Sorry I didn't get back to you on this one, last time I removed the head there was no damage and the engine worked perfectly after the head was replaced, untill today. Unfortunately this has happened yet again and this time I wasn't so lucky.

This is the inlet valve and guide from number 2 piston, as you can see the guide has been smashed to pieces, fortunately there was no damage to anything on the head.



The following two pictures are of pistons 1 and 2 after a bit of a clean. You can clearly see where the rest of the guide went, are these two pistons now ruined and in need of replacement? I would rather not do this if possible as the engine was rebuild only 2 months ago.

The head will be going in for new guides tomorrow and ill ask shop how the seats look

What is so special about bronze guides and are they suitable for a road car

Bronze guides conduct heat better. Hence why Mk1 Cooper S, Alfas, Fiats etc use them.
They are also a better choice for use with SS (uncoated stem) valves.

However there are downsides:
1. They close up a bit when pressed in, and MUST be reamed to size before use.
2. They can close up a bit after a few heat cycles.
3. They (allegedly) wear faster, however I've done 50K miles with mine and 1.5 forged rockers, all are still good.

For racing you are better off getting the original iron guides K-lined. These are a thin spiral bronze bush, you can run them closer clearance on the valve. They cost more than bronze ones but are worth it.

thanks Doc. i think i'll have to hunt down that black book. I have a few different manuals but not that book