TwinAir puts 2x energy into gearbox per thump compared to a four cylinder. Dual mass is there to give the gearbox an easier life. Replacing dual mass flywheels with older type solid can lead to gearbox damage. It's been an issue with diesels for some time so take care.
Quite a number of years ago, when the "new fangled" Dual Mass flywheels started selling in volume, and then appearing, failed, in the workshop, our factor started stocking and aggressively selling, Single mass conversion kits. I was very attracted to the idea of replacing these very expensive dual mass flywheels and, for a while, I would have recommended them to anyone interested. Yes they did make the car a bit more noisy and "rough" and sometimes changed the feel of the clutch pedal to quite a degree but, as long as you were prepared to put up with these small changes, you could save quite a bit of money and gain reliability - or so I thought. Early designs of DMF often failed frequently. They work much harder at low revs so can be expected to give more trouble in "town cars" and delivery vans - Transits seemed to briefly have a problem with this but they sorted it pretty quickly - when compared to a rep's car cruising the motorways.
Then the stories of failed gearboxes and even broken crankshafts started to circulate so I became even more interested in it all. There seems to be two really big problems with converting to a single mass flywheel from a D/M jobbie. One is the engine itself - we'll come back to that in a minute - the other being the gearbox. Invariably, but not always, the gearbox will rattle and chatter at low revs after converting to S/M (especially with the very low viscosity oils being used in todays transmissions) and this gives some idea of the increased reversal of forces going on with S/M. Some of the manufacturers have developed special driven plates with longer more compliant torque absorbing springs to try to control this aspect and they work tolerably well. If you go this route then if you need another clutch later on you must make sure it's the same make as a standard plate sometimes won't fit and even if it does you'll likely have other problems. In general I think you can expect to shorten the life of a gearbox if you change from a D/M to a S/M flywheel - unless you are doing a lot of higher speed open road/motorway cruising.
The engine, and especially the crankshaft, is a different proposition altogether. Most of us who have had engines in pieces will regard the crankshaft as a very heavy and robust piece of kit which would seem difficult to damage? What we perhaps don't think to much about is the forces acting on it. The connecting rods connect along it at different points and sometimes they are transmitting a "push" (power stroke) and sometimes a "pull" (the retarding forces in compression) and there are lots of "weird" twisting and standing waveforms trying to twist it apart. One reason you don't see much of straight 8 engines - longer crank to twist and break, and Ok, they are just long unwieldy engines anyway. A carefully designed D/M flywheel couple to a torsional vibration damping "nose" pulley goes a very long way to mitigating these effects. Take that very carefully engineered flywheel away and stick a big solid lump on in it's place and all these very carefully designed features disappear. You may then end up with a broken shaft - will you though? luck of the draw in my experience, some do some don't. More don't than do though.
Here's quite a good, reasonably non technical, feature about it:
https://www.eeuroparts.com/blog/single-mass-vs-dual-mass-flywheel-why-convert/ It's a topic which many people have opinions on and there's lots on you tube too.
For me though, I'd now always replace like for like. If it's fitted with a D/M flywheel as standard I'd be replacing it with one when needed. There's also a great controversy about when to replace it. There are special tools available:
https://www.ebay.co.uk/itm/115278120054?hash=item1ad71c8476:g:ac8AAOSwenRf3IQ3 which check the wear of an existing D/M flywheel so you can decide whether to reuse it or not. Personally, unless I was going to be selling the vehicle fairly soon, I'd be fitting a flywheel every time. It's going to cost the whole job all over again if the flywheel fails after just a year or so, which, in my experience, they are prone to do.
