Piston ring gap HELP!

E28 technical advice asked and given! Troubleshooting, modifications and more.
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Dirtym30
Posts: 75
Joined: Jan 12, 2020 5:30 PM
Location: Toronto, Canada

Piston ring gap HELP!

Post by Dirtym30 »

Hello everyone, I’m currently rebuilding a m30b34 in my e28. I’m measuring the ring gap and the top compression ring and oil ring are within specification but the middle compression ring is way out of spec.

BMW spec says 0.20-0.40 mm or (0.008)-(0.016)

My feeler gauge is telling me the gap is .660 mm or (.026).
This is way out of spec and I’m not willing to throw it together without knowing first if we’re within range.

To my knowledge the rings I bought new are for an m30b34 and the cylinders were checked at the machine shop and were not bored out.

Could you gentleman give me any insight?
Philo
Posts: 2166
Joined: Feb 12, 2006 12:00 PM
Location: Long Beach, CA

Re: Piston ring gap HELP!

Post by Philo »

I put my custom S38 together with a .015 ring end gap using after market pistons and rings.

Make sure your ring is square in the bore when you check it. Check the bore sizes to make sure it's to your spec. If it is, test all the top rings in the set and see if they all give you a to large of a gap. if they do sounds like it's time to buy a new set of rings as something is definitely wrong.

Hang in there, others will reply, there's a look of engine building talent here. :)
Dirtym30
Posts: 75
Joined: Jan 12, 2020 5:30 PM
Location: Toronto, Canada

Re: Piston ring gap HELP!

Post by Dirtym30 »

Haha I will definitely be keeping a close eye on the forum as I ran into this with the first 2 cylinders middle rings and immediately fled here in panic. I flipped the motor over and started measuring my new main cap bearings to find they were also not even close to spec.

Going to go through all my options here as I’m determined to have this thing on the road for spring time. And then turbo.

Thank you for the reply
:rofl:
Mike W.
Posts: 26872
Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: Piston ring gap HELP!

Post by Mike W. »

That seems excessive, but not extreme. I've got an old BMW big blue book, factory manual and for an E12 528i, it shows .012-.020 end gap on both compression rings. But that's for factory parts, it's possible aftermarket parts might have a different spec and be made out of different materials or something. Maybe. 3 to 4 thousands clearance per inch of diameter is the traditional standard. You might want to contact the ring manufacturer, if you can, and question them. Have you stacked the rings next to each other to compare to make sure it's not the measurement process? I think I'd do that before even phone calls, just to make sure. But .026 does sound big. And I would expect the top ring to need more clearance if anything since it would run hotter, I would think, and need more room for expansion. But the book says the same for 1 and 2.

Now .020 is the first standard overbore size, and since Pi is nominally 3, that would equate to .060 increased gap, plus the clearance gap, so I doubt they are for the wrong size pistons. But that doesn't mean they're ok. What brand are they?

What are your main caps showing and how are you measuring them?
Tiit
Posts: 290
Joined: Oct 06, 2017 6:27 AM
Location: Canberra

Re: Piston ring gap HELP!

Post by Tiit »

I suspect you are assembling a turbo motor? Turbo motors need more ring gap than stock.
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