M20B25 Ring Gap

[BAlbertiX]

Completed a full rebuild with stock components May of this year. New main bearings .0025 gap, new rod bearings .0018 gap and new Mahle piston ring gaps .0135 gap with a 84-89mm 320 grit ball hone. Figured I was rockin with these specs.

Lost compression after 250miles...

Is .0135 too tight for a fresh hone? End ring gap spec is .012-.020.

The cylinder walls were not marred up, but see some gap ring steaks, and what looks like compression ring scraping at the top of the cylinder. Also every single ring was frozen into every single piston.

[MrBurgundy]

Did you lose compression on all cylinders?

How do you know it's the rings? Did you do a leak down test?

[BAlbertiX]

Did you lose compression on all cylinders?

How do you know it's the rings? Did you do a leak down test?

Actually found the issue with a simple compression test, compression ranged from 70 - 90 psi max across every cylinder. So tore the engine back down and above is what I found internally - all the rings bound up into the pistons, no longer free floating. The cylinder walls do not look too bad - but there appears to be stuffs that run up and down the cylinder walls by where the compression ring end gaps were in addition to significant full circumference scuffing by the compression ring at the top of the stroke.

My guess here is that an end ring gap of ~ .013 is too tight for a rebuild, i.e. heat caused rings to expand too much leading to binding in the cylinder.

Curious what you guys spec for new ring end gaps on a 320 grit hone? Any special cylinder wall cleaning notes?

[digger]

are the bores tapered? the end gap needs to be checked at the area of minimum bore (on a used block it is normally where wear is lowest assuming it was done properly before)

The end gap is not something you want to be at the tight end of the spec

[BAlbertiX]

are the bores tapered? the end gap needs to be checked at the area of minimum bore (on a used block it is normally where wear is lowest assuming it was done properly before)

The end gap is not something you want to be at the tight end of the spec

Fair point - I really should make sure I'm finding the low points in each cylinder.

I was doing some more research today and in addition to a tight ring gap, it looks like the ball hone grit size should be between 180-240 grit for cast rings and iron walls in the M20. I used 320 grit... which is apparently not good enough for oil retention.

Math is starting to add up...

[McGyver]

The Bentley specifies the top ring at 0.012-0.020 in, 2nd ring at 0.012-0.020 in, and oil ring at 0.010-0.020 in. Mahle specifies the top ring at 0.020 in, 2nd ring at 0.022 in, and oil ring greater than 0.015.

The general guidance is, "Too loose and no one notices, too tight and everyone knows." It seems like you went too tight, and now you've got to rebuild.

I read a bunch of stuff and watched several videos from Total Seal to get a better understanding of modern ideas about ring end gap. My understanding is that the 2nd ring should always be looser than the top ring, this provides a path for blowby to escape without fluttering the top ring. If the top ring flutters, you lose the seal and your compression. The oil ring should be at least as loose as the 2nd ring, but can be significantly wider (like 0.050 in).

My targets for my 2.7i that will be turbo'd are top 0.020 in, 2nd 0.022 in, oil 0.022 in.

Hope this helps.

[BAlbertiX]

The Bentley specifies the top ring at 0.012-0.020 in, 2nd ring at 0.012-0.020 in, and oil ring at 0.010-0.020 in. Mahle specifies the top ring at 0.020 in, 2nd ring at 0.022 in, and oil ring greater than 0.015.

The general guidance is, "Too loose and no one notices, too tight and everyone knows." It seems like you went too tight, and now you've got to rebuild.

I read a bunch of stuff and watched several videos from Total Seal to get a better understanding of modern ideas about ring end gap. My understanding is that the 2nd ring should always be looser than the top ring, this provides a path for blowby to escape without fluttering the top ring. If the top ring flutters, you lose the seal and your compression. The oil ring should be at least as loose as the 2nd ring, but can be significantly wider (like 0.050 in).

My targets for my 2.7i that will be turbo'd are top 0.020 in, 2nd 0.022 in, oil 0.022 in.

Hope this helps.

Thank you for the Bentley and Mahle specs. I'm aiming for the high end of the spec this go .018 top, .020 2nd, and .020+ oil. The Mahle oil rings are pretty open out of the box - they have to be .030+. Where as the top two rings are coming in between .012 / .013 out of the box, in my block. Top two rings will certainly require a little filing this go around.

I picked through a few of the Total Seal videos last night, good stuff. Also found great how-to content on Uncle Tony's Garage youtube channel.

Good luck with the M20B27 turbo!!

[digger]

After all this effort id be getting a professional assessment of the bore geometry and likely a new rigid hone job would be in order, as those bores look very dark and therefore are likely worn

[McGyver]

Also found great how-to content on Uncle Tony's Garage youtube channel.

Good luck with the M20B27 turbo!!

That's hilarious, I watched a ton of his videos to prep for my build. I followed his guidance for painting the block, we'll see if the rustoleum enamel actually holds up better than spray paint!

If the engine's out of the car, I second digger 's comment about getting it honed to make sure the bore is straight. This would also give you an idea of your actual piston to wall clearance.

[BAlbertiX]

So I got some real numbers tonight on each bore, pulled ring gap numbers up to 3in down the cylinder while also checking for light leakage between the ring and the walls.

.015's across the board and no light leakage, block has 85k, still send to machine shop?

Shown is the top ring position just below chamfer.

Def needs a fresh hone, I'm thinking 120grit will be overkill..? Aiming for 180grit.

[varg]

If you can afford to, and have a shop you can trust, have them machine it for your ring type and be free of any worry. There's just a risk of scope creep any time you walk into a machine shop, make sure you tell them not to hot tank/caustic wash the block because the intermediate shaft bearings are still in there.

[BAlbertiX]

varg McGyver digger come on you guys are no fun... You and everyone else I've talked too so far say go to a good machine shop.

It is decided. I will suck it up and do it right this time. I got a great machine shop 12mins down the road. Thanks for the heads up on the intermediate oil shaft bearings.

I do plan on beating the piss out of this car... the investment now will pay itself back with reliability.

Will report back soon.