need help checking out a miata in L.A.

The airfilter box on my SC had a nipple open to atmosphere for, probably, quite some time. The sky did not fall in, but my SC was sandblasted after

100k. Whether it was from the open nipple or the lousy airfilter, I do not know. Nor how my engine looks inside.

You get what you pay for. I assume you checked at least the crankshaft issue on miata.net?

Leon

No, what is that? I vaguely recall from previous reading that there was some issue with a "woodruff key" sometimes being incorrectly replaced when the timing belt was changed, but I assumed that that had either happened or not, i.e. that whatever damage it would cause if done incorrectly woul by now be manifesting itself.

Okay, been doing some reading and that I least I needn't worry about. The VIN number shows that the car has the newer crankshaft and bolt.

My understanding is that no 1990 model has the newer crankshaft nose. The change was made in mid '91, so some 91s have it; some don't.

If the VIN number indicates the newer crank nose, then, from what I know, it is not a 1990. If it is a 1990, re-read the stuff on Miata net about the crank nose. The newer information suggests that 100% of them will eventually fail.

Ken

I would beg to differ with 100% will fail.

I think what you are debating is that 100% will fail. The "newer information" I reference is the revelation that the design depended on the key to bear the twisting torque. The engineer from Loctite suggested this was bad design. I have no reason to disagree and what I contended is that this information suggests 100% will fail. Mine was OK, till I removed the pulley to change the oil seal at about 150K miles. I have the shop manual and put it together per the instructions. So many thousands of miles later, it failed. I also kept the key from the original repair. Looking at it, it's obvious it had seen a lot of stress. The keyway slot in the crank nose, at

150K miles, was pristine. After it failed, it looked just like the image Harry Sue provided.

There may be more going on, however. I did the Loctite repair a few thousand miles ago and it failed again. I re-used the crankshaft timing belt pulley and that was probably my mistake. When I inspected the pulley, I noted a small amount of wear on one arc section of the flange that faces the engine block. It appeared somewhat minor, so I touched it up with emery paper.

This last time, the entire circumference was involved. Looking at the one I removed and the new one I installed, there was what I believe to be a major difference in the design. The old pulley had a recess of 1mm where it meets the shoulder of the crank nose. The new pulley does not. From what I can see, I believe there is no longer an opportunity for the pulley flange to rub against the engine block. Take a look at the image of a pulley that literally fell off the crank nose

Notice a couple of things inthis image: 1. The very deep concentric groove at the outer flange whereit was making intimate contact with the engine block and 2. The 1mm"recess" I mentioned (this is the first 4mm measured radially from thebore); it's very clear in the image. Consider that the wear on the outer circumference is due to the flange making contact with the engine block only below the crankshaft. This means that the stress on the pulley, which is transmitted to the crankshaft nose, is asymmetrical. As the crankshaft turns, the force is exerted on the nose from one direction, which creates a flexing force. This is likely what caused the crank nose in the image to shear. So besides the key taking all the twisting torque, this business of the flange rubbing on the crankcase seems like another contributing factor, i.e. more bad design.

Harry Sue stated that he didn't know why his repair, and some others, lasted when others did not. I believe Harry used a new pulley, which may have made the difference for the reason I stated. I just completed my second repair, with a new pulley, this evening. We shall see how well it works.

Ken

That, however, is what miata.net says (see Lance's updated info in the garage section.)

Why not have a poll?

Of the two high mileage 1990 Miatas I know of, one failed, and the other had apparently one of the fixes applied (in particular, bolt welded on.)

Leon

I have some difficulty believing that the pulley would rub against the crankcase unless the crankshaft was already damaged. That would be a truly major design blunder.

Any good low mileage 1990 Miatas out there that show the abrasion?

Le>I think what you are debating is that 100% will fail. The "newer

To clarify my comments somewhat:

The first removal and replacement of the pulley was done using the procedure in the shop manual at about 150K miles. The reason for doing so was to replace the front seal. I anticipated needing a new key and replaced it properly. The old key (originally installed when the engine was built in

1989 at the factory) showed definite signs of stress. I can provide an image if anyone wants to see it exactly. The keyway in the pulley and the keyway in the crank nose were in excellent shape. Everything went together snugly. I do not recall if there was any visible wear on the circumference of the pulley flange at that time.

This repair failed in so many thousands of miles in the classic manner. I noted some rub wear on the circumference at that time. It was confined to an arc that was approximately 1/4 of the total. Not really understanding the implications, I used some emerycloth to touch it up and re-installed it using Loctite and a new key. This failed, again in so many thousands of miles and showed complete involvement of the flange circumference similar to the referenced image. I noted that the interference was on the crankcase below the crankshaft. For whatever reason, this area protrudes further. I suppose I am lucky that the nose didn't crack off for the reason I mentioned.

I have a few hundred miles on the new repair using a new gear and Loctite. I will consider it OK if I get at least 5K miles with no further problem.

Regards,

Ken

I am the first owner.

There was no visible damage to the shaft or the pulley when I first put it back together. The only damage was stress to the key, which I replaced. All parts fit well together with no slop. There was no reason to believe this flange would be anyplace other on the crank nose than it was for the first

150K miles. I did not use a new bolt, but belive that this would make absolutely no difference. The manual did not suggest replacement of the bolt; people seem to think it will elongate like a head bolt which is under a lot more stress. That's a stretch (pun intended).

I believe that some sort of deterioration was involved, but don't understand how. Also, it is an absolute fact that the new replacement wheel from Mazda did not have the 1mm recess I mentioned in the initial post. 1mm is a big deal in a situation like this.

Ken

It is however hard for me to believe that the key would be damaged and the keyway not. Regardless whether it is visible damage or not.

And your own facts show that replacing the key did *not* produce another 150 kmi of reliable service. Unless you made a mistake such as improper torquing, cross-screwing threads, putting the key in wrong, etc, of course.

Leon

No need. I believe you.

No, but its existence can be easily inferred from the subsequent failure. It being the only possibility if we believe your claim that you did not mess up the re-installation

Leon

If you look at the stuff posted on Miata Net by Harry Sue, the keyway looked almost exactly like his after it failed. Prior to the failure, the key fitted precisely; i.e. there was no visible damage to the keyway in either the gear or the crank nose.

For the record: I am not a novice. I have been working on cars since I was

1. Since the mid '60's I've owned a Sunbeam Alpine, a TR6 a TR3 and the Miata. I did the timing belt on the '90 around 80K initially and was going to let it go to 160K. I changed it at 150K since the water pump failed. I didn't touch the pulley at the first timing belt change.

I'm also an electrical engineer and have a good background in failure analysis. I manage mechanical design (including high volume sheet metal stamping tools and plastic injection molded tools) as well as electrical design. I don't have a root cause for this failure but suspect it is all that was previously written and likely is complicated by other tolerance issues that Mazda attempted to mitigate by removing that 1mm undercut.

I would be curious to know if the guy who had the gear shear off changed his oil seal and had the same results as me.

Thanks for the scrutiny, Leon.

Best,

Ken