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1986 Mercury 25hp piston fitted the wrong way around

D

dk79

New member
"Hello !

After various stra

"Hello !

After various strange issues with this engine i decided to inspect the internals , so yesterday upon removing the reed block i noticed that the piston in the 2-nd cylinder is fitted the wrong way around (as an whole assembly ,with the conrod ..) -> with the UP mark facing towards the driveshaft instead of the flywheel ..

List of strange behaviour was as follows :
1 Hard starting from cold
2 Rough idle
3 Vibrations/shaking at idle and low rmp
4 Very hard re-starting warm engine
5 Excessive exhaust smoke
6 Hesitating when accelerating from low rmp

Now , can i blame this wrongly fitted piston for all these stuff ?! as everything else on the engine was in good working order (ignition components , carburetor etc.)

Thanks !
Damir"
 
That'll do it. That's

That'll do it. That's a looper with flat top pistons or the motor would have locked up.


Jeff
 
"While it's not a good thi

"While it's not a good thing that you have an upside down piston, I don't think you can blame it solely for all the issues you note.

The motor is a standard cross flow design. The piston is basically a "flat top" so "one side" is not interfering with a intake or exhaust port.

But hey, maybe it is just the piston causing everything.

You need to split the powerhead and turn it around regardless (just because it's wrong). If issues remain, address them when you have it all back together correctly."
 
"Hi !

Well , yes the piston

"Hi !

Well , yes the piston crowns are flat but i noticed that each piston has some kind of ports on the piston skirt ( two ports on each side separated into two sections .. so 4 ports/passages on each piston precisely !) and also cuts on the lower ends of the piston skirt , and , if i understand this , this ports/cuts on the piston must be matched "mirror image" with the cylinder bore ports and if the piston is turned with the UP towards the driveshaft these thins dont quite match up .. and , by rough examination with opened exhaust cover looks like the ends of piston rings where rubbing onto the exhaust port area as well which looks/sound like bad news as well... will tear the powerhead apart tonight and inspect more throughly but i do not fancy using this powerhead again !

Anyway , i have a spare one from early '80 merc 25 which had blue decals (mine has brown) and this powerhead was in good working order so will just re-fresh it a little bit and bolt it on as an replacement .

Thanks for info Jeff/Graham !

Regards
Damir"
 
.."The motor is a standard

.."The motor is a standard cross flow design. The piston is basically a "flat top" so "one side" is not interfering with a intake or exhaust port. "

I think the Great One got his loopers and crossflow designs confused.

Jeff
 
"No , it just looks like that

"No , it just looks like that ! this the holes above the pins didn't break trough the piston skirt on the other side .. also , i've tried to rock/move/push the piston pins an they're solid/tight fit so that's fine as well ..

Thanks for help Jeff !

Regards
Damir"
 
"Yes, Jeff, did get them confu

"Yes, Jeff, did get them confused - was thinking flat top and domed and half domed pistons while answering also an email about an early 90's Force at the same time (explaining that CFI was just a cool sticker that meant it used "old" cross flow design which really wasted gas).

Getting too old to try and do two things at once
"
 
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