Chassis repairs/ welding KW125

Guys, looking for some guidance here. On my 1980 KW the chassis rails are badly rusted around the bogie drive area, no problems from the tanks forward. I would like to either replace the rear end with an air bag cut or maybe just replace the rails and cross members with some new steel. I might add i work for a large fabrication shop, so i do have a grasp of steel grade, weld prep, heat treatment etc.
The plan was to do the spliced cut and alignment myself, but get a someone in with a good size welder to to do the actual welding of the joint. I spoke to Kenworth, they advise that due to heat treatment no cut or weld was possible, i.e new rails required.
From my perspective any steel can be welded given the right pre and post treatments, however i cant seem to get any traction on the engineering standards to apply.

I My previous bus/ motorhome, (1986 model) i was able to contact the manufacturer and they provide an engineering bulletin on what could be done and what could not be done, so chassis repair was easy.

Trucker repairers must do chassis repairs occasionally and cuts off surely get attached, is it an independent engineer report i need? any suggestions welcomed.

I'll stick this here and let you intepret what they say.

You could possibly take to India where they do that kind of repair all the time and could be organised to have it done while you wait.
Sorry, but there must be a valid reason as to why they claim not to de the type of repair you wish to do. There must be something in the grade of steel plus the type of repair that they feel it is unsafe to do. I would guess that an Independent Engineering that is recognised by the Authorities in Your State, RMS here in NSW, will have to pass the repair. Try looking up the list on Engineers they recognise and contact one or two to get their version of what can and can't be done.

More than 2 ways to skin a C@
not recommended for a truck to go onfull work or for sale
slide all the crossmembers and supension out
fit a chassis insert where bogi or rust is
jack chassis out to make larger n easy to slide bits back n slide bogi n cross members back in
run drill thru existing holes
= Dun Deal
this procedure is not recommended
cya

Been following this one, have seen many conversions, albeit probably some years ago, where the chassis itself had been extended.
Most likely short chassis P/Movers blown out lengthwise to make rigids.
Just reading whats been written, if I understand this correctly, this is no longer allowed?
Only way seems that you have to order the correct length chassis, completely swap the components onto the new chassis?
Or, have I got that wrong?
Used to be allowed to insert a section of chassis rail, usually inside the existing one, which had been cut at an angle and rebolted with hi-tensile bolts and washers, even seen it done on double skinned chassis. Non-welding I could understand, once the specially heat treated steel was hit with the welder, affects the integrity of the chassis rails? OK so far?
Well, what about where the original chassis rails were RIVETTED together, be it crossmembers or even double or triple skin chassis?
Once you drill the original rivet out (and they seem to be mostly soft material), even if you replaced those original ones with hi-tensile bolts, lock washers etc, wouldn't that affect the integrity? Even though you weren't heating any components?
I have done a couple like this (NOT chassis extensions, but replacing rivets with hi-tensie bolts) mostly old Jeeps. Both of them had to go through an engineers inspection and nothing was said. However, this was a while ago, maybe the last one about fifteen years or so.
So, has the rule book been re-written?
Dave_64

Hi Dave,
I can't see any problem drilling out a rivet and replacing it with a nut n bolt then tensioned. That process is not introducing the kind of heat that welding does.
Now not being a meatalurgist I would imagine thatthe proceedure taken to heat treat would change the molecular structure of the steel and may put the molecules all in a line for its strength. Introducing hi temps in a localised area as welding does would alter all that original process. Just my uneducated guess here.

I would guess that an Independent Engineering that is recognised by the Authorities in Your State, RMS here in NSW, will have to pass the repair. Try looking up the list on Engineers they recognise and contact one or two to get their version of what can and can't be done.

It would be a gutsy move by an engineer to approve something that is opposite to what the manufacture prescribes.

I remember (long before I had any interest in heavier trucks) often seeing the sticker on the chassis of many trucks saying roughly: "High tensile - do not weld"
This went along with keeping an eye out for 4-digit trailer serial number plates (can't remember what brand of trailers that was, but most had 5 digits)

Jarrod.

wrote:

I remember (long before I had any interest in heavier trucks) often seeing the sticker on the chassis of many trucks saying roughly: "High tensile - do not weld"
This went along with keeping an eye out for 4-digit trailer serial number plates (can't remember what brand of trailers that was, but most had 5 digits)

Jarrod.

As long as I remember, every truck with a high tensile chassis had those stickers, regardless of make. How many of us have heard of a truck being either defected or refused rego because of chassis modifications? How many of us can PERSONALLY recall seeing an altered chassis breaking? Even dealers would do mods to suit a potential buyers needs. As for welding, even cutting the back off a chassis with gas would have some affect on tensile strength.

If done properly and finished well, it should be impossible to tell a chassis has been altered.

I think that the deal with Kenworth is that in order to save weight but still have a chassis that is flexible and resilient they have gone for a steel that responds to post forming heat treatment. In other words what ever you do you won't be able to achieve a smooth transition between old and new.
Cam (who is well qualified to do so) did a post on here a while back on stretching his Dodge 700 series and talked about merits of straight vs slanted cuts and how to go about welding.