confined space of a one-car garage.


Here's how the hinge looks, once everything is installed and done with:


I did notice that, as you close the door, there is some interference between the check strap and some of the internals of the door. The sharp internals rubbed a bit of the urethane off from the strap, but it is still working OK. I'll know in six months whether the strap is holding up to use.

With the new bronze bushings, the door is much easier to close now (I still need to tweak the door lock striker plate a bit.) Which makes me a happier Alfa owner every time I get in the car.

In closing, refurbishing the door hinge bushings is very straightforward, although great care must be taken to protect the door (make sure you don't drop it!) Building the wooden frame to support the door made things much easier (especially when reinstalling the door back onto the car) and worth the extra cost.

Casting your own check strap can be considered a bit of overkill, but I had a lot of fun playing and learning with the whole urethane casting process. (I'll probably refurbish the passenger side strap soon.)

I hope this might help some of you out there (or at least, found it entertaining.) Now, on to the next project...



Enrique
 

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Looks like you've found the right solution! I wish I came across this thread 2 years ago, before I ordered those plastic straps which by all accounts won't work properly. I've tried molding some urethane rubber parts before using Smooth on products but found that bubbles form too easily in the material. Seems that a pressure chamber is required to eliminate this issue....did you not have this problem?
 

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Discussion Starter · #11 ·
Hi Rossano,

Yes, bubbling is an issue. When I sanded the bottom of the strap (to trim back down to size) I exposed bubbles within the urethane material. If you look closely in the picture below, you can see gray dots, which are really bubbles filled in by the sanded-down urethane.



The bubbles did not show up on the unsanded surfaces of the piece (specks on side of the strap are dirt/dust during sanding.)

I am hoping that these bubble defects (which seem minor) will not compromise the durability of the check strap. I will find out in a few months, I guess. (The check strap is not really safety critical, so I figured I'd take my chances...)

But in all honesty, I would not cast structural/safety critical urethane parts (engine mounts, suspension bushings, and the such) without actively trying to eliminate the bubbling - or at least, without a better understanding of this bubbling issue.

Non-structural stuff (firewall grommets, for example) would be fine, though.

(As you can probably tell, I am pretty ignorant about the finer details of urethane casting and production...)
:)


What kind of bubbling did you see in your parts? Was it similar to the bubbling in the picture above? I'm curious...


Hope this helps.



enrique
 

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One of those inexpensive hand vacuum brake bleeders might solve your problem. Mix the urethane in a jar with a lid and a hose fitting, then just pump a vacuum and wait a few minutes. Most of the bubbles should come out.

Robert
 

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Discussion Starter · #13 ·
One of those inexpensive hand vacuum brake bleeders might solve your problem. Mix the urethane in a jar with a lid and a hose fitting, then just pump a vacuum and wait a few minutes. Most of the bubbles should come out.

Robert
That's a great idea! I'll try that with the passenger side strap.

Enrique
 

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That's a great idea! I'll try that with the passenger side strap.

Enrique
Resurrecting an old thread here....but I was curious as to how the strap is holding out? I'm still looking at one my plastic repro strap and one rubber repro strap and contemplating the next steps on these.
 

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Resurrecting an old thread here....but I was curious as to how the strap is holding out? I'm still looking at one my plastic repro strap and one rubber repro strap and contemplating the next steps on these.
Found this thread and I am curious about the longevity of the urethane strap.

Please post updates :wink2: thanks
 
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