FIXED IT !!!
Today I finally got around to taking the (original Bosch) starter out, inspecting the flywheel ring gear and the starter pinion and comparing it to the used CAYC engine Valeo starter I bought on eBay back in November. (The MYP gearbox was originally fitted to the CAYC 1.6TDI engine in the 2011 Passat, possibly the Leon too). It was immediately clear from inspecting through the bellhousing hole that not only was the original starter pinion not meshing far enough in its direction of throw, but also the teeth of the pinion and ring gear were not engaging close enough in the radial direction. Please take a look at the pictures. You can see the bright spots where the teeth are touching and the brown dust / rust where they're not.
I tried to take some measurements but decided I didn't have the right gear to do it with sufficient accuracy in 3 dimensions.
The ring gear on the new DMF has been damaged, I am hoping not too badly.
Putting the new and old starter side by side, it was obvious that the old starter has a smaller diameter pinion than the CAYC starter.
Not so obvious, the centre of the pinion shaft seemed to be in a slightly different position vs. the imaginary line between the 2 mounting bolt holes in the casting.
I tested both starters on the bench. Although the direction of rotation was the same, t seemed as if the throw-mesh would only be 2-3mm more with the CAYC starter. I decided to try it anyway. The solenoid connection was different and I had to improvise a spade connector and tape it up to hold it together, also the solenoid sleeve didn't fit properly so I used several cable ties to hold it in place.
The engine started on the second compression, just like it used to before I fitted the MYP. Several more start-ups were just the same. And the cranking noise was really sweet, just like it used to be. Some air was punched!!
So, after so many blind alleys and so much thought and support in posts from so many members, I'm a happy man.
I wonder if the damage clearly done to the ring gear will be OK. Maybe the throw-mesh is still too small, maybe I will have to change the DMF again (shudder). Also I wonder if a starter from an AMF would have just slotted straight in and meshed really properly....
To anyone who has done the MYP conversion on their 90bhp TDI with ATL engine (at least OKO-Martin): if you didn't change the starter and cranking sounds a bit ropey or is becoming difficult, I strongly suggest that you consider sorting it out. I think your ring gear is getting shredded and you need a different starter (maybe one from a CAYC or maybe AMF).
To those who have done the MYP conversion on a AMF or BHC: which starter did you use, how does it sound and is starting fine?
All the best, and thanks again to everyone.
Matt
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