Piston Rings - Recommendation

[John76]

My adventure into the bowels of my M-10 to determine the excessive oil consumption has left me with lots of questions.

I was expecting to find broken or seized rings and/or nasty cylinder walls. Nope...cylinder walls were not scored.  They all were highly "polished"... no cross hatching at all!

 

Question #1:  With 143k miles on the original engine, would the highly polished cylinder walls cause the oil to slip past the rings and out the tail pipe? Head rebuilt 5k miles ago.

Would proper honing and a fresh set of rings be sufficient?  Pistons, ring lands, pins and big-end bearings all look good (surprisingly so).

 

Question #2: Flat stone or ball hone?  What grit size?  What wall finish? Manual recommends 3-4 microns, but this was the original spec for cast iron high-tension rings.

 

Question #3: What make/type of rings? Modern cars have low-tension rings requiring a very fine wall finish (0.90 - 1.4 microns) for low drag but use very thin oil. My upper and middle rings had no identification marks. Oil ring was marked "ATE".  Do new pistons have a specified ring gap recommendation, or do you use the factory manual spec (50-years old)?

 

Once the pistons were cleaned up, I found identification marks on the tip and inside (see picture). Head is E12.  Mahle "baby grand" pistons are marked 88.97mm with weight group (-).

 

Thanks for your help...I'm sure more ??? will follow!  This is new territory for me, so please be patient with me.  😬

 

 

 

 

 

[rstclark]

So the pistons are stock original size at 88.97 for a 89.00 mm bore   .03 is the clearance   

How big is the bore currently ?  Is there any ovalness or taper in the bore ?  A good machine shop can tell you if you are curious.  At 143,000 miles there should be some wear.

If you want it precise you will have to consider boring the block and buying new pistons and rings

If you want to just try to refresh it, hone the cylinders lightly with a ball hone and install new standard size rings . Check the end gap. If the end gap is within range it's worth a try

The next bore sizes are determined by the availability of rings at 89.50 , 90.00 , 90.50 etc hopefully you won't need to go there

Buy a standard size set of rings and see what the end gap is before you hone the block if it's good, then hone the block lightly and check the gap again  The motor is going to run, even if it's not perfect  and your oil consumption should decrease.

[stephers]

One piece oil ring ?    Check the drain back holes in the oil ring land on the pistons

 

Thanks, Rick

[tech71]

2 hours ago, John76 said:

Pistons, ring lands, pins and big-end bearings all look good (surprisingly so).

With only 143k I would think so especially on a well maintained engine.

you realize you have fallen down the rabbit hole right?

To accurately make the call on how to proceed you would have to measure bores for dia ,out of round and taper as well as pistons including clearance between rings and ring grooves in the pistons.

How was the head? Any worn guides? Crappy seals? 
you can lose a lot of oil through a worn head

Is that an actual pic of your engine? If so then wow!

If you end up replacing rings only, buy either Goezte or Deves, or maybe Sealed Power.

Goezte seems to be the most readily available these days

 

Edited July 13, 2023 by tech71

[Mike Self]

Coupla thoughts...

 

I did a valve job on my '69 (121 head, stock 8.3 compression pistons) at 157k due to a water passage which had eroded into #1 cylinder (successfully repaired).  Since I was experiencing the classic oil smoke on the overrun, while I had the head off I replaced worn valve guides/seals and ground the valves.  That completely cured the overrun smoking, but it also made the rings the weak point in the compression situation, so it began smoking a bit on startup and a slightly increased oil thirst. Not smoke screen bad, but a quart or so every 800-1000 miles.  I suspect that the same thing has happened to your engine.  Nice tight valves have made your rings the weak point.  My engine now has 227k miles, oil consumption hasn't gotten worse, the engine runs just fine, doesn't foul plugs, fog for mosquitoes etc.  And for the 500-1000 miles I put on it a year it's not worth going down even a chipmunk hole, much less a rabbit hole.

 

When you have your pistons out preparatory to doing a little honing...

• check the ridge at the top of the cylinder--where the rings don't reach.  If it's significant, you can break a ring while installing.  It should be cut
• check the ring lands on your current pistons for wear.  If the ring can flop around in its land, it will either break or quickly wear the land even more
• take a new ring , set it in the bore (after any ridge has been removed) and measure the end gap.  That'll determine whether you can avoid a rebore and new oversize pistons.  

