Ask LRO: 16 tech an­swers

Un­ri­valled ex­per­tise at your dis­posal

LRO (UK) - - Contents - Peter Galilee

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

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