Chassis Swap for MPC ‘69 Grand Prix.

[bisc63]

Since the trans cross-member needs to be replaced to a BOP type anyways, you may want to consider the Revell '66 GTO as a chassis donor. It compares favorably in its floor-pan stamping detail to online reference photos, but may need the rear pan shortened a little. The engine cross member is a separate piece, which might simplify modifying the front suspension also. You would of course still need to lengthen it just a little, I'd say in the front floor/firewall area, as it looks to me that the Grand Prix has lots of legroom up front.

[StevenGuthmiller]

47 minutes ago, bisc63 said:

Since the trans cross-member needs to be replaced to a BOP type anyways, you may want to consider the Revell '66 GTO as a chassis donor. It compares favorably in its floor-pan stamping detail to online reference photos, but may need the rear pan shortened a little. The engine cross member is a separate piece, which might simplify modifying the front suspension also. You would of course still need to lengthen it just a little, I'd say in the front floor/firewall area, as it looks to me that the Grand Prix has lots of legroom up front.

Just out of curiosity, I checked the GTO chassis, and this is my assessment.

 

First, the chassis is a little too long in the rear.

This would create a problem with shortening as the fuel tank would be affected.

The chassis is a bit too wide in the rear, which wouldn't be a problem, and a little too narrow in the center, which could be a little bit of a problem.

The biggest issue is the fact that the floor pan and frame are molded as one piece which would make it more difficult to stretch in my opinion.

 

The floor pan stampings appear to be relatively close, (at least the Monte's is close enough for me) but I could cut the transmission mount from the GTO and install it in place of the Monte's.

 

I'm beginning to get a picture in my mind for what will likely need to be done.

At this point, I'm probably going to go with the Monte Carlo chassis, and just to ensure that parts will mate more easily, I'll use as many of the Monte's parts as possible.

I will likely need to start with removing the rudimentary cowl vent section from the Grand Prix body and scratch building a new one as the Monte's is not the same as the Grand Prix's

The firewall from the Monte appears to be pretty much a spot on match for the Grand Prix's.

I'm thinking that lengthening the floor pan from the Monte Carlo may not be necessary, so lengthening the frame directly behind front suspension may be required.

I'll know for sure when I see where the joint between the floor pan and firewall comes together in the body.

 

Another issue will be the inner fender wells.

The front quarter panels on the Grand Prix are much thinner across the top than the Monte, so a simple engine bay swap will likely not be possible.

I will probably need to remove the fender wells from the Monte Carlo, widen them and then mount them into the correct position on the frame, so that when it all goes together, there won't be any issues.

There will be room to fudge position of the fender wells between the radiator bulkhead and firewall, but they will have to sit correctly over the suspension.

 

Anyway, I have time to work all of this out as it will still be while before my '68 Coronet is finished, but I wanted to get a jump on the situation.

I'm not even sure that the Grand Prix will be my next project.

I've been contemplating whether it will be the Grand Prix, a '64 Bonneville convertible, a '67 Plymouth Fury or possibly a '66 Bonneville.

Decisions, decisions.

 

 

 

 

Steve

[Oldcarfan27]

More Pontiacs, PLEASE!!!

[bisc63]

A lot to consider, for sure. Not having a GP body in hand, I just squeezed the GTO chassis into the Monte for a quick reference. The rear overhang is indeed the first thing I noticed, but it didn't look like a difficult chop. The GP really is kinda short in the rear, it would appear. I hope you can get on this one soon, it will be an interesting project of a subject seldom seen. Excellent detail photos at classiccars.com, including chassis and engine bay. Beautiful cars!

[Can-Con]

Well Steve, at least it's easy for you to get references when you own a 1/1.

I think a lot of people are missing that point. ?

[StevenGuthmiller]

10 hours ago, bisc63 said:

A lot to consider, for sure. Not having a GP body in hand, I just squeezed the GTO chassis into the Monte for a quick reference. The rear overhang is indeed the first thing I noticed, but it didn't look like a difficult chop. The GP really is kinda short in the rear, it would appear. I hope you can get on this one soon, it will be an interesting project of a subject seldom seen. Excellent detail photos at classiccars.com, including chassis and engine bay. Beautiful cars!

I will have all of the reference that I need.

I have one sitting in my garage!

 

 

 

 

 

Steve

[StevenGuthmiller]

2 hours ago, Can-Con said:

Well Steve, at least it's easy for you to get references when you own a 1/1.

I think a lot of people are missing that point. ?

