Offered is our replacement for the Mercruiser 863903A03 Mercruiser Heat Exchanger - ID of appx 4" diameter with OD of about 4.125 inches.   This includes the heat exchanger, end caps with gaskets and the OEM seals that are used in the crossover.  This part is made with OEM equivalent materials and is guaranteed for fit and function. 

A while back we decided to include the OEM seals for the crossover.  The seals tend to take a set or get a little brittle over time and you could end up with a glycol leak where the heat exchanger presses into the crossover if you try to use the existing seals.   In addition, each heat exchanger is made with a fixture and checked for fit prior to shipment to confirm that there will be no fit issues interfacing with the crossover. 

There is a 3.5 inch version and a 4 inch version.   The change over from 3.5 inches to 4 inches occurred around the 2006 timeframe.   You may want to take a look a the pictures to see which one you have.  It's a simple measurement.  It is our experience that a 3.5 inch version cannot always be replaced by a 4 inch version.  There are special instances when it works, but for the most part if you have a 3.5" version, replace it with the 3.5" version.  4 inch replaces the 4 inch.   If you want to assure you're not going to experience interference issues, order the part you currently have on your boat. 

 Some details to note:   

• 863903A03 is a 3.5 inch part -   
  • You are reading the advertisement for the 4 inch model.
  • This is the link for the 3.5 inch model https://www.mrcool.us/863903a03-mercruiser-heat-exchanger.html
  • a 3.5 inch inner diameter part has an OD that is approximately 3-3/4" to the outer diameter.  
  • Inner diameter of the shell is very near 3.5 inches.
• 863093A01 is a 4 inch part
  • You are reading the product page for the 4 inch version.
  • A 4 inch part measures about 4-1/8" to it's outer diameter

Part Number that this part replaces:

• Replaces MerCruiser 863903-T, 863903 T, 863903T, 863903A01, 861945T
• Replaces Quicksilver 863903-T, 863903 T, 863903T, 863903A01, 861945T

 Engines that this part goes on:  

• 496 MAG (Base Model)
• 496 MAG (H.O. Model)
• 496 MAG EC (Base Model)
• 496 MAG EC (H.O. Model)
• 8.1L INBOARD (Base)
• 8.1L INBOARD (H.O.)
• 8.1L TOWSPORT (Base)
• 8.1L INBOARD (Base) EC
• 8.1L INBOARD (H.O.) EC
• 8.1L INBOARD (Inline)
• 8.1L INBOARD (V-Drive)
• 496 H.O. BRAVO