'85 MCM-185 - Pistons?

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.

Not 100% sure but the original piston is 27 years old.........they have come A LONG WAY with piston and material design since then.........there probably have been a few changes along the way........if the part number was good and that is what you got then...........

As long as the piston pin center line to piston deck is the same and the skirt is not any longer then I would say the same as NIKE...

JUST DO IT!!

That was my thinking too.

Personally, I'd go see your local trusted machinist. He will have the knowledge to tell you how equivalent the new is to the old. The notch is used to indicate piston location so the thrust face goes to the correct side. The piston pin height is another key dimension. he should be able to give you a thumbs up or down and can migrate the rod, too.

I trust you measured the cylinder before getting a standard sized piston.

makomark said:

Personally, I'd go see your local trusted machinist. He will have the knowledge to tell you how equivalent the new is to the old. The notch is used to indicate piston location so the thrust face goes to the correct side. The piston pin height is another key dimension. he should be able to give you a thumbs up or down and can migrate the rod, too.

I trust you measured the cylinder before getting a standard sized piston.

Click to expand...

Ayuh,.... Or start lookin' somewhere other than the Mercruiser parts books...

It's a GM/ Chevy part that has a GM/ Chevy part number....

I'd be concerned with what appears to be a weight issue,...
They don't appear to weigh the same,...

Balance is important...

You overbored just one cylinder ? What i really mean you have it all apart and you only want to do one cylinder. Really how many american dollars is it going to cost to replace all 6 overbored say 30 thou. New rings and your good to go.

Overbored, no, deglazed/honed, yes. Took it to a machine shop and had the heads magnet-flux tested before milling, #4 was cracked in 2 spots.

I have used the Silvolite piston reference before to ID pistons
your originals are on this page
http://webpages.charter.net/beckracing/slvpg26.htm

I went to shop.evinrude.com and pulled up the 1985 4.3 from OMC because they use the Chevy part numbers instead of Mercruiserizing everything
pretty sure GM sold OMC the same 4.3's in 1985 as they sold to Merc and VP etc




If you decide to go with the one you bought, I see those pistons on line for $11
I'd probably just replace all 6
http://www.behrents.com/mm5/merchan...40P&Category_Code=FEDEngine-Piston-CastPiston