has any one replaced the neck in car?

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It's easy once you know how.

You should just need to resolder it as its leaking around the bottom of the neck yeah? Through perforations in the solder?

I have done this in car with a butane burner and some solder. Often just having it re-melt is enough.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

If it's damaged inside the neck, pull the rad out and take to a rad repair shop. They have the necks and gear to do it properly.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

it has just come out from one and the cracks were not spotted.
a heat gun works for removal of a filler neck in about 5 mins.

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It's easy once you know how.

If they repaired the rad and missed that, I'd take it back and complain.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

this crack has a brother on the opposing side

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It's easy once you know how.

Those cracks wont actually affect anything unless you're running an expansion tank, they may leak if the radiator is over filled and overflows, but the overflow pipe does that anyway. Under normal use without an expansion tank the radiator seals at the ring at the bottom of the neck.
If you still want to change it you need some radiator solder and flux liquid (usually obtainable from radiator repair shops) and a decent butane torch or an oxy acetylene set.
I guess a heat gun would be fine if it can get the neck to about 400 degrees C.

It's pretty easy to do, I accidentally took mine off when I replaced my broken overflow pipe. (I'm using an expansion tank)

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The fleet:
1949 Fiat Topolino
1965 Morris 1100
1966 Mini Van (project)
19?? Morris 1500 (for sale)
1979 Chrysler Regal
1990 FSM Niki

If you're getting a new neck fitted, get a normal height one, so you can convert to closed system.