Originally Posted by Stu @ M Developments

Over the years i have read literally hundreds of people asking why Cosworth specified a maximum head depth of 138.68mm and can they risk going thinner?

Originally Posted by Tony S Mannock

the valve seats are removed by heating the head to around 200 degrees C at this temperature they normally fall out on an aluminium head with the aid of a very expensive piece of equipment called gravity.
Valve seats normally fall out/ come loose for the following reasons
1 head has been run hot at some time in it's life
2 long term heavy detonation at some point in it's life
3 over bored throat during the modification process of porting
4 poor machining of the head at production
5 insufficient interference fit at production
6 4 and 5 after market valve seat replacement.
The interference for the seats on a yb head is 8 thou.the seats must be frozen and the head heated to install them.
skimming slightly below the recommended limit is not normally a major factor
the exhaust seat is more likely to give problems than the inlet seat, as the inlet is cooled by the incoming charge and is on it's seat during the hot exhaust cycle, unlike the poor exhaust valve