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OMC did make a casting change after the initial run of the 56 cubic inch production [sand cast block]. There is a hole in the inside water jacket to improve the water flow around the middle cylinder. IT IS A BETTER BLOCK! The T850 guys would appreciate some one posting a picture of this block. I don't believe you can add this hole...you need this block.

Don, can you explain what's going on in this image?..

Don, can you explain what's going on in this image?..

Matt, that is the second version of the 56 sand cast or production block. That is NOT a reworked block according to the owner. The factory was trying to solve the sleeve twisting/turning problem with the number 2 cylinder. The sleeve would turn making the intake ports smaller because of the aluminium casting blocking off part of the port. The closed port caused the motor too run lean in that cylinder and blew. This change provided better cooling for the number 2 cylinder and stopped the issue. The one with an * is visible when assembled .

ANY 56 OMC motor running on a T boat has better cooling because of the boat running much faster than a normal family boat so there is more water flow. Many motor re-builders heat the block and remove the fishing sleeve in one piece to insert a new SST60 sleeve. I would bore the original sleeve out leaving a few thou of the original sleeve in the block, clean up the block and then insert the SST60 sleeve.
The sleeve fit will be tighter in the block compared to the original. I would then check the new SST60 bore for size and straightness and bore it to make sure it was perfect.

This is a lot of time and money in my view for little or no gain. A race prep foam cast SST 60 will be 98% as good compared to the perfect reworked 56 above. AND a well used stock fishing 56'r where the sleeves have not turned will work just fine going over size.
BUT YES, HAVING THIS BLOCK IS A BONUS! Looking at the picture this block looks like a replacement block.