chriscraft168
New member
I have a 1985 Mercruiser MCM185 / 4.3L V6 - all original internals until recently. Engine started running with a slight and brief hesitation at WOT (RPMs would drop then pick back up then drop again). So, I replaced the old Thunderbolt IV Ignition with a new Delco Voyager (new distributor, wires, spark plugs) and it ran better but kinda the same (I think the damage was done) as it started burning oil like mad (3 qts in 8 hrs). Tore it down and found #4 piston damaged but the cylinder looked decent enough to hone, so I caught it before it got out of hand.
I purchased a new piston for that cylinder (FED640P). When I compare that to my old piston they are nearly identical except; 1) the top edge of the new pistons are squared off and the top "hole" is not as deep and doesn't also have a "slot" toward the outer piston edge, and 2) the new piston is simply a bit shorter than the old pistons. From the top down to the piston pin the heights are the same but from the pin to the bottom, the new piston being about 3/8" shorter, see photos.
QUESTION: Are my old pistons simply obsolete and replaced with the new style or do I have the wrong piston for my motor? I see there is another piston for Mercruiser motors (MER735-808661). I've asked the Parts guys the same thing but you guys are like Mercruiser Yodas from what I've read.
Any help would be appreciated.
I purchased a new piston for that cylinder (FED640P). When I compare that to my old piston they are nearly identical except; 1) the top edge of the new pistons are squared off and the top "hole" is not as deep and doesn't also have a "slot" toward the outer piston edge, and 2) the new piston is simply a bit shorter than the old pistons. From the top down to the piston pin the heights are the same but from the pin to the bottom, the new piston being about 3/8" shorter, see photos.
QUESTION: Are my old pistons simply obsolete and replaced with the new style or do I have the wrong piston for my motor? I see there is another piston for Mercruiser motors (MER735-808661). I've asked the Parts guys the same thing but you guys are like Mercruiser Yodas from what I've read.
Any help would be appreciated.
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