My strainer only has about a 7mm hole in\out due to the type of hose connector. If this was increased would this cause a problem ? Engine is a Yanmar 3gmd
Not sure what type of strainer you have, but the raw water hose for the Yanmar would be 19mm and if using a Vetus type strainer the spigots would also be 19mm so there is no reduction in flow. 7mm sounds very small. Even the little 1GM engine requires 15mm.
When I re-engined froma Bukh 20 to a Yanmar 3GM30 I left the original inlet seacock for a season. This was 1/2 inch rather than the preferred 3/4 inch. The engine ran hot and seemed rather rough. Changing to the correct size made a big difference and the flow rate from the exhaust appeared doubled. 7 mm bore is very small and I doubt that the engine will receive sufficient coolant through it.