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Old 03-21-2011, 07:40 PM
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'76 Grand Prix converted to convertible

my brother wants to remove the top of his '76 Grand Prix, it's clean and in good shape, I'm gonna help him doing it, any special reccomendations about stiffening the frame? I was thinking of installing an X seccion like the ones some convertibles had from the factory, also fully boxing the frame between the axles, what else will be needed? how about the door sills? should I reinforce them also?
thanks.

Augusto.

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Old 03-21-2011, 09:36 PM
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76 GP Ragtop

In addition, you will also have to stiffen the cowl and the 'rear cowl" between the trunk & pass compartment. You will have to stiffen it A LOT. Those cars were not all that stiff to begin with. Any one who has one of those with a sun roof or similar will experience the creaks and groans of the body flexing at every opportunity.

While it is a great concept and I can visualize how great it would look, I'm gonna go on record as saying that I predict that you will regret it later
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Old 03-21-2011, 09:42 PM
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I know the risk and I'm willing to take them for the looks of it, how would you stiffen the cowls? braces toward the corners or something like that?
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Old 03-21-2011, 11:16 PM
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76 GP Ragtop

I guess I'd be looking at doing a fairly large "X" from Passenger-side top of cowl to Driver side cowl bottom, tacked to the firewall, and vice versa, both front and back, just to stiffen it up for side-to side cowl shake. Then the angle braces to keep front-to back cowl shake under control. You WILL need to stiffen the frame a bunch as well, and likely also need to put braces from both front shock towers to center of firewall.
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Old 03-21-2011, 11:16 PM
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One thing that will be a big help is to box the open section of the frame between the front and rear factory boxed sections. The frame under the rockers is just an open roughly c-channel shape. In the 60's/early 70's convertables and El Camino's/Station wagons came from the factory with fully boxed frames to help prevent flex.

Polyurethane or solid aluminum body mount pads will also help a bunch compared to factory dried out/rotted rubber.

I've seen a mid-70's Monte Carlo(73 or 74 I think) turned into a convertable a few towns over from me at a car show a few years back. He used the top and mechanism from a '68-72 Chevelle and it looked very good.
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Old 03-22-2011, 05:46 AM
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Quote:
Originally Posted by Dave57210
I guess I'd be looking at doing a fairly large "X" from Passenger-side top of cowl to Driver side cowl bottom, tacked to the firewall, and vice versa, both front and back, just to stiffen it up for side-to side cowl shake. Then the angle braces to keep front-to back cowl shake under control. You WILL need to stiffen the frame a bunch as well, and likely also need to put braces from both front shock towers to center of firewall.
OK, I didn't think of side to side movement, now I understand the need for the X's you talk about, I was only thinking of twisting and front to rear movement, thanks for your help.
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Old 03-22-2011, 05:55 AM
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Quote:
Originally Posted by ericnova72
One thing that will be a big help is to box the open section of the frame between the front and rear factory boxed sections. The frame under the rockers is just an open roughly c-channel shape. In the 60's/early 70's convertables and El Camino's/Station wagons came from the factory with fully boxed frames to help prevent flex.

Polyurethane or solid aluminum body mount pads will also help a bunch compared to factory dried out/rotted rubber.

I've seen a mid-70's Monte Carlo(73 or 74 I think) turned into a convertable a few towns over from me at a car show a few years back. He used the top and mechanism from a '68-72 Chevelle and it looked very good.
that's what I was talking about, that C channel is very flexible, when I did the chassis for my '46 chevy using a malibu's donor car, I found that with only light pushes I was able to shape and align the frame, it flexes a lot, now you made me think I should also box the frame of my '46, that body is heavy.

polyurethane mounts is a good idea since the ones it has now are almost gone I can make them myself with the lathe, I've done this before using Grilon a hard nylon that is very machineable and long lasting, a few hours at the lathe is kind of fun.

I won't use any top at all, it's not a daily driver, I don't want the hassle of making a custom mechanism, besides it will look cleaner without the need of a partly hidden rag top. And finding a donor car will be next to imposible also.

thanks amigo.
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Old 04-09-2011, 09:42 PM
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ok, the top has been cut off, the car looks gorgeous, seems like pontiac screwed up big time by not offering it topless from the factory.

the frame flexes a bit when I lift the car on a 2 post car lift and the door gaps increase by something like 1/4 of an inch, I'm gonna have to stiffen a lot the frame.

I left 5 inches of sheetmetal from the top next to the windshield, I don't know yet how thick that section should be, I made the cut leaving some material to bolt the sun visors on, but it's too much, I may leave it like 2 inches or so.

since ther will be not ragtop I'm gonna build a reinforced sheetmetal rear deck, you know where the speakers go, that blends smooth with the trunk section, so people could eventually sit on it, the speaker will go on the side panels.

I'll post some pics soon.
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