As 60,000 miles approaches or has already gone by, timing belts must be replaced. To neglect this vital item of maintenance risks a tow or a long walk home. As stated in the National Miata Club's technical bulletin (Miata Magazine Fall 1996), 1990 and early 1991 models have a different crankshaft nose (VIN 209446 or less) from the later 1.6 liter engines. On the earlier models it is easy to change the timing belt by removing the V-belt pulley with four small bolts (10mm head size) and slipping the timing belt guide off the shaft, over the large diameter crankshaft bolt. This bolt should not be messed with unless there is other damage or your oil seal is leaking A LOT! However, what the article does NOT tell you, is that on the later 1991 model cars (BRG's), 1992 and up, you MUST remove the large diameter crankshaft bolt in order to change the timing belt! The nice, thin sheet metal timing belt guide that slipped off easily on the early cars has been replaced with a heavy forged belt guide that is held on by a bigger crankshaft bolt. This creates a problem. The force required to remove and retorque this bolt to 120 ft-lbs is more than you can exert without the crankshaft turning, even in gear, with the brake on! I discovered this on a 1991 BRG. The car just moved! To make this procedure safe and reliable, a special tool is required to hold the engine still.

I fabricated a bracket that bolts to the timing gear with the four pulley retaining bolts and attaches to a large bolt on the air conditioner bracket to lock the engine safely in place. With this bracket in place, there is no trouble loosening the crankshaft bolt and retorqing it to 120 ft-lbs. With this tool, changing the timing belt was no big deal. The torque wrench loads the 4 bolts in shear. Since all four bolts bear, they handle the torque load just fine. Caution: If your Miata has been up North or at the beach a lot, corrosion has probably made these bolts harder to remove. Use plenty of penetrating oil on these small bolts before loosening. A broken bolt here is a real chore to get out. It just takes a full afternoon to pull off a bunch of parts, change the worn belts, clean off the covers etc., and put it back together. No other nasty surprises. Everything is step by step by the Miata Shop Manual or the Miata Enthusiasts Shop Manual. I have also used this tool to change the timing belt on a '94 1.8 liter Miata. The fit is identical to the later 1.6 liter Miatas. See Figure 1a & 1b and Drawing 1.

However, on the early Miatas you do need to remove the large crankshaft bolt to change a leaky crankshaft seal! Here is the problem. The tool I made for the '91+ does not work for the '90 model as one of our members found out. The timing gear is smaller and recessed, so he made a new tool for the early model Miatas. The flat stock must be bent and notched to clear obstructions and line up properly. The '90 model only requires 85 foot pounds of torque since the bolt is smaller. So now there are two special tools for loosening and retorquing crankshaft bolts. See Figure 2 and Drawing 2.

To fabricate either tool, the crank pulley is used for a template or guide when cutting a piece of 1/8" thick or greater flat stock. Measurements are taken to clear obstructions for tying the other end to the AC bracket. If you car doesn’t have AC try to find a large bolt in that area to tie off the free end of the bracket. I haven't looked at any cars without AC. After a few trial fits and trims you end up with a useable tool. It usually takes about an hour to fabricate. The attached pictures shown are the homemade prototype brackets. The real neat one was done in a machine shop by one of our members. It is not yet trimmed to clear obstructions. The drawings are in EXCEL'97 and are to full scale on my printer. Different computer systems tend to produce slightly distorted drawings. Adjust the print size in the fit to page window to produce a drawing with a round center hole. Your drawing should then be close to full scale. All dimensions were rounded to the nearest 1/16" to facilitate US standard tooling instead of the true metric dimensions. All holes have enough clearance for hand built tolerances.

As always, use these plans at your own risk. Any breakage of bolts due to the condition of the car or the skill of the mechanic is totally your responsibility. This method has worked well on at least 7 cars so far. Take proper precautions when using power tools. Break all sharp edges and round all sharp corners for safety. Be prepared to adjust for fit with a file on each unique situation. Take care on tightening the bolts. The AC bolt does not go all the way into its bore due to alignment differences so all the threads are not engaged. Snug this bolt down but do not tighten. It is there for lateral shear support only.