Juste received the first batch od Shimo3 engines and indeed this is how it looks like.

I wonder how well this works.....
There is already a 1mm space on that gap where the shim needs to be placed. The space is needed to give the manifold some flexibility, not only the movement but also to deal with a slight different mounting angle. Not knowing how thick the shims are but the first will for sure not give an extra length as the shim suggest because the 1mm gap.
The shim will also loosen the sealing on the large ring of the coupling rubber.

2mm of material has been removed from the crankcase.
Shims are 0.5mm thick.
When you mount without any shim it is like you have a shorter manifold.
2 shims are supplied.

Oh wow, that is a change!
So basically the shims will put the manifold on its normal length. But still the sealing will be more critical if the manifold is not pushed to the large ring of the coupling rubber.

Helped a friend run in a Shimo 3. Put 1.5ltrs of 25% (12% oil) thorough it . Opened the engine afterwards and the piston was scratched.

It's now hard to start and idle tune changes wildly as the temp comes up (we preheat the engine).

Then I heard that a friend has 3 2106 engines with scratched pistons.

I was close to buying the Shimo 3, but it seems the break-in problem is still there.
What were the running-in conditions like, was it cold outside?

1.5 liters isn't much fuel through a fresh engine; all these 9 port OS engines seem to require a perfect break in that includes good fuel with plenty of castor as well as the proper temperature. I am still in the process of breaking in a 2105 but as we know the Beautiful polished cooling head gives false readings, I have replaced it with a 2102 head to help monitor more accurate readings..So far I have 2200ml through it with 11% oil package. 7% Putoline Castor R (My break in Castor, run in castor is Blendzall Green) and 4% Motul Rc1 after I did an oil bath on it. The engine has been in a buggy with a 16T bell to help keep rpms down but knock on wood no scratches yet. Once I get to 4000ml I will put a 13T bell on it to finalize the break in process by turning some rpm while being in the temp ranges of 220-240F. This is my first OS 9 port, so I am being overly conservative with. I can't help to believe if you run the best fuel (castor heavy), monitor the ACTUAL temps, keep the rpms in check and have a tune that is conducive to a well lubricated engine while retaining temps and altering/varying rpms you will end up with an engine that will accept a "race tune" without damaging the piston via cold weld spots .... I think and hope lol

Any on track feedback on this shimo 3 compared to 2106? And also comparison to previous shimo 2.

I did hear the Shimo still need a long break in. In the last week I did break in my 2106 and after about 5 hours oilbath and about 800ml fuel on the track it looks broken in. I will do 2 or 3 tanks carefully next time before going on full race tune. The 5 port R21 also took 5 hours oilbath and about 600ml fuel to be fully broken in.

Side note, I also have an Hasi set in an R2105. That one took more than 6 hours oilbath, has run 2 liters to break in yet and is still not broken in but it is almost there, maybe an half liter more.....

When they all are broken in I will inspect them.