Chassis Swap for MPC ‘69 Grand Prix.

[StevenGuthmiller]

Hi folks. 
In the very near future, I’m planning on getting to work on a full detail version of the MPC 1969 Pontiac Grand Prix to represent my 1:1, and while the AMT ‘70 Monte Carlo is the most obvious choice for some of the guts, the wheel base is too short for the Grand Prix and will require a chassis stretch to fit.

While that’s not a big deal, I keep thinking that I might be missing something.

So, the question is, does anybody know of a modern kit with a detailed chassis that would be compatible with the Grand Prix that I’m missing, other than the Monte Carlo?

 

 

 

Steve

[Juanderful One]

MPC 1969 Olds 442.

GM A-bodies of the era used the exact same chassis components as the G-bodies (Monte, GP) of the period. The G frames are nothing more than stretched versions of the A frames.

[Snake45]

At 116", I think the Monte Carlo (or '68 El Camino) is as close as you're gonna get to the GP's 118. 

[StevenGuthmiller]

10 minutes ago, Juanderful One said:

MPC 1969 Olds 442.

GM A-bodies of the era used the exact same chassis components as the G-bodies (Monte, GP) of the period. The G frames are nothing more than stretched versions of the A frames.

Either way, it would need stretching. 
I was wondering more along the lines of something like a Riviera?

 

 

 

Steve

[SfanGoch]

1966-69 Rivieras had a 119" (119mm in 1/25) wheelbase. That extra 1mm is negligible if you want to use the chassis. I'd suggest the AMT '66 Riviera chassis since it has separate suspension and exhaust parts.

[Rodent]

Didn't the 66 Riv still use an x-frame?

[SfanGoch]

The '66-'69 Rivieras are X-frames, which wouldn't be accurate. The AMT '67 Impala (119" wheelbase) uses a box frame similar to the Grand Prix. Changing the front upper and lower control arms would be required, at the very least.

Edited November 2, 2021 by SfanGoch

[Mark]

I'd still go with the Monte Carlo, and stretch as needed, in an area where the frame is the widest.

[Can-Con]

1 minute ago, Mark said:

I'd still go with the Monte Carlo, and stretch as needed, in an area where the frame is the widest.

I agree. It would definitely be the closest.

[Mark]

I wonder where the 2" difference is on the 1:1 cars.  Maybe the rear control arms are different, moving the rear axle back?  Or is the frame just 2" longer in the middle?

[Snake45]

20 minutes ago, Mark said:

I wonder where the 2" difference is on the 1:1 cars.  Maybe the rear control arms are different, moving the rear axle back?  Or is the frame just 2" longer in the middle?

I'd be willing to bet it's just in the middle. Remember that at the same time, they were also making 112" frames. Simplest change would be in the length of the main frame rails. 

[64Comet404]

21 minutes ago, Mark said:

I wonder where the 2" difference is on the 1:1 cars.  Maybe the rear control arms are different, moving the rear axle back?  Or is the frame just 2" longer in the middle?

I believe most of the difference is found between the front suspension and the firewall. Monte Carlos nad the GP's were known for having engines 'set back' in the chassis compared to the other A bodies.

[SfanGoch]

Frame dimensions of 1967 Impala, 1969 Grand Prix and 1970-72 Monte Carlo:

1967 Impala

 

1969 Grand Prix

 

1970-72 Monte Carlo

 

The '67 Impala looks like the closest match.

[Plowboy]

I think the '70 Monte Carlo would be the best donor. Especially since you'll probably be using most everything from it. The 2" difference in the wheelbase can easily be fudged by simply moving the rearend back and lengthen the control arms. The inner wheelwells could also be moved if they look out of whack. As always, do a check fit first. The difference may be even less in scale. 

[Mark]

You sure that's the Grand Prix frame?  One of the measurements is labeled as being different for the convertible, but there was no GP convertible after '68.

[Mark]

Fitting the kit parts into a '70 GP body...

-'67 Impala frame is too wide, would require spreading the rocker panels.  A minute amount to be sure, but still should not be necessary.  Rear of frame hangs out way too far, rear frame rails (between rear wheels) are on the wide side and could make fitting rear wheels problematic.

'70 Monte frame is closer, wheelbase is short however.  I would make the stretch somewhere in the side rails, but would fit the Monte floor to the body and then fit the frame.

[1972coronet]

I've long been under the impression that the 1970-1972 Monte Carlo ( G-Body ) was based on the A-Body platform , with the Chevelle inner wheel wells being the exact same as the Monte's . I'm fairly certain that the 1969 Grand Prix was the first downsized G-body , with the the-new 'Carlo following for 1970 -- perhaps the Revell 1968-1969 Chevelle could work as well ? 

What plans do you have engine and transmission-wise ? Differential ? IIRC , the Pontiacs axle housing widths were Wide Track ; that the Chevrolet 12 Bolt would be incorrect (?) .

