Originally Posted by
Roelof
There is a purpose.....
At maximum pinch there is also a maximum compression. When the engine is broken in and the pinch is (almost) gone the piston is actually floating on an oil layer in the sleeve. The huge pressure of the combustion can push some along the side of the piston so there is a tiny compression loss. That can be compensated by removing one shim..
Carbon build up on the top of the piston could also be "compensation" for more compression. That dosent mean if you dont want to clean the build up off the top of the piston, you have to ADD a shim either. If you have to remove a head shim to make up for power loss due to a worn sleeve, (in the end thats what your saying to do) then you would HAVE to run a lower % of nitro and change to a lower heat plug. If you dont you could have some pre-det happening. You dont want the top of the piston to start to pit. Not to mention the unnecessary stress and wear you will be putting on the rod and wrist pin. In the end it's beter to have the sleeve pinched, not remove a shim on the same plug and fuel.
Originally Posted by
Roelof
But it is all about how the engine is configured at the start, some engines have that extra shimm from the start, some do add an extra shim in the box, some do advise to use an extra shim and others do not anything.
Now, if there is a engine out there that requires you to remove a head shim after break in, thats a first to me. I have never used one.