Crank thrust bearing problem
 

Has anyone had any issues or know of any issues with the crankshaft thrust bearing?

Twice, the rear bearing has disintegrated with very little run time. Upon assembly, the crank end play is within Mazda's specs. Front washer is fine. Seems like maybe it's being loaded from the rear somehow. Clutch/flywheel is a supermiata race twin clutch from 949 with the throughout bearing from them as well. Stock 6spd tranny.

I've not had an issue with them. I use ACL thrust washers and a VERY heavy pressure plate.

Maybe im way off base here, but what damper and rev limit?

Do you still use the shift interlock? iv used acl thrusts with no issues on various setups. Also although unlikely but we should present the question, are they being installed the right direction?

ATI superdamper and never even saw 6k. A harmonics issue would show a problem somewhere other than the thrust bearing anyway, I believe.

Car was never even driven more than out of the garage. No shifting. before it destroyed itself. And yes, bearings installed with oil passages to the crank surface.

And this has happened more than once fot you? Have you had your crank checked for straightness? I have had to straighten the last 3 cranks I have used.

I honestly have no ideas other than that. Sorry

Have you had a machinist look at your crank for protrusions or unevenness? The main cap above the thrust bearing, does it have any wear or markings gouged into it by the crank?

When I hear that an engine with oil pressure (I assume, because this is sort of an obvious thing) has destroyed the rear thrust bearing just sitting in the driveway on first startup, my first instinct is to suspect that there is some sort of problem with the shape or finish of the crankshaft such that it is gouging or tearing up the thrust washer. Or maybe there is too much space between the crank and the main cap such that the washer is getting loose (partially or completely) and wedging between the main cap and the crank? Would installing the cap backwards do this? I've never tried it. Are all the main cap arrows pointing the same direction (towards the nose of the engine)?

I can't think of anything else that could explain this sort of instant destruction. Rear thrust bearing is probably the least stressed bearing on the miata engine unless you're running the engine backwards and using the nose of the crank to drive a propeller or something.

So it was a new build on a different shortblock than we had been using. All parts should have been checked, but honestly, we had some issues with the machine shop we had been using.

After the first time, we figured error on our part. i.e. installing the bearing backward or something. Took it to a different machine shop to check the crank but not sure that they checked it for straightness. It was balanced, however, I would think that would show if it wasn't straight. Triple checked everything on assembly and it happened again.

Now we have another different shortblock at the machine shop now thinking that maybe there was a defect with the other crank or block that wasn't being detected. However, if it's something else causing the issue, it's just gonna happen again. Now the easy way to find that out is to just assemble the newly obtained parts and see if it happens again. But obviously that's a very frustrating and somewhat expensive way to find out. If it didn't happen, we'd know that the other crank/block has a problem. Just trying to brainstorm to avoid it a third time.

Definitely a long shot, but the nb2 bp is machined for a larger thrust bearing. Any chance you have the nb2 block but are using the nb1 thrust bearing?

I had a machinist look over his parts books and swear that I needed the cheap nb1 thrust bearing while I knew it was an nb2 block and required the expensive oem mazda bearing...

No, thrust bearing diameter is correct, but keep the ideas coming.

Do you have to press the clutch to start the engine? I noticed some galling on the back thrust bearring on my last engine with low miles from loading the cranck without oil pressure, i since put the clutch switch bypass from 949

No clutch switch

Did you put them in the wrong way around? It's an easy mistake. Edit: I know you said you checked it though. Any photos of the bearing / block?

If its being destroyed whilst the clutch isn't loading it, there must be something on the friction surface thats eating it.

Bottom end assembled. Endplay is good. Drug the transmission out from under the car. Installed the 949 clutch assembly. Bolted up the tranny. Zero endplay. Engine still turns over easy, but I'm getting force against the back of the crank somehow and can't quite figure it out. Measured everything like 5 times and there should be enough space for everything. Very frustrated right now.

It's killing the rear bearing? Force against the crank pushing forward should kill the front bearing, no?

No, it would be pushing the crank forward off of or away from the front bearing and pushing the crank into the rear bearing.

Derp, I'm having a Monday.

IMO, it's something in your trans, either the interaction between the input shaft and the discs or the pilot bearing/flywheel. Pull the discs, leave the flywheel in place, and bolt the trans up and see if you still have proper endplay (~.007").

do you have the thing metal plate between the trans and engine? try removing the pilot bearing and see if load backs off. maybe the pilot isnt in far enough. Also, are you using arp flywheel bolts? though that would cause other problems, i have seen issues with arp and stock flywheel bolts with 949 stuff.

It should be a pretty easy thing to pin down. Just keep checking endplay every time you unbolt something.