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  1. #1

    Default What may have caused destruction of crankshaft

    Just completed a top end overhaul on Mercruiser 4.3 ( 2 new valves, seats and valves re-ground, manifolds skimmed, new carb kit, bores honed and new big end bearings) the bearings weren't bad and there was no knocking before doing the work, the reason for the work was because of a serious backfire throught the carb and power loss, found a "tide Mark" in a cylinder bore and intake manifold from water ingress through the winter layup. To cut a long story short, after only 4 hours running the new bearings are toast, especially 2 of them and the crankshaft journals are beyond a re-grind (the engine was a re-con block fitted in 2008 and had
    0.010" oversize bearings) engineer said it wouldn't grind any more?) what might have caused this failure of the bearings. Oil pressure and temperature has been good (except at time of sudden failure when pressure dropped off) oil and filter was new, level ok. What we did find was that the main bearings were stamped 0.25mm and the new big end bearings were 0.010" (calculator says 0.010" = 0.254mm) engine expert said that difference would not cause the failure.

  2. #2
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    Default Re: What may have caused destruction of crankshaft

    I`m guessing it could be the added compression could have something to do with it at least you have a good upper end just get a short block and slap it back together. Did you confirm the distributor was working properly by mapping out the progressive timing advance?
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  3. #3
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    Default Re: What may have caused destruction of crankshaft

    thats called "ten and ten crank" .Did you actually check clearance with plastic gage . Were they properly torqued and was there a bit of sideplay in the rods?

  4. #4

    Default Re: What may have caused destruction of crankshaft

    It has TB4 ignition and before top end job the mechanic checked timing and compression, it was 2-deg BTDC and range from
    160 to 175 on compression, mechanic did use a torque wrench and used the angle figures where stated in the book, he coated the bearings with a special grease and we turned the engine over on starter with coil disconnected for about 20 seconds before 1st start. He did use a parts bath to was down the block and blow dry with compressor before re-building, could it be possible that crap from the bath got caught up somewhere in he block causing the scoring, one of the shells has worn through
    from white to copper to black patches, another 2 are copper coloured and the others are badly scored., what do you mean by a "short block" ? Thanks for the replies

  5. #5

    Default Re: What may have caused destruction of crankshaft

    Just to add, timing was set to 8-deg BTDC and it did advance when revs increased but didn't do a curve map.

  6. #6
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    Default Re: What may have caused destruction of crankshaft

    It is allways a good idea to check the curve if you have performance issues it is too important on a marine engine. A short block is the bare block with a crank, cam and pistons with new timing gear set. Install a new high volume oil pump and complete gaskets and seals. Then do a proper break in period to seat the rings.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  7. #7

    Default Re: What may have caused destruction of crankshaft

    Not found a short block in the UK as yet, only a long block at around 2200, a new crankshaft around 1300. Looking for an engine with say a cracked block to rob the crank if anyone here knows where there is one? I will go down the route of a replacement engine if a repair doesn't work, the engineer is a good guy and won't be adding any labour costs so nothing to loose by trying. Apparently there is only a 7 thousand inch difference in all the journals so perhaps a re-grind might be on the cards, it's 10 thou undersized now so I'm guessing it could be taken to 20 or even 30 and still be ok? The cost of a re-ground crank is around 140 so it might be worth trying.

  8. #8
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    Default Re: What may have caused destruction of crankshaft

    You didnt say if plastiguage was used prior to final assembly?
    http://cgi.ebay.com/ebaymotors/Plast...MAAOxyXDhShR9D
    Put a strip on all the journals and torque them to spec then pull the cap and check for proper clearance for lubrication.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  9. #9
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    Default Re: What may have caused destruction of crankshaft

    From what I see you can go up to .040 under on both the main and rod journals.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  10. #10

    Default Re: What may have caused destruction of crankshaft

    No a gauge wasn't used, the new big end bearings were 0.010" and we think the originals (that were ok) were 0.25mm (same as the main bearings just removed)
    an engine spe******t who does regrinding etc says there is no difference, calculator says 0.010 is equal to 0.254mm?

