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About 1 dozen late model NT34s were built with the Mercruiser QSD 4.2. If you have this model you've discovered by now it's somewhat an orphan child. It's a great engine, we have over 1,600 mostly trouble-free hours on Adagio, but parts and service can be an issue. Case in point; I wanted to replace the thermostat as engine is always running too cool, except at WOT. Price from Mercruiser $320, price from a favorite NJ distributor $286, price right here in Tampa from a Motori dealer $130. These prices hold right down the line, even the oil filter is 1/2 what Mercruiser is asking. Last year I bought a number of spare parts from the NJ distributor. If I had known about the Motori angle, I would have saved $$$. The oil filter is a very common one, found on many German cars. Mercruiser wants over $50, I bought a case from the VW dealer for $12 each.

If you need service or parts the best source I've found to date is a dealer who services MV Motori engines. Motori is who built our Mercruisers. If you look at the Mercruiser serial # tag on starboard side it even shows model as MR706LS. The Motori dealer will likely ask for a Motori serial number. It is found on port side, stamped into the block, approximately at the bottom of the primary fuel filter. You may have to scrape away some white paint, but it is there. There is a good diagram in the Motori manual.

I've attached a Motori manual that is helpful for several general service items; filters, belt, engine fuses, bleeding fuel system, engine zinc location, etc.