Thread: What caused this 2000 25hp mercury

The fellow that bored it to .030 over, did the the cross hatch with a flexhone and it looked good. I started it and used about a 35 to one ratio of oil to gas, and run to hooked to muffs for about 20 minutes before checking the compression. When the weather breaks I will
look inside and see what it is. Thanks for the info!

Some clarification on mercosil for the o/p and others that might some day face the same issue. The mercosil bore is nothing more than aluminum. It does contain a high amount of silica that acts as a friction reducer. The standard cast iron sleeve of common is much harder than aluminum as we all know. The cross hatch is a method of promoting wear of the rings to form a good seal. With the advancements in technology in the mfg of rings, it's really not necessary anymore. Now, when you cross hatch aluminum, it greatly increases the friction in the cylinder. But unlike a cast iron sleeve, the cross hatch aluminum cylinder is going to fail as the rings cycle back and forth. The visual signs of the failure will be best described as a smearing of the aluminum cylinder wall which will immediately have a similar effect on the piston that rides the portion of the bore. A correctly prepped mercosil block will be finish honed on a bar. The flex hones with an extra fine grit can work if not over used. The flex hone can and will 'out of round' the cylinder if over used. (That's why the finish is normally completed on a bar.) The final finish of the mercosil block should be very smooth. Perhaps not a reflective mirror finish, but a very smooth one, never the less.
As for the 20lb difference of the o/p's motor, that amount of difference does indicate a significant issue is present. Unlike the cast iron cylinder type motor that normally improves it's compression with some run time and the 'seating' of the rings as they wear on the cross hatch, merco blocks don't go through that phase and should be optimum from the time it's assembled. Good luck to the o/p, I hope you get it lined out before it goes down.
Also, the standard compression reading of a new 25 merc is 120psi. The difference between gauges will give slightly higher or lower readings. But an accurate gauge on a new motor will show 120. If a non modded motor begins to show higher numbers, it's a good sign that the motor has developed a mass of carbon on the bottom side of the piston and effectively "stuffed" itself.

hi, I have a 95 mercury 25hp 2 stroke. I have had it since new. I had the carb rebuilt for the first time about 4 months ago. the mechanic(a merc authorized mechanic) said that one cylinder showed a little scoring. he asked if it had ever been run without oil. it has not. I have always run it 50-1 with quicksilver/mercury oil. he also said that compression was 120 in each cylinder. I winterize it every year, change plugs/impeller every 3 years. the motor has probably seen less than 60 hours in its lifetime. I always treat my gas with either startron or seafoam. it runs real well, pushes my 1648 about 31 mph, starts in the cold right away and idles nicely. so my questions are...do i have a problem developing inside my motor? if so, what should or can I do to head off a potential problem?

lattehead. Have you ever washed your engine block. I'm talking about the engine itself? I see alot of relatively nice and clean looking outboards that have a LOT of dirt on the engine block. Since it has no air filtration system, any dust/dirt that's allow to build up inside the cowling is subject to being pulled in. That may explain the light scoring. If that's the case, a simple wash down is all that is needed to prevent future scoring. Also, your motor is a 'mercosil' block. That year model has a history of being problematic. I would suggest having the exhaust cover gaskets either replaced annually or have the spark plugs 'read' for signs of water entering the bottom cylinder. If water is detected, for sure change out the exhaust gaskets. If allowed to run with water intrusion, it will be it's demise.

sliderman, thanks for your reply. the engine inside the cowling is super clean and shows no dirt, dust or grime. I hand clean it with a damp rag every time I do maintenance on interior(cleaning out fuel filter, grease shift cams, etc.). I have never used a hose to clean interior of engine. can my engine still run with the light scoring without it getting worse, as long as the scoring is not being caused by something else currently failing within the engine? is it possible that the light scoring is age related to the mercosil block, or was there at time of production? does the scoring have to be the result of the exhaust gasket failing? I will check the plugs after the next use to see if there is any water on them. if there is, is replacing the exhaust gasket something i can do on my own(I have installed an electric starter on the motor, replaced impeller and do all winterizing myself) or is this a job for a professional outboard mechanic? also, do you know what years mercury used the mercosil blocks in their 25-60hp outboards ?

You can clean the engine with a cleaner/degreaser and water hose. Nothing will be damaged by getting wet. Stay clear of the ultra high pressure water. Tilt engine or place in shallow water drive and be sure not to spray water directly into the carb. The scoring is not from age alone. Since the mercosil block's bore is aluminum, any debris that makes it way into the cylinder is subject to leaving light scoring. I don't suspect it will increase unless more debris or water enters the cylinder. Mercosil blocks officially started in production in 94 if memory serves. Before that, a similar but different block known as nikasil was used. To read spark plugs. Run engine on hose or in a tank for several minutes. Shut down and remove bottom plug. It will be wet with fuel which is normal. What you are looking for is either a light foam color (water/oil mix) or water droplets. You should be able to replace the gaskets. Simply remove the exhaust cover, clean mating surfaces and re-assemble with new gaskets. While the cover is off, you will be able to view most of the cylinder, through the exhaust ports. Since you have good compression, I don't expect that you will find anything more than light scoring.
In the 25-60hp lineup, the 25 is the only motor to have the mercosil block that I'm aware of. Mercury did use Nikasil, Mercosil and chrome linings in some of their big v6 engines in an attempt to reduce internal friction and increase power output.

Sliderman, I have a quick question. Not very familiar with outboards. I am looking at purchasing a boat that has a Mariner engine on it (115h.p.), who makes this motor. It is the all one where the cylinders are stacked on o top of each other like the old Mercury's...Thank you... and rather than tie this mans thread up you can PM me. Thank you, Eric.

Originally Posted by sliderman

You can clean the engine with a cleaner/degreaser and water hose. Nothing will be damaged by getting wet. Stay clear of the ultra high pressure water. Tilt engine or place in shallow water drive and be sure not to spray water directly into the carb. The scoring is not from age alone. Since the mercosil block's bore is aluminum, any debris that makes it way into the cylinder is subject to leaving light scoring. I don't suspect it will increase unless more debris or water enters the cylinder. Mercosil blocks officially started in production in 94 if memory serves. Before that, a similar but different block known as nikasil was used. To read spark plugs. Run engine on hose or in a tank for several minutes. Shut down and remove bottom plug. It will be wet with fuel which is normal. What you are looking for is either a light foam color (water/oil mix) or water droplets. You should be able to replace the gaskets. Simply remove the exhaust cover, clean mating surfaces and re-assemble with new gaskets. While the cover is off, you will be able to view most of the cylinder, through the exhaust ports. Since you have good compression, I don't expect that you will find anything more than light scoring.
In the 25-60hp lineup, the 25 is the only motor to have the mercosil block that I'm aware of. Mercury did use Nikasil, Mercosil and chrome linings in some of their big v6 engines in an attempt to reduce internal friction and increase power output.

sliderman, thank you for all the advice. once we begin to thaw, I will check out the plug and exhaust. I am presently in the market for a mercury 40-60 hp 2 stroke. are there any production years I should look for or avoid?