Short nose crank on a 1991 Miata?

Hi all,

I bought a 1991 Miata yesterday to serve as a donor car for my next project. I noticed that the crank pulley has 4 slots on the front face, which I believe signifies that it has a short nose crank. Is this assumption correct? Also, I thought that the short nose crank was limited to the 1990 model. If both my assumptions are correct, it would appear that that the engine was swapped at one point with a 1990 engine. Another clue is that the cam angle sensor is on the cam closest to the intake side, and I thought that on later cars, the cam angle sensor was moved to the cam on the exhaust side.

If it makes a difference, the car is a Canadian car with manual steering (base).

Any feedback you have is appreciated.

Short nose has four slots. I am not 100% sure, but I thought early '91s had short nose.

The short nose crank was switched in the middle of the 91 model year, there is something on miata.net that gives the vin split.

All 1.6 cars have the cas on the intake cam.

if the crank pulley isn't wobbling I wouldn't worry about it.

I bought a '90 with 155k miles; no crank issues yet. I did replace the nut and retorque it for a bit of peace of mind.

from miata.net:

If your Miata is a 1992 or newer, forget this article. You have nothing to worry about. If you have a 1991 with VIN 209447 or more, you're safe. If you'd like, verify the stronger crankshaft and larger bolt are fitted by measuring the bolt shoulder diameter to be 1.455 inches. If your Miata is a 1990 or 1991 with VIN 209446 or less, you have a crankshaft with a smaller diameter nose. (Note: according to Mazda, the European spec MX-5 breakpoint is at 127442.) The pulley bolt shoulder diameter is 1.128 inches. There is an intermediate fix in the form of a longer nosed crankshaft with the same small diameter (Figure 3). Unfortunately, we can not predict whether you have a long nose crankshaft or earlier short nose crankshaft. The High Road as been unable to verify whether the long nose crank was fitted to any cars at the factory or if it is only a service part. If you order a new 1990 crankshaft from the dealer, you will automatically get a long nose version. There is no reliable 5 minute technique to check the condition of your keyway. Verification of these parts requires partial disassembly and examination of the components.

If you replace the bolt and retorque, you may want to investigate the torque spec. It is different then the later BNC cars. It is also rumored that Mazda changed the torque spec from what is listed in the original factory service manual on the SNC cars through a TSB or something similar, but I have never seen the actual documentation from Mazda. A search on miata.net should bring up info.

Does this.... http://raleigh.craigslist.org/cto/4518113958.html

+ this.... http://raleigh.craigslist.org/cto/4485327890.html

• turbo = cheap, fun swap for a DD???

pappatho wrote: If you replace the bolt and retorque, you may want to investigate the torque spec. It is different then the later BNC cars. It is also rumored that Mazda changed the torque spec from what is listed in the original factory service manual on the SNC cars through a TSB or something similar, but I have never seen the actual documentation from Mazda. A search on miata.net should bring up info.

There was a printing error in one of the factory manuals per the years. The torque spec for the small ones is 80-something, the big ones is just over 100. I forget the specifics, it's in Mazda Miata Performance Projects. The one thing people overlook from the factory manual is the use of blue Loctite.

mrwillie wrote: Does this.... http://raleigh.craigslist.org/cto/4518113958.html + this.... http://raleigh.craigslist.org/cto/4485327890.html + turbo = cheap, fun swap for a DD???

Could be - although check the pricing for 1.8 cars that aren't covered in nasty rattle can paint and haven't been hit by a bus. It may not be worth your time wrenching.

Keith Tanner wrote:

mrwillie wrote: Does this.... http://raleigh.craigslist.org/cto/4518113958.html + this.... http://raleigh.craigslist.org/cto/4485327890.html + turbo = cheap, fun swap for a DD???

Could be - although check the pricing for 1.8 cars that aren't covered in nasty rattle can paint and haven't been hit by a bus. It may not be worth your time wrenching.

From the swap info I read last night, I was assuming that I could use the engine from the newer miata w/ the rear-end damage and swap that and tranny into the one that has the crank issues.

Keith Tanner wrote: There was a printing error in one of the factory manuals per the years. The torque spec for the small ones is 80-something, the big ones is just over 100. I forget the specifics, it's in Mazda Miata Performance Projects. The one thing people overlook from the factory manual is the use of blue Loctite.

My '93 FSM (factory workshop manual, soft cover version) lists 116-122 ftlb (BNC) My '90 FSM lists 80-87 ftlb (in a least two different places) (SNC) If I remember right from the '91 FSM I use to have it just seemed to cover the BNC cars. Neither mentions Loctite or any other thread locker in the pulley lock bolt section, though I definitely would use some.

http://forum.miata.net/vb/showthread.php?t=471082 This is one of the threads saying a SNC should be 97-104 ft*lb Here is another http://forum.miata.net/vb/showthread.php?t=501662

I'm not really sure what is correct for the SNC, just posting what I have found.

I bought a '90 Miata with crank keyway failure. I didn't know at the time, it was just dog slow. I pulled the crank pulley and the keyway was eroded about 1/4"...no pulley wobble. If I were in your position, I would go ahead and do a timing belt, timing gear, new woodruff key and bolt. Chase the crank threads and use a threadlocker and good torque wrench. If the keyway is damaged, get the miracle loc-tite stuff.

Had good luck with the repair. Sold it and the next owner autocrossed it successfully for several seasons. AFAIK, the repair is still holding.