Rusty frame crossmember replacement

[ymatt]

Hey all. Greetings from the old country.

With the last few days exhibiting the first signs of decent, dry weather since last August, I decided to make the best of it and head out to resume work on the truck. What a difference some sunshine makes to the usual gloom... the air begins to smell good (except when being passed), and you begin to emerge from hibernation mode. Anyone in Arizona want to do a good deed and adopt me?

This post would probably be better off in the 73-91 K5 forum, but I don't seem to have the required permissions to post there anymore. Anyway, here's my problem:

I have a '78 K5 (extended front floor). The third frame crossmember from the rear is very rusty (the one underneath the front part of the rear floor that's in line with the spring eye and middle body mount). It's about 1/3 the original thickness in some places. I have already blasted part of it, but as I cannot completely remove the body from the frame, some parts of it are very difficult or impossible to get at, especially the areas behind the two side braces.

Apart from it being a VERY time consuming and awkward process to blast the crossmember, I'm not happy with the weaker, thinner metal that is left, particularly for a major structural section like this. I figure it would be much easier to grind the rivets out, and replace the crossmember with a new one. Has anyone already done this? If so, did you fabricate, or is someone out there selling a bolt-in replacement?

I would weld something together if that was the only option, but I want something that is at least as effective and strong as the original, and I have zero knowledge of chassis structures. Would anyone care to share their thoughts? I notice these frames flex a lot (don't ask me how I discovered that ), and in my book, that's not a good thing.

 

[dremu]

Parts like that are not made aftermarket, and it's highly unlikely you could get a new one from GM.

If you were on this side of the pond, it would be easy enough to get one from a truck being parted in e.g. Arizona, and bolt it up through the old rivet holes.

Given the stupid shipping costs across the pond and the relative lack of these trucks over there, I think fabbing your own makes sense. If it were me I'd start with 3/16" wall square tube, maybe 2x3 or 2x4... but others may chime in as to details.

-- A

 

[ymatt]

Thanks very much for the reply. As you say, these trucks are rare over here, and rust free ones even rarer, so sourcing a part here is not an option. I've shipped quite a bit of stuff over in the past using USPS (usually economy), which wasn't too bad, but they've recently increased their prices significantly. The worst bit though is when you get hit another 30% on top of the lot by customs. Legalized theft.

Can you (or anyone else) give any guidelines as to how I should build the crossmember? Should I try to copy the original as closely as possible, or are there improvements that could be made? For a start, it looks like there is room to center it a little more, without hitting the prop/driveshaft. Any Colin Chapman type chassis experts out there?

By the way, I love your 'brochures' site; I first came across it years ago. Very cool!

 

[dremu]

Oh lordie ... the USPS. They're a frighteningly psuedo-governmental thing -- their web site was initially "usps.gov", but they masquerade as a private company (thus "usps.com".) If so, they're the world's LEAST profitable private company, as they're largely government-subsidized, even with postage prices as they are.

That said, I can drop something small like a DVD into a box and have it on the other side of the country in a couple of days or the other side of the globe in a week, and not have to pay Fedex or UPS prices... so we can't complain. The USPS must move *billions* of pieces of mail six days a week, and it's almost invariably on time and on target. ::shrug::

Thanks for the good word on the Brochures site ... I really need to finish the search thing, maybe Googlify it... and there's always more brochures to buy -- it's a labor of love =))

Anyway, on the subject. Note I've not had to remove the body from any of my trucks, or do frame repair, so I'm largely talking out of my butt.

Depending on the level of wheeling you're gonna do, i.e. how much abuse it's gonna see, I would first design the thing to fit easily into the place of the old one, and be simple to fab.

If you're really gonna be flexing the thing, you might over-design it some ... though my concern there would be that if this guy doesn't flex the rest of the truck will prolly take more abuse... and then you end up replacing all the cross-members and/or the frame rails and suddenly you're building a totally custom buggy. (The 'creeping feature creature' is not limited to the software world!)

-- A