  11. #11

    Default Re: What may have caused destruction of crankshaft

    Yeah I was told that this morning by an engine supplier in the UK, however the crank re-grind guy said it could break the crank and would I want to risk it in a boat!!!
    he said it would be ok in a car? How tough are these cranks? If my mechanic or me cant find another crank I think we will be taking it to another re-grind firm for a 2nd opinion, How important is the use of a plastigauge?

  12. #12
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    Default Re: What may have caused destruction of crankshaft

    It is real important for final assembly. If there is not enough clearance for oil you will get good oil pressure but not enough oil in the journals. You want it close to spec as possible. Most everyone here uses plastiguage before putting a motor into service.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  13. #13

    Default Re: What may have caused destruction of crankshaft

    Quote Originally Posted by kimcrwbr1 View Post
    It is real important for final assembly. If there is not enough clearance for oil you will get good oil pressure but not enough oil in the journals. You want it close to spec as possible. Most everyone here uses plastiguage before putting a motor into service.
    Thanks for that info, I guess even though the shells were replaced like for like they could have been too tight as the old ones (8 years) would have worn
    slightly? or perhaps as the mechanic thinks the originals may have been 0.12mm (he has 12 in his head when he ordered the new ones) is it possible to have 0.25mm mains and 0.12mm big ends on a re-con engine? If so then that IS the cause. Unfortunately the old shells were binned 3 weeks ago.

  14. #14
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    Default Re: What may have caused destruction of crankshaft

    Using the plastiguage gives you a visual on the actual clearance. There is allways the possibility the new bearings be slightly different than it says on the package or the machinist micrometers be out. You sound capable of doing the final assembly if you dont plastiguage its like putting it together hopeing it is right.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  15. #15
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    Default Re: What may have caused destruction of crankshaft

    Get the long block and call it a day.

    Max cut on a 4.3 crank will insure its destruction. Especially if there is no radius. They are poor boat engines.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

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    Default Re: What may have caused destruction of crankshaft

    2x on the long block....

    and I'd strongly encourage you to prime the oil system using the correct tool before initial startup...

  17. #17
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    Default Re: What may have caused destruction of crankshaft

    Ditto on long block,the 4.3 was not a good marine engine,ran fine in a lot of chevys

  18. #18

    Default Re: What may have caused destruction of crankshaft

    "Prime the oil"? Can't find that in he book and I've not heard of it before, sounds a sensible idea though. The more I think about this I think the bearings were too tight, the mains and corresponding journals are in good condition, it's just the big end she'll,s that are toast, definite signs of heat stress on them, one big question mark is could the original bearings have been 0.12mm? The mains are 0.25mm. The mechanic said the mains and big ends are usually the same size on a re-manufactured engine. The new shells were 0.010" (0.25mm) which would make the twice the thickness if this was the case.

  19. #19
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    Default Re: What may have caused destruction of crankshaft

    Sounds to me like your mechanic should stick to mini coopers and stop working on boat motors. Do you have a good repair manual for that motor if not get one. The second opinion sounds like your best bet. Get a good machinist to mic out that crank and see if it is salvageable. What year is your motor? are you sure the bearing caps were put on in the right direction?
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  20. #20
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    Default Re: What may have caused destruction of crankshaft

    Before you start the motor adapt a small steel rod to fit the oil pump shaft and with a drill pump the system up then install the distributor and fire it up.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  21. #21

    Default Re: What may have caused destruction of crankshaft

    Just seen video of the oil primer, easy! Thanks. As for the caps being on the right way, I can't be sure, but the mech did show me the main caps with the"arrows" cos I asked him the same question, he was quite meticulous on laying out the components
    and using wood blocks with holes etc when he did the top end, year is 1995 (re-con block and new manifolds fitted in 2008)
    I have had the boat 4 years. I have the Seloc manual and mech has both. Just a big mistake not using a plastigauge on hindsight. I can get a long block for around 2k here in the UK, But having already spent 1k + on top end I,m a bit reluctant to throw that away without trying a repair on the crank or get a re-con one 1st. Do you know if it's possible that the mains and big end bearings could be different sizes on a re-con motor? The machinist that looked at the crank did say that because it's already been cut then there will be little or no hardness left in the metal?

