Extending kit car chassis

[CompoSimmonite]

I've got a Marlin Roadster based on a Marina. It's a SWB model but they also made a LWB which had an extra 6" in the cockpit area. The chassis is a simple box section affair (I think it's 4" x 2") and extra area is in a straight / parallel section. People cut the chassis in half and add the extra to existing cars so I'm considering doing the same to create better legroom.
So what is the proper way of adding such an extra section into a chassis ? I assume not a simple cut and weld in. Is a stepped cut the way to go ? If so does the step go in the top and bottom or sides ? Is it necessary to add any inner reinforcement ?

Thanks.

Paul H

 

[GordonC]

If its box section you could measure the inner diameter and see if you could get some box that will fit snug to sleeve inside it. I reckon that would be pretty strong if welded properly mate. Just my thoughts but I'm sure other people will have other suggestions aswell.

 

[mylesdw]

Cut the chassis, add the box section with a simple butt weld being careful to get all the dimensions correct and then plate across the join with an overlap. Vertical sides of the box section only so four plates in total. This will be more than strong enough.

 

[pedrobedro]

The Marlin looks like a heavy box section chassis. I would cut out the whole piece out replace it with a longer piece instead of bodging a section into the middle of it. You need to extend the centre tunnel and the propshaft, brake lines and wiring as well as the bodywork and chassis, it's not going to be a two minute job.

 

[CompoSimmonite]

The Marlin is going to be fully rebuilt. An engine swop (from 1800TC down to a 1275 Sprite lump), suspension raised (too low at present), new doors made with proper latches (Midget items) and suicide opening, Volvo 340 De Dion rear axle on original leaf springs, Sierra diff, narrowed tunnel, raised hood height (touches my head at present) new dash etc. It'll still instantly still be spotted as a Marlin as externally only difference will be door handle at different end !
I've been offered a brand new LWB chassis so if the price is right am tempted rather than stretching the existing.

Paul H

 

[hotrodder]

Best practice is to make diagonal or stepped cuts (not <> for the rails, we want the top and bottom welds to not be in the same plane) and avoid vertical welds with external reinforcement/fish plates. Pics are of round tube but the reasoning behind it the same...

 

[normspanners]

I agree with hotrodder 45deg cuts then let in a piece cut to suit personally for tidiness I would put the support pieces inside but the exact inside size of the box and just on the verticals, with couple of plug welds on each end, the supports could be welded into the extension then slid into the original chassis the 45's butt welded and then plug weld the end of the supports afterwards, that way it would ensure the chassis would remain straight and you would end up with no actual weld shear point.
A little tiding with a flap wheel and you would not know it had been modded.