Rebuilding an engine can be a very simple or very involved operation. If the crankshaft bearing surfaces are still round, it's just a matter of replacing old bearings with new. The piston bores will probably need to be cleaned up which will usually involve new pistons. The head
may need to be resurfaced if it is warped. If not, it's a matter of doing a valve job and replacing the valve guides. Rocker arms, cam, etc usually don't need replacing but can. Rebuidling an injection pump is costly but doesn't always need to be done. Until you or the mechanic
tear the engine down and check all the bits, the cost of a rebuild can't be determined but certainly is not a cheap
operation. If done properly a rebuilt engine will run for a very long time though probably not as long as a new engine. Most sailboat engines fail from neglect and disuse than running too many hours. Many of the parts
that should be replaced in a high time engine may still be in excellent shape on a relatively low hour sailboat engine.
rebuild houses can do thorough rebuilds at relatively low cost because they rebuild engines all the time. The mechanics are familiar with the engines they are working on and have the tools and machinery at hand to do the job quickly and efficiently. Having said that, have seen engines advertised as rebuilt that have had little more than a clean up and new paint
. Quite often an unknowing owner thinks a rebuild is the same as doing a valve job. May have cured his starting issues but has done nothing for wear on other parts of the engine. If you go for a rebuilt engine, find out what parts were replaced and what was done on the rebuild. The reputation of the rebuilder is very important. There are quality people and not so quality people in every business.
One thing in the rebuilt/replace decision is the cost of installing a different engine over dropping the same make of engine back in. An engine change can cost as much or more than the engine in if there is considerable work needed to accommodate the new/different engine. Be sure that it's a drop in or you have to figure in the cost of making the different engine fit. Spent over two boat units for a mechanic
to help me to do a proper install of the Yanmar
in my current
boat. The PO had done a crappy job of replacing the A4 with the Yanmar
and the engine couldn't be aligned so ate shaft log bearings at an astonishing rate. Had to fabricate new brackets for the engine mounts, use different engine mounts and modify the engine bed
. Can't imagine what a yard would have charged for the work that the mechanic and I did to get the engine to fit.