How hard (moreso costly) is it to put a modern drivetrain on a vintage mtb?

Right. Your best bet would be to go for a complete bike, pop a wanted ad as said. It'll be a lot cheaper to buy a complete bike than build one up. Less faff than moving everything over as well. Ease yourself into wrenching, get a nice serviceable bike that suits your needs and maintain it yourself, get tools as you need them and don't overthink it too much. You can also make a thread asking what to look for, occasionally there's good bargains posted in the Marketplace/eBay watch section too.

https://www.retrobike.co.uk/forums/mtb-ebay-marketplace-watch.4/
And here's the wanted section:

https://www.retrobike.co.uk/forums/mtb-wanted.3/
Throwing good money after bad is never the right approach. Impel cranksets are solid and I like them a lot, but if I were I'd put the money aside and look when somrtbing pops up on here in the 150ish quid bracket. Occasionally some real bargains cheaper too. What are your needs specifically?
Thanks. Yes as mentioned above, as I move soon I want to hold off on purchasing substantial stuff until settled at new place so just want to research and line my ducks in a row for the next couple weeks. Get a feel for my shopping list of parts I will want.
 
Thanks. Yes as mentioned above, as I move soon I want to hold off on purchasing substantial stuff until settled at new place so just want to research and line my ducks in a row for the next couple weeks. Get a feel for my shopping list of parts I will want.
Sounds like a plan. Depending on budget you'll be able to get something that meets your requirements quite easily. I see a lot of sub 200£ bargains on here from time to time. I'm not in the UK so I can't speak for the various marketplaces, FB or Gumtree, but I reckon there's a lot of good stuff there too. Certainly better than over here. When you see something you like the look of, you can always ask in here if people think it is a worthwhile investment. Building will always end up being more costly than a full bike starting out, until you've hoarded enough and amassed a big pile of ju....*coughs* a fine assembly of parts that certainly will come in handy someday.
 
If you use an MTB 5 arm 94/58mm crank set (Shimano M563, M737 for starters) then the 58mm inner will go as low as 20T. Fit a 28 or 30T on the back and you have around a 19" gear - which is all I need on a loaded tourer on Dartmoor.

If you have a 7s hub you can fit a 9 or 10 speed cassette, just leave off the smallest sprocket and run 8-from-9 or 9-from-10. Obviously you need a matching shifter and rear derailleur (7/8/9 speed derailleurs are interchangeable, despite what Shimano may tell you). 10 speed is totally different.
Oh that is good info thanks!

Regarding the second paragraph, won't the older derailleurs struggle with the huge 'dinner plate' sprockets of like 46+ teeth though?
 
Oh that is good info thanks!

Regarding the second paragraph, won't the older derailleurs struggle with the huge 'dinner plate' sprockets of like 46+ teeth though?
You won't find any 7/8 speed derailleurs that can handle a 46t. The cheapest *older standard* wide range derailleur you can get is the SunRace M9 for 9 speed. RDM900. 51t max. Also the only clutched mech that'd be compatible with the older spacing.

(I have got a m735 mech to clear a 46t with a goat link but my god it was janky)
 
You won't find any 7/8 speed derailleurs that can handle a 46t. The cheapest *older standard* wide range derailleur you can get is the SunRace M9 for 9 speed. RDM900. 51t max. Also the only clutched mech that'd be compatible with the older spacing.

(I have got a m735 mech to clear a 46t with a goat link but my god it was janky)
Ok, so would be looking at contemporary derailleur at that point?

Anyhow let me see how I go with 22 (or even 20)/34 first!
 
You won't find any 7/8 speed derailleurs that can handle a 46t. The cheapest *older standard* wide range derailleur you can get is the SunRace M9 for 9 speed. RDM900. 51t max. Also the only clutched mech that'd be compatible with the older spacing.

(I have got a m735 mech to clear a 46t with a goat link but my god it was janky)
Is this one you linked to: https://www.retrobike.co.uk/threads/sugino-impel-chainset.478531/

The same as this: https://bicyclist.cc/product/sugino-impel-350-chainset/

Asking as the second link states 58 bcd. That would be great as it would give the option of getting even a 20t as mentioned by @hamster in https://www.retrobike.co.uk/threads...ivetrain-on-a-vintage-mtb.484041/post-3625628

Probably not worth paying £23 (https://www.spacycles.co.uk/m8b0s11...ompact-58-BCD-(5-Bolt)-inner-20-22t-Chainring) for the 20t chainring just for an extra -2t.
 
Ok, so would be looking at contemporary derailleur at that point?

Anyhow let me see how I go with 22 (or even 20)/34 first!
Yes. And you'd need to replace the cassette to fit one of the wide range 9 speed ones they make. There's a lot of Chinese offerings for 8/9 with up to 46/51t but I don't really see any of this as an upgrade. The m9 isn't a very longevous derailleur. A 3x is vastly superior if range is what you care about. And low cost. To go the modern 1x route you'd end up replacing wheels and all. It isn't worth it.
 
Is this one you linked to: https://www.retrobike.co.uk/threads/sugino-impel-chainset.478531/

The same as this: https://bicyclist.cc/product/sugino-impel-350-chainset/

Asking as the second link states 58 bcd. That would be great as it would give the option of getting even a 20t as mentioned by @hamster in https://www.retrobike.co.uk/threads...ivetrain-on-a-vintage-mtb.484041/post-3625628

Probably not worth paying £23 (https://www.spacycles.co.uk/m8b0s113p906/SPECIALITES-T-A-Compact-58-BCD-(5-Bolt)-inner-20-22t-Chainring) for the 20t chainring just for an extra -2t.
58 bcd, yes. Didn't you say 22 would yield you a 16" low? Should be fine. But yes you have the option to go 2t down eventually.
 
58 bcd, yes. Didn't you say 22 would yield you a 16" low? Should be fine. But yes you have the option to go 2t down eventually.
Nice, so to confirm those are replaceable chainrings, not the rivetted cheapo ones :D?

I could buy this now and fit on my British Eagle to test it provided it has a square? I guess it would if it is a 3 part crank and that would have been standard around 2003?

I just gotta get a crank puller...
 
Nice, so to confirm those are replaceable chainrings, not the rivetted cheapo ones :D?

I could buy this now and fit on my British Eagle to test it provided it has a square? I guess it would if it is a 3 part crank and that would have been standard around 2003?

I just gotta get a crank puller...
Yep. Replaceable rings, fits on square taper, works fine with 113mm spindle when I've run it. Undo bolts, thread in puller, tighten it down to pull it off the taper. Insert new arms, put bolts and torque it down unto the taper. Job done. Obviously need to make minor adjustments to the front mech.

Oh and might need to size the chain accordingly.
 

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