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MG MGB Technical - cracks in doors near vent windows
|Hi looking for info on new door skins and not having the Cracks come back Thanks Don|
|dab donald a baker|
|Weld a plate behind the panel.|
I have mounted Miata Mirrors on my doors (which are heavier than the MGB mirrors) and no cracks.
This crack is lovingly known as "The Crack of Doom".
Check out the following website for fix details.
MGBV81 is correct about welding a doubler in the crack area. The crack is caused by the vent window seal to the windshield being too tight at the top causing the door skin to flex and crack. Work-hardening is the term for this. Loosen the windshield bolts and try to increase the gap at the top. I had to relieve the mounting holes in the body slightly to get the clearance needed on my B. If you are careful one can do this with the windshield in place, but be mindful of the threaded casting.
|If that door skin came out of an aircraft lofting office the draughtsman would have been drummed out of the Brownies. The front part before the crack is stiffened by the quarter light assy and the way the door is continous to mount it, and behind the crack by the return flange where the weather seal rivets in place. Where it cracks a single skin is on its own and the stress concentration in this very small area results in a crack propagating through the skin. Weld a plate on as advised, I tapered the ends of mine to ensure smooth stress level changes.(i.e. a trapezium) I made mine about 6 x 3 cms overall 16 gauge I think, long time ago. Of course it fits with the short end at the bottom . On the n/s door I tapped the edges in and inserted a tapered shaped rib 3 to 4 mm at its widest, spotted it and filled over it, that has worked well from about 1995 up till now, but there it had cracked already.|
|If you follow that link above you will see that there was a service bulletin as early as 1963 for a fix and saying a production change was being made. My 73 came to me with a crack so it obviously didn't solve the whole problem. It seemed to me there was a weak point where the door skin turned round from the side of the quarter-light frame where it had a flange, to the front where it didn't, and the crack on mine had propagated from the point the flange ended. I extended the flange round to the front as well as using a stiffener behind the skin that the door mirror screwed into. Subsequently I saw a Heritage shell complete with doors that had the same mod, when that was done I don't know. The stresses come from scuttle shake waggling the top of the quarter-light from side to side over anything but perfect surfaces, and transmitting that movement into the opening in the skin. As yet another protection against it happening again I made sure the side of the quarter-light frame had a clearance to the side of the screen frame rubber, and the front of the frame only just touched to the rubber, to reduce even further the chances of the screen waggling the quarter-light.|
This thread was discussed between 06/03/2011 and 18/03/2011
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