Would it be better to group the valve as a single item? The valve’s so far are my main hold up in the production queue. It seems to always take long to order and deliver even with a stock pile nearby.
My plan is to split the engine making into its own line, so instead of splitting things into ‘fit valves’, fit valves’ will become ‘install engine’
install engine will require a new ‘engine’ resource
There will be ‘make engine’ which will require…
valves
pistons
crank shaft
And of course then we will have
make valve (already in the game)
make piston
make crank shaft
all of which take steel as an input.
This will allow you to build engines separately and avoid a bottleneck.
I think at this point the requirement for valves is too high and the stock pile doesn’t hold enough. If you could either raise the stock pile of the valve stage or lower the amount of valves needed, i think it will remove this bottle neck. Storage of a valve shouldn’t be an issue.
see attached for the valve values.
gyazo.com/8d5950bb456ed8ba34218c2efcff4a44
I see you will change it but this could be a quick fix potentially. 
I love this ideal. Sub-manufacturing items is a great ideal, and could speed up flow on the main assembly line.