Mercruiser 260 pistons

[whiskeyRichard]

I recently pulled the engine from my boat and was suprised to see the pistons that were in there. This is a 1980 Mercruiser 260.

Do you think these pistons are stock? The motor was rebuild a number of year ago, but the previous owner said he used all stock replacement parts for the rebuild.

 

[whiskeyRichard]

and a 4 bolt main and what appears to be a cast crank.

 

[Bondo]

Ayuh,.... Looks like what I would expect to see,.....

Let me Guess, the block castin' number ends in 010,..??

What were you expectin' to be different,..??

 

[Scott Danforth]

yes, stock s#!tty GM 4-relief pistons. the worst pistons on the market one could use (and actually more expensive than a good set of pistons), however they make it easy for the idiot with a hang-over to install them on a monday.

 

[whiskeyRichard]

Yes, the casting number is 3970010

I was expecting a dished piston to keep compression low but after I punched the numbers into a compression calculator, I see this motor was 8.5:1 with the 76cc heads.

Out of curiosity, what makes these pistons so bad?

wR

 

[Scott Danforth]

they suck at propagating the flame front. there are better flat-top pistons out there (only 2 reliefs - however require someone to be half awake to install properly). BTW, the same pistons with a 64cc head would be 9.4, which is what the later motors had.

 

[wrench 3]

they suck at propagating the flame front. there are better flat-top pistons out there (only 2 reliefs - however require someone to be half awake to install properly). BTW, the same pistons with a 64cc head would be 9.4, which is what the later motors had.

If you had a piston with only 2 reliefs you would need different ones for port and starboard banks. You can't turn the piston around because the wrist pin is off set to the thrust side.

BTW whiskey, a backhoe works better. Gives you more lift.

 

[Scott Danforth]

no, you just have to pay attention when assembling the rods and pistons.

4 get assembled one way, 4 get assembled the other.

 

[DLNorth]

A story from way back.

many years ago I worked with/in a diesel development group as we developed a new family of engines.
This engine was an IDI, meaning a directional pistons. (in addition to the offset pistons)
We were to the point of building the 1st prototypes in and by the future production plant/guys.
I was sent down to oversee the builds (4), but they started early and had the short blocks together and heads on by the time I got there.

We started testing on the dyno and while they ran OK the smoke numbers were too high. Nobody knew why.
So we removed the heads and found the pistons in backwards. They should have run terrible but didn't except for the smoke.

no, you just have to pay attention when assembling the rods and pistons.
4 get assembled one way, 4 get assembled the other.

 

[wrench 3]

no, you just have to pay attention when assembling the rods and pistons.

4 get assembled one way, 4 get assembled the other.

For regular use I think I'd rather go with the off set pins and get less wear on the thrust side on the pistons.
Anyway, with the four relief pistons you still have to orient them on the rods so that the dimple on the top goes to the front of the engine.