Valve guide install problem

[bdonally]

3.2 Carrera, 175000 km
Valve guides were worn enough to merit replacement.
It looks like guides were original, but were treated to a copper insert sometime around 50000km - possibly to rectify the rapid wear or original guides.
New guide cracked the head.
Bore in head is 0.5121", OD of guide is 0.5151".
Head heated to 200F and the guide soaked in liquid nitrogen.

Couple of questions:
-is the intereference of 0.003" too much? I have reference to using that interference for pressing in - maybe the heat shrink causes more stress?
-is such a crack repairable? And if so, how?

Thanks

[racing97]

Yeah that is too much interference .0015 is all you want, if it has gone in crooked due to the interference and effort to put it in you are in a bit of trouble cutting the seat and it will just make things worse and you will have to weld it up and re-bore the guide boss. If you are relatively close to being concentric on the seat and the guide is in tight cut the seat and don't mess with it. It is my opinion of course but I have done it and it is not uncommon to see cracks especially when folks get a little carried away with slimming down the guide boss.
Just don't compromise the seat trying to make it work.

regards

[bdonally]

With the heating/nitrogen cooling the guide went in easily so I think it is square.

I've had some opinions about whether or not to do a superficial weld without removing the guide- not sure I see a lot of gain.

Its odd that there are instructions indicating 0.0025" or more interference for a cold press fit.