470 Merc metallic rapping sound

[Rod Holder]

Hi all,<br />I have a 21' Glasply with a 470 that had 1,200 hrs on meter when I bought it. I have since put on around 25 hrs of trouble free use. Ran great and strong although a bit of piston slap while cold.<br /><br />While cruising on home last week (3,200 rpm) all seemed well until all of a sudden it started a horrible rapping sound that is timed to rpm. I shut down and pulled valve cover and all rockers appeared ok (I was hoping for a collapsed lifter or stuck valve). I ran it in at normal cruise with no power loss (someone at the dock asked me if I had a monkey with a ball peen hammer under engine cover, to which I replied that if I had, I would spank it) : <br /><br />I have since pulled the timing cover (hoping once again for a simple exploded chain tensioner). Besides finding a sloppy impeller, it was fine and still rapps after dealing with the impeller.<br /><br />what would cause a sudden situation like this ??<br /><br />Does anyone know who has the best price on re-mfg<br />long blocks in the USA ??<br /><br />I want to stay with the 470 as I'm an avid do it myselfer and I do not want to mess with the mounting/alignment situation of going to a 4.3 or such.<br /><br />Thanks guys...........

 

[bomar76]

Re: 470 Merc metallic rapping sound

Wow, 1200 hours....kinda long in the tooth!<br />My guess from across the continent is you have a rod that wants out really bad. Hosed rod or main (but I bet rod) bearing.

 

[johnkc]

Re: 470 Merc metallic rapping sound

have you done a compression test ?#3 cyl was notorious for getting antifreeze in it,due to the riser gasket leaking, and siezing the piston pin and developing the piston slap noise that you knew existed .and maybe this #3 piston let loose causing the louder noise .

 

[Rod Holder]

Re: 470 Merc metallic rapping sound

Thanks for the ideas. With as many hrs that it has you may both be right.<br /><br />When I was running with plug wires removed cyl by cyl, #3 had the most notible difference. If I remember right, the gasket between the manifold and the riser blew out during the first long test run (in the lake) after I got it. At the time I thought that was a blessing as once I had the riser off I could see that it was toast so I replaced it.<br /><br />I'm not going to put another penny into it and will replace with a rebuilt version.<br /><br />Does anyone have any pros or cons to cross country engine purchace and core return ? I have seen alot on the East coast (I hope Boston smokes Oakland), but nothing here in the upper left.<br /><br />Once again, thanks for the info..........