I know , I'm putting the cart before the horse . 

Edited November 3, 2021 by 1972coronet

[Plowboy]

Here's a visual for you Steve.

I would definitely go with the Monte Carlo for a donor. Personally, I would add the length in front of the firewall. That way, the floorpan and firewall should hit where they need to. If you center the front and move the rearend back, you'll probably have to add some length to the floorpan at the rear. Either method will work.

 

[Snake45]

Great pics, Plowboy! 

[Bills72sj]

As noted above 69-72 Grand Prixs had their own 116" wheelbase. 70-72 Monte Carlo had the 114" WB 4 door A-body frame. The Monte and the GP share the same roof skin which was later authorized for Oldsmobile Cutlass Supreme in 1972 (Maybe 71). The front windshield and all 4 side windows are the same as the Chevelle, GTO and their GM cousins. The back glass was unique.

Good donor kits with decent detail are the 70 Monte, 68 El Camino and 69 Olds 442 and Hurst Olds. You will have to figure out whether to add to the frame in front of the firewall or behind the rear seat.

I will be following this one with great interest as I have owned at least one 1:1 70-72 Grand Prix continually since 1982.

Below is a pic when I owned 3 at the same time briefly.

Edited November 3, 2021 by Bills72sj

[Oldcarfan27]

At the time, the 69 Grand Prix was said to have the longest hood in the industry. And since the main body shell/understructure was basically a Chevelle/GTO/Monte Carlo/Cutlass fuselage, the frame length had to be made up forward of the firewall, behind the control arms.

I had started a project using a 72 GP resin body and the aforementioned 70 Monte kit. After extending the frame under the floorboard to match the wheelbase (a very small amount, BTW) and using the underhood parts along with the floorpan of the Monte Carlo, I was able to get a presentable set up for the chassis. My stumbling block turned out to be the upper cowl-to-hood fit and the bottom fender liner/frame interference, which pushed the suspension down too far for a satisfactory ride height. So the project stopped. 

Steve, I would be excited to see you tackle the 69, as I have a vested interest in seeing how you deal with these issues. Can't wait to see you start this one!

I can post pics of my stalled project if you need them.

Edited November 3, 2021 by Oldcarfan27

[StevenGuthmiller]

Thanks for the input everyone.

I guess I suspected that the Monte Carlo was going to be the most likely candidate, but I thought that it would be good to pick your brains to see if there was a kit out there that I don't have and was missing.

 

I checked the frames on several cars, including the Monte Carlo and '67 Impala and found that the Impala was too wide and far too long in the rear, including a much too large gas tank, so that's out.

My inspection of the Monte's chassis leads me to believe that not only will the frame itself need a slight stretch, but the floor pan might need a little stretching as well, depending on where the firewall falls in the body.

It's pretty obvious when doing a side by side comparison, that the added length is in the section between the front wheel well and door. (front suspension and firewall) so in order for the patch to fall in the correct location, it looks to me as if the stretch would need to take place directly behind the front suspension, (red circles) unless of course, the floor pan needs stretching, in which case it could be done further back. (black circles)

 

I'm going to have to ruminate on this one for a while.

There are going to be a whole bunch of changes done on this model in order for it to match my 1:1.

My car is a "Model J", not an "SJ", so along with a couple of minor trim modifications, (ie, removing the "S" from the lower front quarter script) there will be major interior upholstery pattern changes.

I already began sanding off all of the vinyl top trim and detail some time ago.

 

It's going to be a time consuming project, but I really need to have a model of my 1:1 on my shelf, and if I'm going to do it, I just as well go all in.

 

 

 

 

 

This is where the frame would need to be stretched if the floor pan is not.

If the stretch took place here, there's a possibility that the tranny brace would also need to be moved forward.

 

 

If the floor pan needs stretching, it would probably actually make everything easier!

This is where I think the cuts and splices would be needed with a floor pan stretch.

 

 

 

 

 

Steve

 

 

[1972coronet]

Is the transmission crossmember / brace different between the Pontiac and the Chevrolet G-bodies ? 

You're definitely on to something with your proposed mods ( not that I've ever questioned your works ) .

It would seem to appear that the cowls ostensibly line-up between the kits ( just as likely , on the 1:1 examples as well ) . 

[Mark]

I would fit the floor first, doing whatever is needed to put both the inner rear wheel houses and the firewall in the correct places.  After that, stretching the frame behind the transmission crossmember but ahead of any rear suspension attachment points should be simple.

[Oldcarfan27]

6

3 hours ago, StevenGuthmiller said:

This is where the frame would need to be stretched if the floor pan is not.

If the stretch took place here, there's a possibility that the tranny brace would also need to be moved forward.

Steve, I think you just need to stretch the frame in front of the transmission mount and move the motor mounts back to match.

The engine setback on the GP is significant, look at the underhood especially the fan shroud. Quite a bit away from the radiator.