Ask LRO: 16 tech answers
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QThe rear crossmember of my 80in Series I was replaced by a previous owner some years ago and is still good. But the last bits of the main chassis rails that join on to the crossmember are now looking rusty. Is there a particular procedure for replacing the back bits of the chassis rails? B Gifford, Cambridge
AIf the rot isn’t too bad, you could try to cut out the bad bits and replace just those sections. You’ll have to remove the back body to do a proper job, but you won’t disturb the body-mounting position (rear crossmember tabs).
If the rot is really bad, measure the position of the spring-mounting points – they’re saved and reused. Then cut off the whole of the back of the chassis and angle-grind the remains of the longitudinals from the back of the rear crossmember so it can be re-used. Reconstruct the rear sections of the longitudinals, lightly tack-welded for now.
Next, you have to get the rear crossmember tabs in the right place. You’ll have removed the back body anyway, but remount the body at its front end and wedge or prop its rear end until the back body is properly aligned, then bolt on the rear crossmember.
Now that the body and rear crossmember are in their correct positions, adjust the newly made sections of rear longitudials to fill the gap between old chassis and rear crossmember – you only tack-welded them, so they’re easy to adjust. Then, tack the rear crossmember to the longitudinals, take off the body, weld properly, and finally attach the spring mounts.
Don’t plate over rust – it festers out of sight. Cut repair sections to exact size and ‘let them in’ (held with magnets) so they’re flush with existing metal. Repair sections for longitudinal sides should have angled ends – that way, there’s a greater length of weld and a stronger join. It’s best to practice welding from underneath first before actually doing the job.
Oh, and one other thing – pull out the wiring before starting!
Rear of repaired 80in chassis, upside-down. Notice how the new metal is angled where it meets the old longitudinal