This procedure is not my idea. It was originally posted to the miata forum by a Loctite employee who goes by the handle of Softop. He intended it to be an inexpensive and long term fix for the worn crank keyway. My 1990 miata had this problem. With the alternative being a replacement motor, I had nothing to lose by trying this except my time and some parts costs. Here's what I did.
NOTE: This procedure is for miatas where a worn out
crank keyway has already been discovered. Lamce Schall's miata.net garage article
explains how to avoid this problem on engines that are in good
condition.
INTERNET REFERENCES
| 1. General Article about Timing Belt replacement | http://miata.net/garage/timingbelt.html |
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2. The full loctite procedure is repeated in this link |
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| 3. Lance Schall's miata.net article. | http://miata.net/garage/crankshaft.html |
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4. Plans for Timing Gear Holder.
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http://www.montgomeryauto.net/cams/techtool.html If the above link is invalid, use this this link. |
Using the instructions detailed in link #2, I bought the following parts and supplies. The Mazda parts came from Roebuck Mazda (www.roebuckmazda.com which is now Trussville Mazda). The prices and part numbers from October 2000 appear below. Loctite 242 is available at auto parts stores. The Loctite 660 is harder to find. Mine came from an industrial supply house. If you find a Loctite 660 supplier, get a tube of Loctite 243. It's 66% stronger than the 242, but still allows the pulley to be removed later. By the way, Loctite 660 was formulated by Loctite for pulley and keyway repair.
July 26, 2002 - Here is a vendor with an online site that carries Loctite
660.
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| Crankpulley and crankbolt. | Bolt removed to show the "short nose " . |
This design is found on all 1990 and some 1991 miatas.
You're looking at the crankshaft gear. The gear turns the timing belt that
rotates the camshafts, A pulley also bolts to the gear. That spins a belt for
the alternator and water pump, and a second belt for the power steering and/or
air conditioner compressor.
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Pulley removed to show worn keyway. |
Heads on view of the assembly showing gap in keyway. |
If the crankbolt loosens, the crank gear is free to start eroding the key and keyway. With time, the crank gear slips backward, which causes retarded valve timing and a loss in power.
The initial symptom is a gradual power loss, making the problem hard to detect. Power loss is caused by the retarded cam timing and is most apparent under 3000 rpm. . Above 4000 rpm, the power seems to come back. Now most 90-91 miatas normally are peaky like this, but have more low end power. Some owners detect nothing wrong and just become used to a slower car. As the keyway wears further, the engine loses more and more power. If ignored, the keyway can get so worn that the engine no longer can run well. That is usually not ignored. I've read that some cars exhibit a wobbly crank pulley. I did not notice that. This was my second miata and I had driven many others. I realized my 1990 had less power than normal. Otherwise, it started and idled well and as the previous owner told me "once you get up to speed, it goes pretty good."
When I read about the loctite fix in October of 2000, I decided to inspect my
motor and see what was happening. The first bad sign was that the crank bolt was
only finger tight. I got my first look at the keyway and saw that it was
worn.
Later in the 1991 production run, Mazda changed the crankshaft design to use a stronger crank bolt.. The later design allows more clamping force on the crank. Traditional repairs are to install the newer crankshaft that will accept the larger bolt, or to swap in a newer engine with the newer crankshaft. These are good approaches to follow if one wants a robust repair that will not need to be revisited. It is probably cheaper to swap in a new or used engine than to take out the old one and repair it.
This procedure, in my opinion, is best suited for a do-it-yourself mechanic because it is labor intensive. If you're paying a shop to do it, maybe your money is better suited toward a traditional repair as there is no guarantee that this procedure is permanent. If you do have it done, I would ask the shop their opinion first. If they see it as a workable procedure, it's likely that both you and the shop will be happy. In short, there is no guarantee. Proceed at your own risk
It's just a reassembly of the crank pulley assembly using new parts on the old crankshaft. The use of a new crank bolt is mandatory, as bolts do stretch and lose their holding strength if re-used. The use of loctite on the bolt threads increases that holding strength even more. Loctite compunds on the crankshaft-pulley contact area also serve to increase the holding strength. This patch probably won't well if the threads in the crankshaft are damaged or if pieces of the cranksg\fat nose have broken off.
.The procedure doesn't repair the worn keyway although it does fill it with loctite compound. . The purpose of the keyway is to provide alignment. The bolt provides the holding power. So the repair should work without repairing the worn keyway as long as one can align the pulley for proper cam timing.
Thanks go to miata fourm user "softop" for consulting with his technical staff at Loctite Corporation and then posting a procedure to follow and which products to use!
CLICK HERE TO GO ON TO PART 2
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Last Revision: July 26, 2002