  22. #22
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    Default Re: What may have caused destruction of crankshaft

    The machinist only needs to grind whats necessary you can grind the mains .020 under and the rods .010 under as long as you install the proper bearings they do not have to be the same. I`m not buying the idea that grinding the crank effects hardness. The pro`s here already weighed in on getting a long block if you do make sure it has a marine cam. You did mention the cylinders had water damage I would hate to have you put together a motor that needs rebuilt before you even start it?
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  23. #23

    Default Re: What may have caused destruction of crankshaft

    The original problem started with a missfire and backfire through the carb resulting in a top end rebuild which is when we found the "tide Mark" in No 3 cylinder and 2 bad valves, top end is now good. High compression, Reevs to over 5k under load now that timing was set correctly, (I know WOT should be no more than 4800 so I guess a re-prop is needed) it's never had the correct timing due to some idiot putting the timing tape on the balancer the wrong way round long before I got the boat.
    I could never get it to rev above 4000 before the rebuild. On another post someone has kindly given me the part number for the crank 429-9517A1 so that gives me better chance of finding a replacement if the old one is shot. I will be catching up with the mech today to see what he has come up with and how to progress, he did say I didn't need to worry about any labour costs though, which I suppose is one good thing. Your input on this issue has been a great help, thank you.

  24. #24
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    Default Re: What may have caused destruction of crankshaft

    How did you check for true TDC? If it has the rubber bushing in the damper pully use a piston stop tool to check for true TDC once you get it back together, then you can set you tape from there.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  25. #25
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    Default Re: What may have caused destruction of crankshaft

    http://www.ebay.com/itm/Proform-6679...9Vixlg&vxp=mtr
    You can make a piston stop with a spark plug and a long bolt. Just bust out the inners of the plug and tap it for the bolt. Put the #1 piston all the way up and then turn the crank about 1/8 turn then screw the piston stop tool in and then touch the piston with the bolt. Make a mark on the damper the turn the crank until it touches the bolt again and make another mark. True TDC is half way between those two marks. Be sure and disconnect the battery you dont want to hit the starter the stop tool will punch a hole in the piston.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  26. #26

    Default Re: What may have caused destruction of crankshaft

    Got it! The harmonic balancer does have a groove about 0.5mm wide all the way across the casting which I assume indicates TDC. But it will be checked. The good news is I have found a local (20miles) company that rebuild Merc engines, they ship worldwide and last month did 7 of them, they will supply a new STD crank, bearings, seals and all gaskets for under $500 with 12 months warranty, Happy days!!! My mech does have an oil primer, plastigauge and end float gauge etc, I will make sure he uses them this time. When this jobs done just got to sort out a little oil leak from the Alpha One which has appeared since I swapped out the impeller and housing 2 weeks ago, I changed the O ring next to the impeller housing but not sure if there is one up in top, I will refer to the manual.

  27. #27
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    Default Re: What may have caused destruction of crankshaft

    When you prime the engine with oil and are using a old dist body to spin the oil pump ,spin the pump untill you see oil in all the rocker arms. I have always done it this way and have never lost a new motor to a dry bearing.

  28. #28
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    Default Re: What may have caused destruction of crankshaft

    Not priming an engine will not be the cause of its failure.

    Yes it is good to prime an oil pump and push oil through the engine, but it will not cause its premature death.

    Cranking the engine over with the starter with the coil wire removed will push oil where it is needed.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  29. #29

    Default Re: What may have caused destruction of crankshaft

    Which way do you spin the pump?

  30. #30
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    Default Re: What may have caused destruction of crankshaft

    The same way the rotor spins on the distributor.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

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