NB8A main/big end bearings - any known faults ??

[PeterB]

Running my stock NB8A at the track day on road tyres until an unhealthy rattle suggested the main/big end bearings had had enough.

Limped her home, but she is F*****d.

There were no warning signs. It is a 150,000km old 1998 motor running Castrol Edge with oil level on the maximum, no warning lights and temperature was constant, just a nasty rattle when I came off track last run.

It will be interesting to see what caused the failure - oil pump, clogged gallery, oil surge ???

Has anyone any experience with bearing failures in early NB8A engines and if so is there any tricks if I rebuild her - it will be staying stock.

[zossy1]

It will be a big end, unlikely to be mains...

The primary failure mode is oil pump failure, resulting from harmonic vibration. So when you rebuild, use a Boundary Engineering billet oil pump gear and, if the budget can stretch, a 949 or ATI harmonic damper. Shim the pump to deliver a little more pressure.

Also use quality bearings, either ACL (if available) or King race bearings.

Another possibility is oil surge - an Accusump and/or improved Baffling in the sump is the best value option to address this.

[JBT]

Some early NBs had problems with thrust bearings - main and big end bearing failure can follow in short time....https://www.youtube.com/watch?v=bpGikNccHak

[PeterB]

Thanks for the advice. A shimmed oil pump and good bearings was on the agenda, but the harmonic problem is a new one to me and I will follow that through - thanks

I initially assumed big ends, but as the rattle is on float, but silent under load a couple of people have suggested mains, even though this is unusual. A strip down will show.

On reflection there is a big gap in her service records before the last owner, so maybe she went a fair while without an oil change and while she was happy pottering round the street, a couple of lively runs at Wakefield has dislodged some junk and blocked a gallery.

My last few motors were built 1600s with uprated oil pumps and easily took any treatment I dealt them, but I haven't run a stock 1800 hard at the track, although have seen them flogged mercilessly without them giving trouble.

I guess if I go tracking a 17 year old motor I have to expect some failures !!!

[Vat]

I lost a motor at Lakeside in November, that in hindsight was probably on its way out when I bought it 10 months prior. Rattle/knock started at around 2,500 to through to 3,200 or so, motor was happy to wind out, but come Lakeside the knock worsened under load and then got dramatically worse (as in "sounded like a small child with a hammer trapped inside" worse). That was suspected to be a little end or gudgeon pin in the number 3 cylinder.

[chefie]

I initially assumed big ends, but as the rattle is on float, but silent under load a couple of people have suggested mains, even though this is unusual. A strip down will show.

It would be great to hear the outcome of the strip down, my 210Klm NB8A is doing ok at 175Kph on the straight at LS and hitting the limiter in 4th , that is wringing its neck at 7000+, I do have a building nest eggs for that day

look forward to the feedback

[Okibi]

Further to JBT's comment - http://www.miata.net/solo/99miatathrustbearingfailure.html

[rodent]

Interesting. My 1994 NA8 motor has died the same way this past weekend. Will be interesting to see what you find in the strip down.

[PeterB]

The NB8A 1.8 is out of the car and initial stripdown has been done.

Big ends 1 to 3 are toast, with number 4 still servicable. Thrust washer shows sign of wear, but no apparent problems with oil pump, harmonic balance or other ancilliaries.

Diagnosis - oil surge through the fish-hook at Wakefield. I must cop responsibility as when I drained the sump, it came up at least 1/2 litre low - bugger. I am usually anal about checking oil level and run it a little above the high mark - so either I have mis-read the dip stick, or overlooked it in the rush of the morning as I was scrutineering and then driver instructing. Note to self - triple check the oil level !!!

[PeterB]

Interestingly when I was removing the motor it was obvious some-one had been there before me.

The two top bellhousing bolts were missing and the short bolt from the rear gearbox mount to the PPF was replaced with a mis-fitting bolt and nut. Makes me think nervously about my enthusiastic hillclimb launches !!!

The sump gasket sealant was not original and the bellhousing was filthy with caked on old oil.

Maybe a quick rear main seal replacement had been done in the past, or some other repair requiring the sump off and gearbox out ??

A quick measure looks like a hone a crank polish will be sufficient machining, but a good clean and measure will show.

It has to be a stock rebuild so I can continue in Clubman class - Zossy's short motor was sooo tempting, so lucky it sold before I lost my rational thinking.

[manga_blue]

Even for Clubman class there are still things you can do to improve durability and to get the best out of it. Engine balancing, ACL bearings, stronger studs, etc don't improve performance but give you peace of mind. Matching ports and removing any obvious casting flaws in the ports without reshaping or polishing are just correcting manufacturing faults.

[sailaholic]

Might pay to check your dipstick. Different Mazda engines ran an identical looking dipstick with different oil level marks. Even BPs were different between fwd and rear it is believed.

The issue has come up before of oil volume not matching dipstick.


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[TrackAttack]

Might pay to check your dipstick. Different Mazda engines ran an identical looking dipstick with different oil level marks. Even BPs were different between fwd and rear it is believed.

The issue has come up before of oil volume not matching dipstick.

And a follow up to that, the dipstick tube that seals in the sump has many different options also. If the car has been played with, its possible it has the wrong length dipstick tube or dipstick.

I wouldn't bother with studs anywhere. I would rather spend the money on an NB8B sump and MBSP, with the engine mount brackets to suit. Genuine gasket kit through-out will save you hassles in the long run also.

[PeterB]

The other little mystery was a 2cm square piece of hard plastic in the sump, must have come in with an oil fill at some time. All in all it was lucky we had to pull it down before something really went bang.

The motor is the parts cleaner then off to the engine builder for a measure up and head hardness check.

Thanks all for the advice, much appreciated.

[PeterB]

The final part of the puzzle has been revealed :

The front of the sump has been welded, presumably to fix a hole. The car also has evidence of a small front end repair.

Guessing - left road with front left and hit a rock or other object holing sump. Gearbox and sump removed, sump welded and re-fitted. Gearbox re-fitted leaving out two top bellhousing bolts (too hard to fit) and replaced lost rear mount bolt with dodgy nut and bolt. Then sold on quick !!!

How long was it run after sump holed, etc ??

Next lady owner used is a daily and it probably never got a rev and was very happy with it.

I then got it and flogged it around Wakefield with oil level low and goodbye big-ends.

Conclusion : no inherent design or component faults, just a series of user errors culminating in the rebuild.

Whatever - the end result is the same and I will have a fresh engine and clutch for the future, but minimal motorsport budget so she will have an easy life for a while.