Have fun with your project...and while you're at it, check the timing chain and sprockets for wear, replace the chain tensioner, check the oil pump for wear and its chain for wear, plastigage the bearings, etc etc...welcome to rabbit-holing 😁.

 

mike

[tzei]

Q3: piston ring material depends on block material (bore surface metal). P/ring material & intented use (stock, race, turbo, nitrous etc.) determines ring gaps. Block & piston material and intented use determines bore to p/wall clearance.

So near stock is where you want to be. I think.

Before you buy some fancy nos pistons super cheap check that there is rings available. Modern rings are thinner and 4 mm oil rings are not nessesarily on every manufactures production any more.

[uai]

3 hours ago, rstclark said:

So the pistons are stock original size at 88.97 for a 89.00 mm bore   .03 is the clearance   

How big is the bore currently ?  Is there any ovalness or taper in the bore ?  A good machine shop can tell you if you are curious. 

No. Standard Bore is mostly more than 89.00 - Piston clearance default is 0.045 + 0.005 
Just measure Piston and Bore and I guess you'll have to bore hone and use 3 Piece oilscrapers

Edited July 13, 2023 by uai

[jimk]

9 hours ago, stephers said:

Check the drain back holes in the oil ring land on the pistons

+1  Hold a small light inside the piston and peer in the oil drain holes.  If closed up, the oil couldn't get back to the crankcase and then works it's way into the combustion chamber.

[John76]

On 7/12/2023 at 8:35 PM, rstclark said:

the pistons are stock original size at 88.97 for a 89.00 mm bore   .03 is the clearance   

The manual shows the standard pistons are originally 88.97mm for a bore of 89.015 +/- 0.005.

Bore tolerance is therefore 89.010 to 89.020. My piston to wall gap for #1 cylinder is 0.012 mm. Is this good?

23 hours ago, stephers said:

One piece oil ring ? 

Yes, 1 piece oil ring. I did check the holes and they were a bit dirty, so I cleaned them up.

 

23 hours ago, tech71 said:

How was the head? Any worn guides? Crappy seals? 

Rabbit hole indeed!

Head was rebuilt (valves, seats, guides, seals (new style), 1 rocker shaft, 2 rockers) 5k miles ago. I think the head is solid.

Did not touch the lower end 'cuz I figured it was good with 3k mile oil changes since new.

23 hours ago, Mike Self said:

check the ridge at the top of the cylinder--where the rings don't reach. 

Did this Mike. No ridge detected or measured. Ring gap consistent throughout full range of stroke.

Timing chain OK. Oil pump chain was really sloppy. Needs shims?

 

Thanks for all the feedback.

Here are a few pics of the pistons and bearings.  Do you see any issues with the scuff marks on the thrust side?

 

 

Oil deposits on intake valve face suggests oil entry into combustion chamber is coming from the crankcase, not the guides/seals. Is this a correct assumption?

 

 

 

 

#1 big-end bearing top. Any issues?

 

 

 

 

[rstclark]

Cranks are bullet proof. For the price of all bearings just install new ones Particularly if you do not have oil pressure issues

Don’t overthink this

[uai]

1 hour ago, John76 said:

Bore tolerance is therefore 89.010 to 89.020. My piston to wall gap for #1 cylinder is 0.012 mm. Is this good?

You really have to measure the piston and bore. And you have to have the right tools to do so.
But it's a bore/hone new piston job at 140k miles anyway

Edited July 14, 2023 by uai

[Mark92131]

13 hours ago, John76 said:

Needs shims?

 

Yes, the shim(s) will correct that sloppy chain.

 

Mark92131

 

 

[John76]

Middle picture on my last post (above) is labeled incorrectly.

 

Here's the Intake and Exhaust valves on cylinder #4.

 

Time for a vacation!   😎

 

 

 

[John76]

5 minutes ago, Mark92131 said:

Yes, the shim(s) will correct that sloppy chain.

Thanks Mark.

My pump had 2 shims (0.005" each).

Do these come in various thicknesses, or do you just keep stacking them up to remove the slop?

John

[Mark92131]

2 minutes ago, John76 said:

Do these come in various thicknesses, or do you just keep stacking them up to remove the slop?

 

They come in 1mm and 2.5mm from Ireland Engineering.  You stack them until the slack is gone, but the chain deflects slightly.  I think I have a couple of 1mm's left from my S14 if you need some.

 

Mark92131