Ring Failure--second time
 

As stated I'm having ring failure problems on my build.
1.8 swap, '99 head, mspnp2, 2560, rods
Intentions with this car is a fun daily and built as such.

First build was with forged pistons, gap was set by me to specs given, actually bottom ring was a bit bigger so I didn't change them. top .016 bottom was .020-21 so I didn't touch them. Mainly bottom ring failure. Ran for all of 5 min before extreme ring failure. the specs called for 16-18, but conversations with the manufacturer said not enough end gap. . . I really wasn't convinced as we followed all specs given to a T, even ripped it apart and double-checked our measurements.

Ran out of money so I slapped stock pistons in the next motor, new rings, .016-.018 gap. This setup ran for 500 miles, then when I introduced boost, got failure again, no more than 9-10psi, worked up to that over 50 miles, obviously the last 50 miles. I haven't ripped the motor apart yet, but it's not bad, but rings are failing.

Startup was very straight forward as I had been running a conservative tune before the bottom end build. I really can't say I missed anything as I started right away, set timing, no issues came up. My question because I am at a loss is, what could I be missing? Even with end gap being smaller than it should have been, 10psi from a 2560 should not have given ring failure even if it was not enough. Any help with ideas would be greatly appreciated.

What type of pistons. We tend to like Supertech pistons with Weisco rings.That seems to be the winning combo.

They were Supertech. Didn’t want to bash anyone for whatever I’m missing which is why i didn’t mention it.

What size pistons and compression? Mostly asking just to get more info.

I have only used 84mm Supertechs and Wesico rings. I will try to find the paperwork of ring gaps I have used.

Thanks so much, they were oversized, I believe 84mm which is one step up, 8.6:1. I forget as it has been a while.

What was the failure? What was the condition of the bore? How was the bore conditioned, eg boring, honing? How were the the rings installed? The napier hook rings are very easily damaged from installation. There is only one ring option for these engines and that is the NPR 21-GNH-08400.

Bore was honed properly by shop. Bore was scored, rings were stuck closed and worn, pistons scored enough to bucket them. Debris got into the ring lands and stuck the rings. Supertech’s conclusion was end gap, but as far as our measurements we’re concerned we were within specs given. The fact that second ring failure happened with Mazda piston/rings with gap provided is why I’m scratching my head wondering what I’m missing. I was meticulous with end gap, double checking halfway down the bore. Machined by a shop that is a spec miata Race shop. I’m confident in my machine shop, I thought I was confident in my assembly skills, or at least I’m scratching my head now. Just looking for options as rings shouldn’t be this problematic.

.017 top ring, .018bottom ring are what I found in my emails as possible ring gaps. I need to do some more searching later.

It was not a ring end gap issue. Most likely there was not enough piston to wall clearance. You say ring failure, but it was a piston failure. Not enough clearance and maybe the cylinder temps were to high. The 4032 alloy of the ST/Malhe pistons have a lower rate of expansion compared to the 2018 alloy commonly used but they are still a forged piston and need at least .0035" clearance for an 84mm dia.

We made sure we had the proper clearance on pistons, had that issue on a different motor unrelated. Assume proper clearance on piston to wall. Can’t tell you how many times we checked and double checked that. Top of cylinder wall, midway, and bottom.

How much boost, timing and AFR's? Also not the cause but what oil? Pics of the pistons, crowns and skirts?

Didn't read thread, but are you 1000% sure the cyl bores are straight, true, and appropriate size?

I’m working on getting the old block measured again, but I did a quicker check with no issues to slow the build down. The new motor, yes I’m sure, but will check when I have it apart.

the boost was tuned out, EBC, First motor complete ring failure was about 4 min after setting timing. Second motor, I didn’t see boost until 450 miles on the motor, then worked up to 10lbs until failure, or the start of failure. Timing Map set conservative from my running setup, same components, new blocks. Fuel was not an issue, leaned that over the years. Just wanted to get it Ready for dyno tune.

It's been years, but I think I ran 0.017 and 0.021

Shit....i asked the question :rofl:


https://www.miataturbo.net/engine-pe...-follow-81272/

I read this thread, on my supertech pistons, I was to specs on my bore per the xx rings. The OEM piston/rings I may have went a little small, but I went bigger than stock and still got failure, I can see a problem with high boost, but not where I was. My next set of rings gaps will be bigger, but still wanted to fish around for some more info to see if I’m missing something.

If it add's any value, that motor I built in 2014 using those ring gaps is still running strong on someone else's car. It's seen over 315whp it's entire life, peaking up into the 400's. Of course YMMV and it could have been some black magic voodoo by Bob the master machinist in the basement of a NAPA Auto.

Im thinking maybe the bores are ovalized? I run wisecos and i forget my gaps but they are on the loose side for certain.

How does someone bore an oval hole? Sounds impossible.

Dunno just throwing out ideas you got any?