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Still waiting for my bottom end to come back from rebuilding, so in the meantime I'm thinking about the top end.

I've had many M20s over the years, some have had very quiet valve gear, some have been noisy. In general the lower mileage ones have been quieter but that's by no means the rule. Sometimes adjusting the valves improves matters and sometimes it doesn't.

So what's the secret to quiet valve gear, do I have to replace everything?

Perhaps wear on rocker arms where they move over the rocker shaft, and the rocker shaft? Sometimes you adjust valves real tight and they're still noisy, must be freeplay on the other side of the rocker I reckon?

OK I'll add a little background.

When I rebuilt the head (first time before it cracked) I used original camshaft with new rockers and shafts (pattern parts, believe they were Febi) . It was noisy as hell.

Second rebuild I used a matched set of camshaft, rockers and shafts that came with the head, still noisy.

Having sorted through all my bits I have the following:
Unused pattern Camshaft (manufacturer unknown)
Full set of pattern rockers (500 miles)
I don't have a pair of shafts so could buy new ones.

So for the third rebuild I could use the parts above which would all be new or close to new but all pattern parts. I don't want to go through this a fourth time, particularly as it involves removing the head again. Genuine BMW new parts are really expensive.

Check the eccentrics have not got worn flats they may cause some noise, you only mention the rockers not sure if you had new eccentrics in them.

The eccentrics are all new (pattern parts I suspect).

Really weird if you have a noisy valvetrain while pretty much everything is new, and while noisy valvetrains are pretty rare on M20 engines as far as I've experienced, a lot better compared to S14 engines for example.

The machine shop called me today and the bottom end is rebuilt, I'll collect it on Tuesday, I expect it will take me about 2 weeks to get the old engine out, build up the new engine and get it installed (mainly because I am old and decrepit).

The top end I'm going with new cam (genuine) new rockers and eccentrics (done 500 miles, pattern), new rocker shafts. I'll check everything carefully and see how it works out.

Just a thought, make sure you clean out the cam spray bar, lack of lubricant will make the valve gear noisier and let it wear more.

I'm thinking of a new spray bar as well just to be belt and braces

And install the excenters the right way around. Wrong way installed excenters ruin the valve stems.

Install the eccentrics the right way round, could you explain this to me as I didn't know there was a right/wrong way?

Check out this topic i wrote about the excentrics a few years ago:
http://www.bmwe21.net/forum/viewtopic.php?f=61&t=23266