should pistons be loose in cylinder??

[baxtr]

Ok, motor is apart, bad piston is out. Remaining 3 pistons seem to be loose though, isn't the purpose of a ring to keep the piston tight to the cylinder wall to keep pressure from escaping and to dissipate the heat? the rings are gapped for expansion, but not touching the cylinder wall at all, I can grab each piston and wiggle it back and forth inside the cylinder. I always thought you needed a ring compressor to get your pistons back in the cylinder, mine just drop right through. I understand that under heat they expand, but that is the purpose of the gap in the ring, correct? Maybe re-ring all cylinders? anyway, does anyone know how tight it should be when I install them?

1989 Force 125 4 cyl 1251X9B

 

[The Marine Doctor]

Re: should pistons be loose in cylinder??

The pistons should move slightly when jiggled.

The pistons will fall right through without the aid of the rings.

Rings and pistons must be put back in with a compressor...If the rings just fall through, the engine is either bored too much or worn.

Cameo from TMD

 

[ewbish]

Re: should pistons be loose in cylinder??

Ok, motor is apart, bad piston is out. Remaining 3 pistons seem to be loose though, isn't the purpose of a ring to keep the piston tight to the cylinder wall to keep pressure from escaping and to dissipate the heat? the rings are gapped for expansion, but not touching the cylinder wall at all, I can grab each piston and wiggle it back and forth inside the cylinder. I always thought you needed a ring compressor to get your pistons back in the cylinder, mine just drop right through. I understand that under heat they expand, but that is the purpose of the gap in the ring, correct? Maybe re-ring all cylinders? anyway, does anyone know how tight it should be when I install them?

1989 Force 125 4 cyl 1251X9B

Check your ring gap, book should list the spec. To do this, pull the piston, ring out of the cylinder. Remove ring from piston, carefully place ring in cylinder. I like to measure about 1 inch from top and also at intake. To slide ring down, use the piston to push it down and make sure it's even. Measure the distance between the ends of the ring with a feeler guage, if it's wider than the book spec, time to rebore/resleeve. It it's just a little out of spec, or right on the line, see if the manf. offers a B or C piston. If it's in spec, no worries, but a rering never hurts.

 

[baxtr]

Re: should pistons be loose in cylinder??

Yeah,........top ring had 4 gaps, it was torn to shreds along with the top of piston. Allready have powerhead completly torn apart and block at the machinist to over bore. replacing 2 cyl and all rings right away.

 

[pecheux]

Re: should pistons be loose in cylinder??

I am not a mecanic, but been told by a friend who is, that outboard motors are built with somewhat loose pistons, and that the 2 cycle oil fills in the gap while the engine is running ... which could explains the difference in running performance from one brand of oil to another.

Personnaly I have found that using Amsoil HPI (premix) made my 9.9hp Johnson run thighter and quieter ... while the results in my 15hp Force just makes it run smoother at wot with a few more rpm.

Cheers