Replace pistons Merc V6

[1954bmw]

Currently purchasing a 1990 135 horse Mercury v-6 that supposedly has low compression in hole number 6, owner says cylinder may be scuffed. Is it possible to R n R the rod and piston without removing the complete powerhead???

[Glastron1987]

I picked up a 150 rescue around 2018. One cylinder was down. I pulled out the one piston and cleanup up the cylinder with 3" nylon wheel. Then replaces the piston and rings with new ones. Put it back together and the compression came back at least mostly. Have been running it since with no problems. Just did 61mph on a river run yesterday on a 1986 Glastron CVX-17. It's really propped more for skiing.

In my case I think the cylinder walls do not look too bad, it was more of a matter of some stuck rings. Would be good to pull the head and look at the cylinder. Be careful not getting junk in all the other cylinders if it's just a matter of cleaning the one.

[skialot2]

It is possible to pull and change a piston without pulling and splitting the powerhead. But it is not easy. Getting it out is a lot easier than getting it back in. If the 4 long powerhead studs are not corroded in the adapter plates, the powerhead comes off really easy. 10 nuts total and it lifts off. But first pull the cylinder head. See how bad the cylinder looks. If it rough to the touch you probably need to bore it out.

[rock]

There is only one way to do it right. Nearly impossible to clean cylinder without introducing waste into engine. If it is only aluminum caked on cylinder walls you might get lucky and be able to remove it with muratic acid. Do NOT do this with an assembled engine as you will kill the crank with the fumes. If it requires a hone to clean it garbage will go everywhere. If you plan on keeping this motor and want it dependable, take it apart. Not at all hard to do.

Rock

[blackmaxonly]

On a similar topic, anyone know the max difference in grams between pistons?

Needing to bore .030 out and the .030 ProV pistons assemblies are a full ~30g heavier than the others (makes sense since it's bigger). Wondering if that's fine or if I should look for a different brand. Piston assemblies including piston, wrist pin, bearings + washers and rings

[LakeFever]

I wouldn’t be the least concerned of weight difference on that motor. Plenty and I mean plenty of fishing motors with one or two holes oversized and the others stock. If you’re planning on building some 8000+ rpm screamer then worry. For stockish horsepower do the pistons that need replacing and leave the rest is fine imo

[blackmaxonly]

I wouldn’t be the least concerned of weight difference on that motor. Plenty and I mean plenty of fishing motors with one or two holes oversized and the others stock. If you’re planning on building some 8000+ rpm screamer then worry. For stockish horsepower do the pistons that need replacing and leave the rest is fine imo

awesome, thanks for the knowledge. ill drop 2 new pistons in and run it. dont plan on anything over 6500 range anyway so I didnt think counting each gram mattered but wanted a reality check!

[rgsauger]

I’ve got 4 stock holes and two that are .015 over. Runs awesome!

[skialot2]

I have three .015 over pistons in my 2.5 200. I have only a 4 gram difference between the heaviest and lightest rod/piston assemblies. I don't think the oversize matters. I wouldn't worry too much about it. I have weighed a set of rods from a 2.4. One rod was 30 grams heavier than the lightest rod. I ran my Bridgeport for years before I blew it up the first time. When I took it apart it had 2 different rods in it. Some butchers put in 4 narrow beam racing rods and 2 wide beam fishing rods in it. You could not tell from the way it ran. Ran 7000 RPM all day long.

[tlwjkw]

we had a shop clean up years ago.. probably '90/'91... decided ta do a little experiment with "junk".. found a 200 2.4 block with 4 steel, 2 30 and 2 15 over and 2 chrome holes.. put bp arts in tha chrome and oem in tha rest.. assortment of big and small rods.. we didn't WEIGH NOTHIN'.. plastic and steel rod bearings, metal mains.. seven petal front.. practically gave it to a friend who put it ona fly weight 16' laser runnin' 2 ta 1's with a 26 hoss.. it ran as strong (stronger than most) as anything 200 out there for years till we lost track of it... shot tha "balance theory" all ta hell on sub 8 grand motors............ jmo.

[blackmaxonly]

Thanks for the replies you two, I have a lot more confidence with the mix and match of different sized pistons now! I figured I was worrying about nothing but wanted a quick check. My shop balanaced my chevy within a few grams but figured these mercs dont care about the weight difference!

[tlwjkw]

My shop balanaced my chevy within a few grams but figured these mercs dont care about the weight difference!

absolutely... car motors are a different animal..... i do know a little 'bout those also from "back in tha day".............

[LakeFever]

Two strokes fire every time the piston goes up so the load is constantly down on the rod, into the crank. The piston is never unloaded pulling up on the crank its constantly in power or compression which negates most of the issues related to weight differences in the rods/pistons. Four strokers ride two full strokes up/down with the load pulling up on the rod/crank so the balance is a lot more critical to offset inertia loads.

[tlwjkw]

Two strokes fire every time the piston goes up so the load is constantly down on the rod, into the crank. The piston is never unloaded pulling up on the crank its constantly in power or compression which negates most of the issues related to weight differences in the rods/pistons. Four strokers ride two full strokes up/down with the load pulling up on the rod/crank so the balance is a lot more critical to offset inertia loads.

[David - WI]

I think it has more to do with the (relatively) short stroke, compared to something like a small block Chevy motor. The forces involved in turning 7k rpm with a 2.65" stroke compared to a 3.48" stroke car motor.