When to rebuild the bottom of an engine?

[BrettBBonner]

I have a 1990 ski-boat with a 175 (4.3L) Mercruiser Alpha 1 I have owned since new. It has 400 hours on the engine and running only synthetic after 1993. Wasn't running well, ran a compression test and it failed on CLY 2-4-6 with 130, 90, 90 respectively. (when the plugs were pulled and water spurted out, good indication there was a problem!!!). Pulled head and found bad valves on 4 and 6 and a bad head gasket between 2 and 4, and some water - not much - in oil. The gasket could have been leaking water into cylinder 4 and into the oil. I Put a dial indicator on the pistons. Piston number 4 is about .004 to .007 below the other pistons at TDC. I?m assuming a bent rod, although I really don?t know what the spec is. Cylinder walls look great, no rust, no piston ridge ring. Engine has a new alternator, starter, fuel pump, one relatively new exhaust manifold. Took the heads to an engine shop and two sets of valves on both sides would not hold vacuum.

One other complicating issue ?The glassed-in wood under the motor mounts I should pull the engine anyway as I need to cut out and re-glass in some engine mounting wood. It?s all temped up now with injected epoxy wood flour and might hold forever. I'm doing all the pulling of the engine and the mounting on the long block, but my time is worth something.

Questions:
1. Is this rod bent enough to REQUIRE a bottom rebuild or rod replacement.
2. If I test the exhaust manifold and riser with vacuum and they hold vacuum, should I replace anyway (they are 20 years old).
3. Should I
A. Install a new 220HP MPI for about $6.5K, which is a complete engine with manifolds, engine coupler, mounts, etc.
B. Put in a rebuilt long block with new manifolds and risers for about $3K
C. Take a chance on a head job and gasket kit for about $500 and hope the rings are still good and the water in the oil came through the bad head gasket or some minor blow-by?

I'm only using the boat maybe 20 to 30 hours a year now because I'm getting older and I have other boats. But I might have grandchildren someday and my kids might use it more in the future.

Any advice is greatly appreciated.

 

[Todd157k]

Re: When to rebuild the bottom of an engine?

Most of your questions, only you and your pocketbook can answer however as for the .004-.007 piston.. you probably bent the rod when you hydrolocked the cylinder by running it with water in it. It will never run smooth and you'll eat up the cylinder if you don't pull it out. I'm sure a re- manufactured long block would be far cheaper in the long run.

 

[Simoniz]

Re: When to rebuild the bottom of an engine?

I guess you think the rod is bent because of the water which you had in the cylinder?

How much water came out when you turned the motor with the plugs out?Enough to really cause a hydrolock?

Could it be that the difference in piston height is just due to a difference in the height of the piston crown? 4 - 7 thou doesn't sound like much and could be the thickness of a layer of carbon.

If you measure the height of the piston at TDC at 4 points around its crown are they all equal? Are the other pistons equal?

I think you've found the cause for the rough running, low compressions and water in the cylinders in the form of the bad head gasket. I would be tempted to check the heads and block decks for flatness and put it back together with new gaskets and see how you get on rather than spending several thousands on new engines.

 

[TJS]

Re: When to rebuild the bottom of an engine?

I have a 4.3 V-6 mercrusier engine that was sized. Took it apart and the same thing. The previous owner did not take care of it. It has roller cam and the crank is in great shape. The rods are not bent cause he did not run it. The block needs at least a .030 overbore and new pistons(I think the block is junk). By the time I do this I could have done a 350 v-8 for the same price. Remember the v-6 you have is just a v-8 with 2 cylinders cut off. All the mounts are the same as are the accessories. I already have a 350 Mercuiser laying around I just need to go thru it. Might want to think about upgrading to a V-8.
T.J.

 

[EddiePetty]

Re: When to rebuild the bottom of an engine?

....I'm in total agreement that you can not determine a .004-.007 rod bend with the pistons still in the bore. Piston rock, imperfections in the crowns and being slightly over rod dwell can easily account for this small amount.
Past experience has proven to me that half an engine re-build only transfers the weak link in the engine (do the heads: new valves/guides/decking/restore dimensions) and pretty soon the rings will blow-by followed by rod knocking.
Given the other repairs that you discribed, should you choose to retain this engine, I would vote for a COMPLETE overhaul.

 

[BrettBBonner]

Re: When to rebuild the bottom of an engine?

Simoniz - You ask if I think the water in cylinder 4 was enough to hydrolock - yes. Especially given the fact I just replaced the starter when it "stuck." And you ask if I believe it was enough to bend the rod - possibly. And I really don't know if .004 to .007 is much at all. The water and the fact that this piston is lower seems to support the premise.

Eddie Petty, TJS, and Todd - It was about $2K for a local rebuilder using my core, or about $100 less for a totally new rebuilt long block not using my core. Oddly, it seems that a gamble on my block - even if it ruins the block - costs me $400 (rebuilt heads, new gasket kit). even if my block is eventually ruined. Does this change your opinion?

I can't put in a new V8 because I would have to completely rebuild the interior - otherwise, I would. It started out life as a 175 2bbl, and now sports a 4bbl Edlebrooke and performer manifold (a massive improvement) for the last 10 years or so. The best I can do is the 225MPI.

If you guys can direct me anywhere to a reference about checking a block like this, it would be very helpful.

And guys - thank you very much for taking the time to answer.

 

[Todd157k]

Re: When to rebuild the bottom of an engine?

Your decision.. and it really depends on who built the motor vs. who is going to rebuild yours. Any monkey can slap the parts together, but is it balanced? end plays checked? deck height? compression ratio? bearing clearanced? line bored? on and on and on. If the place is reputable, and offers a warranty, it's always easier to replace as a unit.