 

 

 

Steve

[Bucky]

3 minutes ago, StevenGuthmiller said:

 

 

 

Steve

'69 and '70 were the best looking GP, in my opinion. My cousin had a '69 that was gold, inside and out. I tried to trade a newer Oldsmobile for it, but he said, " No way!"

Yours looks great in that color, Steve.

[bisc63]

10 hours ago, StevenGuthmiller said:

I have one sitting in my garage

That's just cheating.?

[Bills72sj]

On 11/3/2021 at 12:49 PM, Oldcarfan27 said:

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. 

You are correct on the appearance of the engine setback. On my 1:1 I have actually changed camshafts without even having to unbolt the A/C condenser from the radiator support. There is so much room with the fan, shroud and radiator out, you can climb in and sit on the sway bar to change the timing chain.

[Bills72sj]

Steve, To add some input on the engine bay I looked through some of my builds in regards to the firewall and cowl. The Monte firewall with the A/C evaporator box is pretty close. The top of the cowl has the 3 vents in approximately the correct size and shape but lack detail as far as the eggcrate vent pattern over the openings. With your skillset I am sure you could overcome that obstacle with some screen material. All my Chevelles of that vintage have the same lame, detail-less cowls. The 69 Olds 442 cowl has some grille detail in the openings molded in, but it is too fine and the openings are a bit too narrow.

On a separate subject, the A/C compressor from the Monte appears correct but mounts to the wrong side of the engine. This will make all the kit's hoses all wrong. But since they are just hoses, I am sure coming up with flexible substitutes will be no issue for you.

The large fan shroud will likely have to be made from scratch. The Monte shroud has too much ribbed, support detail and also appears to be too small in diameter. The fan shroud is a serious fail in all the MPC Grand Prix kits of that vintage.

 

The pics are from a gluebomb restoration of a 70 I did years ago.

[bill-e-boy]

Steve

Your real car looks almost as good as one of your models, lol

Looking forward to a build up of it and will follow along like I have with your other builds

Always inspiring

[Rodent]

18 hours ago, Bills72sj said:

On a separate subject, the A/C compressor from the Monte appears correct but mounts to the wrong side of the engine. This will make all the kit's hoses all wrong. But since they are just hoses, I am sure coming up with flexible substitutes will be no issue for you.

There was a little discussion of this on the "Models with A/C" thread. It might be easier to dig up the A/C parts from an AMT '68 El Camino. The compressor is on the passenger side and the rest of it, including the evaporator case, should be fairly close to the GP as well.

[StevenGuthmiller]

37 minutes ago, Rodent said:

There was a little discussion of this on the "Models with A/C" thread. It might be easier to dig up the A/C parts from an AMT '68 El Camino. The compressor is on the passenger side and the rest of it, including the evaporator case, should be fairly close to the GP as well.

Yup, this was my plan.

The engine will need to be a complete swap anyway, so unless there’s some other issue that I don’t know about, I’ll plan on using the belts and pulleys from the El Camino on a yet to be determined Pontiac engine. Probably the Revell ‘68 Firebird’s. 
 

Anybody have a suggestion for the best automatic transmission to use?

 

 

 

 

Steve 

[NOBLNG]

18 minutes ago, StevenGuthmiller said:

Anybody have a suggestion for the best automatic transmission to use?

Steve 

That would be a TH400 I imagine. The AMT ‘70 Monte Carlo one is fairly decent.

[StevenGuthmiller]

54 minutes ago, NOBLNG said:

That would be a TH400 I imagine. The AMT ‘70 Monte Carlo one is fairly decent.

Well, that’s convenient, considering that I’ll be using a good portion of the other parts from that kit. ?

 

 

 

Steve

[Rodent]

Kris Morgan makes a nice one as well, with a separate pan. It might be worth the few dollars to preserve the Monte's engine as trading fodder.

B.O.P. bellhousing bolt patterns are different from Chevrolet. I am not sure if you are that much of a rivet counter or not. This is obviously a manual bellhousing, but you can see that there is a huge difference in the shape vs. the Chevy. I don't have the Firebird, but maybe you could use part of its bellhousing with a M.A.D. TH400? If it's that important to you.

Edit: Also, Pontiacs have the starter on the driver's side, Chevys are on the passenger side. That affects the shape of the bellhousing area as well.

 

Edited November 6, 2021 by Rodent

[1972coronet]

The AMT TH-400 is a nice replica . My only complaint about it is that the oil pan is moulded-in , and is split with the trans' case .  

I'm sure that you could modify the poop out of it if need be , Steve . 

I second the M.A.D.  TH-400 . I have a few in my stash for conversions of a few of the Revell 396-427-454 engines .