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My name is XXXXX XXXXX Im happy to help / advise you ...
To be honest - I strongly suspect this would have been a failed ( early ) Dmf.. ( you say Smf... but its a dual mass flywheel... Dmf ;)
That would certainly cause your noise and clutch rattle issues - but - it wouldn't cause the engine to lock up when you turn it ! -- That is - if you are saying you are turning it with the Dmf etc stripped off... If its locking up in that situation - then I would be concerned about internal engine issues too yes..
Yes you can get a snapped crank - but that would normally result in a real heavy noise - inc oil loss - as it oscillates and lets oil out the seals - and to - lock up of the engine eventually when running - but it would be a heavy
noise from the engine when running..
I wonder if you have valve to piston contact - through a fault with the timing belt / gear... thats a possibility...
I say Im wondering - because obviously I cant be there to test / feel or hear it - so you are going to have to inspect these areas visually / manually..
As I say - Im saying the above because what you say about the engine locking up - you need to be sure of that really before much else - and that its not just compression stopping you turning it.. compression will stop it turning (by hand) and then as it escapes - you can then carry on turning... It could be just that ... in which case - no engine issues -and you are back to a premature failed Dmf.. To be honest - I think you will find this is the case.
So consider the above - and if you determine it is down to the Dmf you will have to complain to the suppliers - I know they are not favourable to Taxis.. but it would be worth a complaint.
I assume you ruled out any gearbox
failure issues ...
Let me know how you get on / if you need anything else-
Very best Rgds, MIKE.
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Best Rgds, MIKE.