...supposed to have copper washers under them?

I've removed the pain in the arse peg bolts from the rockers (to replace them with allen bolts) and they had copper washers in place which made me think.

Lenny,
yes, although they usually look more like brass washers.

The odd one out is the lower cylinder head stud, which has a (thick) steel washer.

ken

Smashing. Thanks very much.

and brass ones under the lower rocker shaft bolts you replaced ( even tho' they aren't cyl head nuts )

in situ by Sean602, on Flickr

This engines been in bits a few times before and the washers under the peg bolts were definitely copper rather than brass (along with no washers at all on the actual head nuts).

I've been and got some brass ones to replace them with but (apart from the slightly prettier colour) I wonder what the difference is and why one is fitted instead of the other?

case hardened steel bolt on a case hardened steel rocker shaft can cause galling the brass acts like a lubricating element. On the lower stud its on to the soft ally so a steel washer is used to stop the nut marring its seat.
copper is just too soft

So are the brass washers re-useable or should they be replaced like copper washers are?

You see I've only gone and mangled me bloomin pushrod tubes while banging the heads back on haven't I, so it's all had to come apart again.

Bugger.

re use
never bang anything
anneal copper washers to re use (heck i even anneal new ones)

push the head on by hand and watch the springs on the tube bouncing push it on as far as you can then spin on the bottom nut by hand then the top nuts - checking the pushrod tube is still free to move the nip up all 3
once its all snugged down centralise the barrel
fit the manifold nuts and bolts hand tight
first torque head nuts.
torque manifold nuts and bolts
2nd torque nuts

have a coffee/tea or beer

Thanks very much.