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MG MGA - Fender Challenge (cont'd)

Well, I got the bent bulkhead out in pieces, straightened, and joined back in 1 piece. Removed all the bondo, beat out the PO dents, ground off all the braze and now doing a trial fitting against the door, which is my master template for the front and rear fender profiles. I got a reasonable match to the door profile, but I've got a high spot which needs shrinking, centred about 12" rear of the door and about 2/3 way up on the panel. I've tried a bit of heat and quench, but the magic doesn't seem to work for me. Can anyone advise me on how to do this? I have a MAP gas torch. I'm trying not to overheat, just heating till the colour darkens a bit and steam comes out of the quench. Also, anyone have experience using a "shrinking disc"?

Art Pearse

To shrink, you heat the spots to bright red hot, then quench. Or you heat the spots to red and then hammer it flat while red on a dolly, Only hammer until it is flat - no clink-clink against the dolly or you stretch it again. The hammering flat will drive the stretched metal into the panel (increasing thickness but reducing area), and the subsequent cooling will shrink it more. The heat and quench approach does the same thing - the hot metal is restrained by the cold metal around it, so it compresses into itself. Anything less than red heat just anneals the panel, which is good for rework but will not shrink it. The shrinking disc works the same way, by heating spots red hot while the surrounding area is cold. I've never used one, since I learned to do it the basic way. A problem with the disc is that low spots are stretched just like the high ones, but the disc does not affect them, because it can't contact/heat them.

If you overshrink, or make it flat, the subsequent planishing will restretch it a bit and recreate the curve it should have. But as I told you earlier, do not be surprised if you get cracks anyplace there was braze, as it is still in the steel.

FRM
FR Millmore

An addition to the above instructions: use a wooden mallet to do your hammering.
David Werblow

I think it would be worth trying out a shrinking disc. A couple of years ago I decided to make one from 16 gauge mild steel. It was 8” diameter and fit into a 9” grinder. I tried it out on a fairly large area of the body covered with lots of smallish high spots and it worked a treat. You don’t have to heat the high spots too much, certainly not glowing, and I usually stop when they go blue (about 300 deg C) followed by a quick spray with water. They say when you see steam after spraying you’ve got the temperature about right.

The beauty of the shrinking disc is that, unlike hammer and dolly working, there is no possibility of stretching the metal and you are only shrinking areas that need shrinking i.e. the high spots.

One disadvantage with a large disc is the weight of the grinder and the work is quite ‘hairy’ with the thin, unguarded disc revolving at high speed. Also access in concave areas (insides of wings) is limited. I recently bought a smaller 5” disc to use in my small grinder and although access to concave areas is a lot better it doesn’t seem to work as well as the larger disc and I think it’s because the heating effect is less due to the smaller diameter.

I have heard of body shop men using a worn out abrasive disc for shrinking which also might be worth a try ...............................Mike
m.j. moore

This thread was discussed between 19/08/2011 and 20/08/2011

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