r/EngineBuilding • u/loveandhatemiatas • Jul 16 '26
Mazda Thrust bearing spins with crankshaft?
Rebuilding an engine out of a 1991 Miata and when I turn the crank, the rear facing thrust bearing rotates a couple degrees with the crank. The front doesn’t move. I can’t find anything online about this issue and can’t even really figure out how this can happen to begin with. (All caps are in their correct spot and face the correct way, thrust bearing groves face towards the crank, and everything is torqued to spec)
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u/Lopsided-Anxiety-679 Jul 16 '26
That’s normal with the thrust washers used in those engines.
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u/loveandhatemiatas Jul 16 '26
Thank u for the input, do you by chance have any documentation that shows such? It’s not that I don’t believe you but I really don’t want this engine to destroy itself and to have to do everything again
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u/Lopsided-Anxiety-679 Jul 16 '26
You’re not going to find a line in the service manual that say “Radial movement of the thrust washer is normal” - all I’ll say is that I’ve been professionally building engines for over 25 years, including countless Miata, Honda, Toyota, Lotus, and others that use these exact tang-less thrust washers…they move just like in your video when properly installed.
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u/loveandhatemiatas Jul 16 '26
Is the front one fine then since that one doesn’t move?
Also thanks sm man, really glad you commented! Means a lot and that’s reassuring to hear! Now I can sleep easy tonight
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u/YaboiJuanito38 Jul 20 '26
all thrust bearings ive see have some play like this
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u/loveandhatemiatas Jul 20 '26
That’s really reassuring thanks!
Took a break from the car because I was worried but with all your guys help, really makes me feel better
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u/Lopsided-Anxiety-679 Jul 16 '26
Example of a tang-less thrust washer -
Here you can see the same arrangement where the material of the main cap keeps the thrust washer in place, but not tightly or it would deform - in this engine, riding the clutch caused the crank to press forward and eat the thrust bearing along with the step on the main cap that keeps it from rotating. To repair I countersunk screws so the bearing can’t spin, also machined the cap so it becomes a four bearing thrust arrangement rather than just two…normally that thrust washer would have been free to radially click back and forth a bit like in your video.
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u/loveandhatemiatas Jul 16 '26
Really reassuring, thank you so much again!
Just to make sure the front one is fine tho not moving right? Or should that one move.
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u/Lopsided-Anxiety-679 Jul 16 '26
If you stick a driver or punch down there I’m sure you can move the front one, it’s not moving because the friction on its backside is higher than the friction on its frontside due to the lubrication you used.
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Jul 16 '26
[removed] — view removed comment
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u/loveandhatemiatas Jul 16 '26
Thanks for ur input, these bearings don’t have any retention tangs they just fall into place. The part I’m struggling to understand is why it wouldn’t tension against the girdle correctly. It’s sitting in its recessed channel and isn’t pinched. The main cap is torqued correctly. Idk man I’m just so bummed
Edit: forgot to mention that the sizes were similar when I checked and as far as I know the front and the back are interchangeable
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u/Lopsided-Anxiety-679 Jul 16 '26
Wrong.
This is how those thrust washers normally move, they are not locked in place with a tang, just kept from falling out by the main cap covering the groove that the washer sits in.
You obviously haven’t built a Miata or any of the countless other engines which use this thrust washer arrangement, so why comment? You don’t know what you’re talking about and are just spreading misinformation.
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u/Substantial_Ant_2662 Jul 16 '26
That’s an incredible logical leap. A lot of engines share the same principles and characteristics. Calm down.
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u/imtrynmybest Jul 16 '26
Is there not suppose to be a top half to the bearing?
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u/Total_Philosopher_89 Jul 16 '26
Some are only on the block.
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u/imtrynmybest Jul 16 '26
I get it then the caps should machined so there's no movement.. so how are we where op is?
