Mercury Piston question

[jbjennings]

Pistons in merc 650 have "boost ports"???? I'm rebuilding it due to detonation and other problems. I've got some used pistons in good shape that fit good, but one of my new pistons does not have the hole in the piston skirt that I believe is a boost port.

Question, can I still use the piston without the boost port, or do I need to drill one in the piston that does not have one, or just get another piston?

Also, the newer used pistons fit perfect, but have holes drilled into the wrist pin hole in a different place. Is this newer design compatible with my old '74 merc 650? They did not come out of a 650, but another merc of a different horsepower.

Thanks fro the help,
JBJ

 

[wired247]

Re: Mercury Piston question

They all need to be the same. Boost port or no boost port.

 

[jbjennings]

Re: Mercury Piston question

Thanks for the reply. Would drilling my own boost port and chamfering the edges of the hole be a ridiculous idea? Would the piston without the boost port have a piston skirt that was not thick enough? Is the mass difference of the piston the problem or the lack of a boost port, or both?
Do I need to just bite the bullet and get a different piston with a boost port?
JBJ

 

[wired247]

Re: Mercury Piston question

The boost port works with recesses in the side of the cylinder wall to create "sort of" a looper engine effect. If you run boost port pistons in a non boost port cylinder wall it does nothing. If you run non boost port pistons in a boost port cylinder it does nothing. If you run boost port pistons in a boost port cylinder it increases the intake charge into the cylinder. If you mix them up in a boost port engine you will have some cylinders creating more power than others. It will create harmonic vibrations that could damage the crank bearings and/or the block.

To be honest I dont see any problem with cutting the piston. Its just a round hole in the skirt.

 

[Chris1956]

Re: Mercury Piston question

Does his 650 even have the boost port cylinder design? I thought that was restricted to Merc 1500s, and later 115HP motors.

I think it comes down to the mass of the different pistons. Can the two be run in a mixed format?

 

[jbjennings]

Re: Mercury Piston question

I sure do appreciate your time to answer! In looking at my cylinders, I don't see anything special in the cylinders associated with the boost ports. I'm going to try to drill out a hole the same size and location and dremel/sand out the edges so they don't snag. I'd like to just leave it alone if possible if mass won't be significantly effected.
Thanks,
JBJe

 

[wired247]

Re: Mercury Piston question

If you don't have boost ports in the sides of the cylinder don't worry about it. They look like sort of a wedge shaped recess.

 

[Faztbullet]

Re: Mercury Piston question

Do not drill them as the piston will be out of balance!! Boost port pistons have a thicker wall on the ported side as told in school. Also if wrist pins in different location they will not work as piston may not reach full TDC or actually hit the top of cylinder. That should be a boost motor and use the boost ported pistons. Also measure the piston skirts as as they make look good but be past wear limit...

 

[jbjennings]

Re: Mercury Piston question

I'll order another dadgum piston.... Seems like I will never get this thing back together. I will take another look at those cylinders and post a pic and see if you guys can tell if it's boost or not. It probably is I'd think, because those are likely the original pistons in there and they're all boost port pistons. I've never heard of this boost port stuff..... I learn something new every time I tear into a merc.
Thanks for all the responses! Much appreciated. I will measure the skirts as well. I plan on keeping this motor for a long time. I like the power and weight of it.
Later,
JBJ

 

[jbjennings]

Re: Mercury Piston question

I checked out the cylinders and it does indeed have boost port cylinders. I guess I'll have to get some boost port pistons. Right now I only have one, possibly one on the way, and will need one more. At least, the cylinders have virtually 0 scoring and hopefully are not out of round. All new rings.
Thanks for the good advice by all.
JBJ