225 Promax Smoked Piston Need Advice

[TNRoostertail]

Buddy just bought a beautiful 209TE Gambler yesterday. It has a 99 Mercury Promax 225 on it. Checked compression, all cylinders were 118-121. Well today it was running at 5800 rpms and just fell on its face. Got it back home and ran a compression test right off the bat. Pics will show what we found. It melted the port side middle piston. My question, is it rebuild-able? The port seams to have got slapped and nicked off some of the actual engine casting behind the sleeve. If rebuild-able we will of course bore it and just refresh the whole engine. As always guys thanks for your help.

[lpugh]

a great excuse to upgrade to a optimax or four stroke

[TNRoostertail]

a great excuse to upgrade to a optimax or four stroke

ewww

[OldTimer57]

I've not seen a motor like that without sleeves. So you can certainly re-sleeve the block to get rid of any nicks or metal transfer / damage. I got pretty good at repairing my old 2.5 racing motor. Managed to burn a number of pistons and sleeves / liners. Of course, occasionally I got to replace a block when a rod pulled/broke away from the piston and punched a hole. If you like the pro max, it can most likely be fixed. IIRC, that was a 6400 RPM motor? sounded pretty good. Not like a 7500 (and well beyond for many of us) 2.5 racing motor. They sounded REALLY good. :)

[Riccochet]

Can be re-sleeved. Certainly cheaper than replacing it.

Might also want to figure out why that cylinder gave up the ghost. She leaned out pretty good to melt like that.

[revlimiter]

That certainly looks rebuildable to me. Once you get the aluminum out of the cylinder you can measure and see how it looks. From the pictures the worst case scenario would be a new sleeve or maybe get lucky and hone out to .015 over, throw a new piston in and go. Of course you’ll have to check everything to see if there’s a smoking gun as to why it did that.

[OldTimer57]

One thing I would add, re-phrasing what others said above. Do NOT assume this was a random event in the universe. If you do, most likely you will get to rinse and repeat when it goes again. While occasionally a piston will "let go" on its own due to a bubble or crack in the casting, most of the time there is a cause. Running lean, or with too much spark advance have both gotten me in the past. Everything in the fuel system needs to be checked to make sure nothing is restricted.

[TNRoostertail]

Well we found the culprit, seems the #4 injector decided to take a knap. Going to get it mic'd, if its beyond .030 were going to sleeve it. If not were going .030 with all the pistons. Thanks for the help guys.

[EuropeanAM]

Well we found the culprit, seems the #4 injector decided to take a knap. Going to get it mic'd, if its beyond .030 were going to sleeve it. If not were going .030 with all the pistons. Thanks for the help guys.

May I ask how you made that determination?

You might also want to take a careful look at the piston and determine if the LOCATING PINS for the piston rings are still intact. That's about what you would normally see if you lost a locating pin....

Pictures can be deceiving, but it sure looks like there is lubricant sitting in the "unplanned recesses" of that piston.

[OldTimer57]

Top piston also looks a little odd. Might just be debris from removing the head, but I had several 2.5 failures where metal gets ejected from one exhaust port on a bank and goes back in to an exhaust port on an adjacent piston. I would personally completely disassemble and examine everything carefully. Don't want problem A to lead to problem B later because something was overlooked. I am not sure exactly what that looks like. Almost like you had a ring failure since it goes all the way around the piston. Most of my "lean burn" disasters showed a marked loss of metal on the side of the exhaust port where all the heat gets concentrated.

In your case, looks to me like the top ring got hung and pulled the top edge of the piston off starting at whatever port it hung up on.

I will also add that at least once in my multiple repairs, I got a pair of pistons directly from mercury, and after cranking motor, running on muffs for 10 minutes and then removing the head for a quick look, I found grooves in the sleeve where the brand new pistons had alignment pins that were backing out. I took photos and sent them to someone I knew up at Merc (Bob K. if anyone knew him). He sent me a new naked powerhead telling me that they had seen this previously several times, all in the same lot of pistons. Fortunately it didn't rip the piston crown off. But I was not going to run it and give it a chance.

[TNRoostertail]

We were very fortunate in that his neighbor builds motorcycle racing heads out of his garage and had a test bench for injectors. We recieved info on another forum that the injector could possibly be the culprit. We took them over and the number 4 injector barely sprayed when energized. One other also had a weak pattern. I was also told that the number 4 and 5 pistons are the hottest 2 in the block. Either way we are going to do a full work over with a new sleeve and a .015 overbore just to be safe. Thanks again for your help.