Mazda Repair: Miata 1990 rattle; front of engine., model miatas, worst case scenario

Question
On this 1.6L. engine, a rattling/clacking sound seems to come from the case enclosing the timing belt (belt was changed 15,000 miles ago at 60,000 Mi.) Sound is a little irregular.At higher revs it disappears into the engine noise. I'm guessing at some kind of belt tensioner - if so, will this lead to problems? Can I relieve it myself without affecting the timing?

Car runs well and recently passed Virginia inspection.

Any advice gratefully received.  

Ray

Answer
Most 1990 model Miatas came outfitted with a crankshaft that had a design flaw.  Though many still remain in use today with no ill effect an improperly fitted boss keyway can lead to crankshaft failure.  The nose of the crankshaft has a keyway slot that is beveled, the keyway has a slight bevel to match the slot bevel.  During a timing belt service the keyway must be set properly or slack will result that will lead to metal fatigue and the eventual shearing of the crankshaft nose at the boss pulley.  Prior to failure in this area belt noses, rattling and even visible wobbling of the main pulley and the presence of rust dust can be common.  As the issue progresses power loss is common due to retarding of the base timing that occurs.  It would seem that this problem should be avoided but you would be surprised at how many technicians not familiar with the Miata do not catch the slight bevel on the keyway itself.

Mazda just didn't make this one monkey proof, it was corrected in subsequent years with a redesign.

Now this is the worst case scenario but one you need to keep a close eye on to avoid issue with.  Some people have contacted me about issues with the main pulley itself which is a rubber supported balancer by design.  I personally have not encountered any failures to the pulley itself but, since it is not a costly part it may be an item to consider for replacement.

The belt uses two bearing assemblies an idler and a tensioner both are subject to wear and should be inspected for replacement at 60k intervals and replaced at 120k.  Use of Mazda OEM parts is strongly recommended.