Having trouble getting a shimano bottom bracket to engage with the thread - and yes, I'm turning it the right way...
It's a plastic cup jobby, the old one goes back in no probs, which suggests the receiving thread is OK.
Any tips on making success more likely, or is it just bloody minded patience?
Got the right and left cups the right way round....? One is RH thread, and 'tother is LH thread.
both sides or just one side?
if one side is ok, put that in first, should help align the second side correctly.
Correct BB? it's not an italian threaded one is it?
I had this on an XTR, one side very stiff. Be v careful not to cross thread. My trick is to put gentle even pressure then turn backwards slowly until you 'feel' the place where the 2 thread tails meet, then start to wind in on. Takes some practice, but just the winding back thing can help make sure you dont cross thread
if you can get the old one in then you're probably turning it the right way which is always a good start 
give the thread in the frame a really good clean with degreaser and a rag and then inspect - doe sit look like there;s any damage? It's possible the old cup has damage too that means it 'catches the thread' further into the frame than the new one.
Check the threads in the frame once again and grease liberally. It can be a pain but patience usually wins out in the end. Also make sure that you have got the cups the right way around...I'm sure you've already thought of that but I have gotten this wrong in the past!
Other cup's already in...
Cheers - will report back provided I've not stormed off in a huff.
there's no particular reason not to just reuse the old plastic cup is there?
rapidly coming to the conclusion I've somehow buggered it up. Can't see any visible damage on the new thread, though.
I changed one the other day, and it took a while to 'Catch' the thread. But yeah, if the old one fits in fine and is the same, just use that.
not the same, unfortunately.
If still struggling get your LBS to ream it through, shouldn't cost too much
you haven't got an italian threaded BB and an English frame (or vice versa) have you.
do the threads on the two cups look like they 'do up' the same way?
Cleaned thread swith toothbrush & solvent?
Put old cup in to check the threads?
Hold the cup threads (old & new) against each other to check the pitch of the threads and their depth. Measure the OD of both too with a vernier/digi/micrometer.
New plastic part may have a fault moulded/machined in to it.
You might be able to increase depth of thread with a needle file or file the tops off the first couple of threads as a lead but be careful if it starts to tighten too much before it is fully home - you might want to remove it one day!
Good advice about winding it back until the ends of the threads engage before winding it in.
Have tried pretty much all of it now. No joy. Including with a different BB. The old one goes in and out so easily that I'm starting to think there's a subtle difference in thread depth, even though I managed to get the other side in OK. It's a Scott bike (albeit a kid's one) so I doubt it would be completely off the wall in terms of compatability.
Finally managed it...then realised that the double chainset required a shim on the BB..and of course that doesn't want to go on.
Now realising what a world of bodge this bike was even before I took it on. The double used to be a triple, and the inner ring had literally been chopped off, leaving stubs which, without a spacer, scraped the frame.
I'd hoped to slot on a new cheapo triple, but of course there isn't enough clearance for the inner ring, precisely what the previous owner found out..
I've now angle ground down these stubs of metal on the old double and there appears to be enough clearance to spin the old 'double' without the shim...
Is it worth trying to extend it out to fit the triple?
Or should I just chuck the thing in the nearest canal and run away?
is it square taper? You can just source a longer axle if it is.
If it's an HT2 chainset then you have fewer options.
It's 68x110 at the moment. Presumably I can go up to 68x117 or further?
110 is fairly narrow.
I may have something longer that will suit in my 'spares I won;t use again box' - what length do you think would give you clearance?
that's very narrow, though it can depend on the type of crank, but Id say 113 - 118 are typical widths for mountain bikes using Shimano Cranks
Shouldnt need a spacer unless its an E-type.
It's a cheapo Suntour triple crankset - wwaswas - thanks for the offer, I'd have to go and measure, but I would have thought that 117 might just squeeze it in.
Just had a look and I've got 3 113mm ones but nothign longer, sorry 
No worries - good of you to offer.
My old Suntour XC Comps from back when they made good stuff used a 68x116mm BB if that's any use(Shimano XT of the same era used 68x107mm for compact or 122.5mm for standard).
Thanks - that's useful. Although this clearly isn't from the golden age of Suntour...
The 110mm protrudes roughly the same distance from each side of the bottom bracket. Would a 118mm extend an extra 4mm on each side, or would there be a bit more on the chainring side?
they're always even - a 118mm would give you 4mm extra each side.
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