crank thrust question
 

Hi all,
I have a question that comes with a bit of a story that I searched for and found nothing. I have a 1999 NB stock engine with 40kish miles on an economy rebuild of the engine, I was replacing my crank sensor a few weeks ago. I was on the side of the freeway (things always go out at the best time.), I didn't have a socket or any thing I could turn the crank with, So I had my buddy bump the starter to spin the crank so i could set the gap some where in the vicinity of the thickness of my business card. When he hit the clutch to start it, the crank noticeably moved forward. not like a quarter of an inch, but i mean enough that i could very easily see it move, easily 1/64th of an inch. So from people who have experience with these cars how bad is this? Is this normal for a lightly worn engine? Should I get the car off the road and start ordering parts for a rebuild? Car runs great no odd noises, It goes into the mountains on a weekly basis as well as being daily driven and out of the 40k, at least 2k of them are on the race track, probably more. I always use a decent synthetic changed very 3k or after a track day(castrol edge or schaeffers if i can get it). so any thoughts or opinions welcome, thanks in advance.

Maximum end play for a 1999 engine is .012"

https://cimg6.ibsrv.net/gimg/www.mia...52673b7290.png

If it truly is 1/64 you're at .015. This is above maximum recommended but is usually not a killer deal. I'd measure with a dial indicator and as long as it was below .020 I would use engine.
Every time you depress the clutch pedal you are mechanically pushing the crank against the rear face of the rear thrust bearing and they will wear with age.
Now if you need an excuse to rebuild one anything over .012 qualifies...
Some engines (Toyota R series for sure) hold the rear thrust bearing on a tiny ledge and it's common for those to wear then DROP OUT leaving you with .200-.250 play.
If the Miata can drop bearings like this then being perfect is more important.
I don't know BP engine well enough yet to advise further.
Hope this helps
Rick

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Every new engine I've ever put together has been .006-.008" thrust. .012 would sketch me out. .015 would have me pulling the bottom end apart ASAP.

This is not the kind of thing you can eyeball. You need a dial indicator.

if you can easily see it, i would bet it is way over spec. .012 isnt much at all and really isnt very noticable.

so the answer is I need to dig my dial indicator out of storage this weekend, find out how bad it is, and probably plan on rebuilding the bottom end

engine has about 40k miles on it, it is nearing time for a rebuild any way, but I was hoping to squeak another 10-15 out of it.

40k and needs a rebuild? You know these engines almost universally last to 200k right?

He said "40kish miles on an economy rebuild of the engine" so no telling what the mileage is on the bottom end.

also stated at least 2k on the track.

I did a full rebuild and had the crank machined, I just didn't have it bored out, and I used economy bearings and rings, also a DNJ gasket, and car has 209k also didn't seem relevant

Measure and know.

I pulled out the old dial indicator, and my thrust is between 12 and 13 thousandth's, so I guess I got alarmed for nothing, thanks!

At or over the Max, but you are happy?

So you're bottom end has 42K miles on it and the thrust bearing is already at or out of spec? Did you not put in a new thrust bearing and check it at assembly stage?

yes, for the amount of abuse this motor has seen, yes I am very satisfied. It will probably be getting rebuilt with forged internals in a year or so, but I really don't have the time or money to do that right now. so I am very happy that I can ignore this for another 10-15 thousand miles till I am ready.
Yes, I put new thrust bearings in it, I have no idea what thrust was at that point. I built this motor in like 2014, And I kept 0 notes out side of my head from the build. The motor isn't using outrageous amounts of oil has no funny noises, I get that it isn't ideal, and that out of spec is bad. I'm willing to bet that if you were to check thrust on every single motor out there at 50k miles quite a few would be out of spec. This isn't the end, I will be checking thrust every couple of oil changes till it gets rebuilt If there is any interest I will post the results here, but right now I see no reason to take action at this time. What i am saying is that I am ok with a 55-60k mile engine life with how hard I am on this car stuff wears out, nothing lasts forever.

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You're butthurt because nobody recognized your redundant comment? The first reply had all the information he needed.