The Tech Tips section of the Mazda Competition Catalog includes the following note about stationary and main bearing clearance: "When using a stock eccentric shaft, allow an additional 0.013 mm (0.005") of clearance at the rear half of the rear journal".



My questions relative to a street-ported 12A are;



1) How is this additional clearance created; grinding/polishing the e-shaft or a special bearing modification?

2) As far as I know the Mazda main bearings (#4801-10-502) are not available in different sizes for clearance. How do you adjust to get the specified clearances?

 

Have a pro. work it in a lathe.

 

I don't have a dial bore gage, and seeing 1/2 a thousandth (0.0005"0 on a snap gage with a dial caliper is just hopefull.



When I have seen damaged main bearings from an over-rev, the damage has been at the end of the bearing under the gears. It makes a shinny ring around the end of the bearing.



So,

I scrape away the overlay with a knife blade, about 1/8" back from the end of the bearing. I run the bearing in the lathe and run some 600 "Wet or Dry" (silicone carbide) paper on the gear end of the overlay. I keep the bearing wet with Kerosene while doing this. I remove an obvious amount of material. Not all the way through the overly, which is about .001" thick.

And only on the gear end ahead of the oil groove. There is no sin at all in sanding some off of both halves. Just keep that oil pressure at 100 PSI or more.



First, the reason for this is that when the crank flexes, the shaft will run at a slight angle through the bearing.

Since the removal of 0.0005" will fix the problem, the crank flex is obviously minimal.



You are not supposed to scream the engine anyway. This is for they few without a rev limiter.



My thinking is that removing the material from the front of the bearing will cause excess oil to move off the bearing toward the gears, rather than toward the Main seal. A small amount of load is removed from the gear end, and gear life is extended. The crank flex would be aggravated by removing material from the gear end, but it seems to work just great. And, you are not supposed to scream the engine[/b].



You didn't say what the engine will rev to, so I cannot comment further. With stock weight rotors, the above is good to 9,600 RPM. Rev limiter and shift point in a road racer.





Lynn E. Hanover