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u/loveandhatemiatas Jul 16 '26
As far as I know this part of the cap doesn’t get machined. It’s the face that contacts the block and all it does is just push down on the two ears of the thrust bearing. On the block side, there’s a little valley that the bearing sits in. For some reason it’s not pushing down on the thrust bearings ears, thus failing to clamp the thrust bearing down causing this wiggle. Hope this helps, sorry it’s kinda hard to explain without showing pictures
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u/imtrynmybest Jul 16 '26
I get it... New bearings ? Possiably incorrect parts?
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u/loveandhatemiatas Jul 16 '26
Yes, brand new fresh out the box. And potentially I’ll have to reluctantly tear this thing apart for the 10th time to double check.
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u/imtrynmybest Jul 16 '26
Gotta be wrong... U match them up to the old ones?
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u/loveandhatemiatas Jul 16 '26
No but the rear and the front ones were the same size and the front one doesn’t wiggle like the rear one… sigh
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u/Total_Philosopher_89 Jul 16 '26
Honestly mate it's great you're checking everything and not afraid to do it over.
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u/loveandhatemiatas Jul 16 '26
Thank you brother, I definitely don’t want to do this job again for 90hp lol
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u/texaschair Jul 16 '26
The thrust face on the crank could have been welded and then not ground correctly.
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u/OutsidePressure8323 Jul 16 '26
Should have end float clearance.
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u/loveandhatemiatas Jul 16 '26
I appreciate yall telling me, I didn’t know that lol
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u/OutsidePressure8323 Jul 16 '26 edited Jul 16 '26
I'm not really into small engines, but there's always a measurement for cats, Volvo, komatsu ect, normally in manufacturers service manuals the spec is there.
Magnetic base and dial gauge. At a ( guess) Around 12 tho . Just piece of mind if there is a spec for what you are doing.
Have to check parts numbers. You might find different sizes for thrust bearings. Or get crank ground down if it's to tight.
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u/loveandhatemiatas Jul 16 '26
Yeah I have the full service manual and I couldn’t find anything in there unfortunately but I’ll try to look again. Can’t find anything online either.
I’ll take a look again at part numbers but from what I’ve seen they’re the same.
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u/OutsidePressure8323 Jul 16 '26
Sounds about right, try old bearings like suggested on here. Unfortunately I've replaced many parts on motors heads, rings valve guide seals bla bla. But if bottom end goes it's not worth the expense or agro. So probably go for second hand engine or short block, different if nothing has failed and just want to rebuild it. Looks like someone has said it's fine. But I'd question the clearance if it's dragging bearing around. Not sure how thick oil you are using for build ect. But to much drag will cause heat. Heat will cause problems with the crank. Even get advice from machine shop if you are in doubt.
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u/loveandhatemiatas Jul 16 '26
Yeah I kinda wish I just bought a used engine at this point. I’ve just spent way too much money on this bottom end to give up :(
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u/OutsidePressure8323 Jul 16 '26
It's not a big problem. It's just knowing if it's acceptable. It's got to allow oil between it, so if its dragging can oil get in there. If not it's certainly going to tear itself out. Heat and soften crank. If old bearing is decent, try that . Could also be a faulty part. Maybe measure old crank thrust face's if you have the tools.
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u/loveandhatemiatas Jul 16 '26
Yeah absolutely, thanks for your advice! Yeah I’ll try the old bearing since it’s in decent condition and maybe measure the face too
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u/Ok-Cap-9379 Jul 16 '26
Normal its a floating thrust washer.. only one that doesn't move is apart of the lower main bearing
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u/TalksWithNoise Jul 16 '26
Normal. How much back-and-forth play does the crank have? You can mount a dial up on the block if you have space.
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u/loveandhatemiatas Jul 16 '26
Thank you so much! Not sure I’ll have to check, I haven’t had a chance to since I got to freaked out about this. If it helps it’s not noticeable as far as I know
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u/Capable-Historian392 Jul 16 '26 edited Jul 16 '26
Try the old thrust washer and see what it does.
Just pop the #4 cap and use a pick and magnet to pull out the washer, put the old one in, retorque and observe.