I put a new piston and rings in my B31 as the existing +.020 bore seemed OK. I was disappointed with both compression and performance and had a look inside after 100 miles and found that the rings were not making good contact all around the bore. I managed to find a NOS +030 Hepolite piston and rings and had a rebore. Compression was improved a bit but now after about 700 miles the rings are bedding in and compression is about as good as I remember it should be from my youth (allowance made for some weight gain in the intervening years). I assume your valve lifter is properly adjusted.

Dennis, I have tried on two occasions to respond to your PM but it does not appear to have worked. If you want to send me an email (martin2555mtl@gmail.com) I will be happy to respond.

Thank you for those replies which are quite reassuring - which I suppose is what I was hoping for.
In response to some of the comments:
I was imprecise about the oil changes - I think there were probably three upto 450 miles and the sample shown is 50 miles after the last change.
I do have a magnetic sump plug fitted and there was a small amount of black gunge attached to it after the last 50 miles which must be ferrous but I am reassured that the consensus is that these are quite dirty engines, particularly when running in. I will be changing the oil quite often and may see some improvement after a few more miles.
Several decades after my last B31, and more recently being more familiar with FireBlades and R1's, I find myself constantly having to recalibrate my expectations of B31 engineering and performance!
 

It's a 5/8" 20 tpi thread, the same as the the cylinder head bolts (in the crankcase). If you have a spare/scrap cylinder head bolt just tap a thread for a 7/16" BSF or whit bolt in it and you've got a mag pinion puller - see photo.

I would try tapping a thread in the bore and draw it out with a short piece of tube (I/D a bit bigger than oil pipe O/D), a bolt with sufficient thread length and a crossbar or washer and nut. A bit of heat may help.

I'm afraid I did not pay close enough attention to your original reply Rog. The M registered but not the 33, I was thinking 20. Your gearbox will be the same as a B33 which has the same cogs as a B31. So if there's any M20 gearbox experts out there .......

I replaced the rocker oil feed banjo bolts on my B31 recently with the correct 65-317 items which have a 1/64" dia hole. The drain pipe bolts have 3/32" to 1/8" dia holes, ie many times the area of the two rockers combined.

Rupert, you think that the taper/keyway are the same as the M series gearbox mainshaft, in which case primary chain alignment would appear to be the main issue. Do you know by how much inboard or outboard your taper is compared to the M series?
I fitted a Triumph-type clutch to my B31 M series gearbox and only to find that the clutch sprocket was 0.2" outboard of the engine sprocket (maybe I have a 1946-48 mainshaft in it - I don't know if that's possible).
If yours is the same issue let me know and I'll explain how I resolved it - I wont bore you with the details if it is not.