M20B25 Ring Gap

[BAlbertiX]

I'm down for dumb shit and shortcuts, but if the engine is already out, and you want it to last for a while, might as well give it a good shot at holding together.

"If you don't have time to do it right now, when will you have time to do it over." is a quote I know well, but have to keep reminding myself :) This motor is going to see abuse and I need it to be as reliable as possible. Gonna do it right this time. Block is being dropped off at the machinist tomorrow.

Could you tell why the rings stuck? As in, galling on the lands, expansion of the ring face, wiping of the ring face, etc...

It just seems really odd that even the oil control rings did it.

t
into the forensics

I'm right there with you. Not sure how the oil ring gets caught up in this either. I've been reading that a light cylinder finish simply does not bring enough oil up the walls during the break in period... that explains the compression ring freezing but not the oil ring - oil ring should be bathed in oil no matter what. My only other guess is the cylinder walls may have been fuel washed between than many no start and long start attempts. Heat was certainly involved.

[BAlbertiX]

You guys will be happy to know we're off to the machine shop today. Will report back.

[BAlbertiX]

Block is finally back from the machine shop. Cylinders only needed a .001 bore to straighten them out. Sticking with stock internals.

Shop was nice and gapped top two rings.016 / .017 and left oil ring as supplied.

I still think I'm going to gap rings further to .018 / .020 and leave oil ring as supplied ~ .020. The shop swears I won't need to open the top rings any further - thoughts? I want to give myself plenty of margin this go around.

[digger]

what pistons and rings?

do the gaps align with Bentley specs? if so the shop is correct.

more likely the original issue was the hone job not the rings butting

[TobyB]

I've had rings not seat for bore finish issues, and then the engine burns oil,
but I'm not following how the hone would make the rings stick in the lands?

That was what was so odd about what happened originally...
the only time I've had rings stick was on older engines that
had gunked up from oil problems... well, or the piston tops
melted, but that's not the ring's fault!

t

[BAlbertiX]

what pistons and rings?

do the gaps align with Bentley specs? if so the shop is correct.

more likely the original issue was the hone job not the rings butting

I agree - if I had to guess the novice ball hone was probably the cause... looking back I also probably did not do a proper cylinder wall cleaning. I power washed and de-greased, but never verified with a paper towel and solvent. Nor did I adequately cover the block between cleaning and final assembly.

Bentley -

top ring: .012 - .020 [0.30-0.50mm]
2nd ring: .012 - .020 [0.30-0.50mm]
oil ring: .010 - .020 [0.25-0.50mm]

The rings are currently in the middle of the spec - according to the shop .016 / .017 / oil ring as is. Will certainly trust but verify.

After break-in I plan on living near temp limits frequently with this car. - autox / hpde track days

I've had rings not seat for bore finish issues, and then the engine burns oil,
but I'm not following how the hone would make the rings stick in the lands?

That was what was so odd about what happened originally...
the only time I've had rings stick was on older engines that
had gunked up from oil problems... well, or the piston tops
melted, but that's not the ring's fault!

t

Angle of the cut, depth of the cut, straightness of the cut, straightness and perpendicularity of the walls are not addressed appropriately with a flexible ball hone... lessons learned.
I was also reading some shops will use two different cuts and some cuts are even proprietary like for racing teams.

[BAlbertiX]

quick question/observation - while I was cleaning up the piston ring grooves this weekend I noticed the pistons felt tight to the rods so I soaked them to PBR. They didn't loosen up much. The pistons swivel on the rods but takes some force by hand. They seem too tight. I want to avoid excessive skirt scuffing or oil retention issues.

I'm wondering if heat from the recent ring issues could have damaged the wrist pin brushings? Should I replace the bushings or is there something else I could try first?

[TobyB]

I ran into that with an M54- there was so much built up oil residue in the bushings and pistons that they were
pretty much stuck. Disassembly and cleaning did the trick on that one. On a new engine, you really
want a 'slip fit' with the rod and the pin, and the pistons will be as the factory supplied them...

fwiw,
t

[BAlbertiX]

I ran into that with an M54- there was so much built up oil residue in the bushings and pistons that they were
pretty much stuck. Disassembly and cleaning did the trick on that one. On a new engine, you really
want a 'slip fit' with the rod and the pin, and the pistons will be as the factory supplied them...

fwiw,
t

Thanks TobyB. That's what I needed to hear. New bushings arrived as needed, but either way will disassemble tonight and report back.