Looking at boxing my frame in. Just wondering if anyone has done this and what needs to be done with the crossmember to get it to work. How are the convertible cross members different from hardtop cross members. Any help or pics would be appreciated.

 

Not sure what year you are working with, but therobski (Rob) 64 post granny build is a great one to look at. he did encounter on reassembly some areas that had to be cut out so read threw the whole build

Hope this helps

 

Let's start with the year and model.

If this is an A-body, do not assume that boxing the frame with flat plates like a street rod frame is what you want to do. On convertibles, GM used boxing rails with a C-shaped cross section. These are welded to the existing hardtop frame, open end to open end, to make a box. The additional C-sections are not constant cross section but are contoured to fit the floor pans. Depending on the year, there are either welded-on tabs on the inside faces for the crossmember, or the inside faces themselves are contoured with a flat spot for the crossmember to sit on. Convertible crossmembers are thus narrower than hardtop crossmembers. Olds did offer the convertible frame under hardtops as a HD Frame upgrade for much of the 1960s.

 

well said, Joe.

If you want to box your frame, then use a donor frame for the box section if at all possible. Use the convertible/ vista crossmember to match.

Otherwise, box in the open side all the way down with your ingenuity and plate steel to suit, and modify a crossmember to fit/ suit.

Cross sectional views

] std frame
[ ] convertible frame (I can't draw the top and bottom with ASCII)

DO NOT do this- which I actually saw done on a '68 once:
]|

They welded 3/8 plate to the vertical rib, all the way down the side. This added a great deal of weight and VERY LITTLE structural rigidity. Torsional rigidity is largely a factory of the area enclosed by the box. This is the goal of boxing the frame, mainly.

Example- paper towel tube- can you twist it?
Not really, w/o crushing the tube.
Now slit the tube all the way down, and note that it is exceedingly flimsy and can easily be twisted.

That is a good representative of the difference that boxing the frame makes.

 

Sorry for the lack of info. It is for my 1971 Cutlass. There are some bad spots in the frame that I need to repair so I figured while I have the frame out I want to box it in now while it's apart. Never saw how the convertible frames use the cross member if it bolts under or over???? I know that after boxing the frame that my cross member won't fit so Just wondering what anyone else did for this.

 

69-72 Pontiac Grand Prix's used the same boxing channels as found on the GM A body convertibles. Find a GP in a bone yard and cut out the center rails from the frame and then remove the boxing sections. I'm pretty sure the trans crossmember from the GP will work too. Just measure where the boxing channels sit to make sure you put them exactly where they should be on your Olds frame so the trans crossmember will install correctly. The channels have dimples in them so they self locate against your exist frame but measure the front/rear locations to get the positioned correctly. Youw ill need to remove the small vertical tab in the center of your HT frame side rails to install the boxing channels.

 

The 69-72 GPs (and Monte Carlos) used a different wheelbase than the other A-body 2dr cars. You will need to do some cutting to get them the fit (same as when using the Vista Cruiser boxing sections).

 

Sorry Joe but that is not correct. The longer wheel base was made up by the longer front section on the frame for the GP and MC. I've not found a MC with a boxed frame, just the GP's. On the GP frame, it is basically the same as an A body frame from the firewall back although the rear section of the frame may differ a little. The center portion of the frame is the same and I have done exactly what I proposed in my previous post. No cutting is needed.

https://classicoldsmobile.com/forums...project-2.html

Brian

 

Elcamino also has boxed frame, but has the longer 4dr and wagon wheel base.

 

Brian,

The 1969-72 GP used a 118" wheelbase, vs 112" for the A-body 2drs. I will agree that the firewall on the GP is further back from the front wheels than on A-body cars, so if the added wheelbase is all taken out forward of the firewall, where the frame kicks in at the front wheels, then yes, the side rails will be the same.

 

On the 70-72 Monte the additional wheel base length is foward of the firewall. The driveshaft from a 70-72 Monte Carlo is the same as a 2dr 68-72 Chevelle.

Interchange manual shows shared driveshafts between69-72 GrandPrix and 2dr Tempest cars so it seems like the additional wheelbase lenghtth will be foward of the firewall.


Henry

 

Here are pics from when I boxed the frame for my 69:
IMG_20131007_193342_351.jpg
IMG_20131007_205518_150.jpg
IMG_20131008_185846_955.jpg

 

In my 69 442 restoration thread, I needed to replace the frame in that car and I used a HT frame and made it into a convertible frame using the boxing channels from a 69 GP. So they are the same as the A body boxing channels. I speculated that the longer front snout on the GP and MC frames introduced more flex in those frames so GM boxed them. But then I noticed MC frames that were not boxed so that ruined my theory.

Someone needs to repop these boxing channels.

Brian

 

What that means is that the engines in the MC and GP were set back considerably as compared to the A-body cars. They were practically front-mid-engine cars. Of course, that explains why they had the world's longest fan shrouds.

 

Just remove the boxed sections from a convertible frame,or similar,and weld the to the hardtop frame.That is all GM did.TYhe boxed frames use a narrower crossmember,due to the frame being thicker.There are holes,in the boxed section,to mount the crossmember.

The ElCamino frames are the same except for the straight section behind the rear wheels.This can be cut & shortened fairly easy,as it is the open "C" section of the rear frame.

Since you have your frame out,and are boxing the rails,now is a good time to strenghen other areas with supports or gussets.These frames were not the greatest thing designed,even when they were new.You just need to make sure all of the suspension,exhaust,and body clear the modifications.

 

Found a guy here in Wisconsin with a frame from Montana. Little to no rust told me $800 takes it frame and what was left to the car. I think it was the whole body minus doors and fenders. Told him I would like to look at it before I buy it. He said that was fine so I lined up a day to go look at it and the guy tells me it's stuffed way in the back of his pole shed behind a bunch of stuff and he won't get it out unless I pay him $850 for it. I tell him we agreed on $800 on the phone then the guy blows a gasket and tells me he's not selling it then. So I'm going to just box mine in and fix the bad spots. After looking at the way the convertibles have the rails for the cross member I think I have a pretty good plan on what I want to do. Thanks everyone for the help.

 

So he lost a sale over 50 bucks he will most likely regret this when he gets 300 for scrap at best