Another thing: What is a good target clearance for a road race motor (stock ports, 8500 rpm max)? I think the OP mentioned .002" as a target and the factory spec is .0016" to .0031" with .0039 absolute max.



My current measurements all show well below the .0016 minimum - but per my previous post I'm convinced my measurement technique is flawed.



Thanks,

Neil

 

Dollars to donuts that after the machine shop turned the journals down, they "polished" the journals with some 600-800 grit.

 

Shoot for .0035 to .004" with 100 plus pounds of oil pressure. My Fiat ran .0035" mains and .0030 rods.



The .004" rotor bearings is still modest as it is a long bearing. With a rev limiter, .0035 is OK.



Lynn E. Hanover

 

Ok,



Is that .0035 - .004 for the rotor and main bearings?



Any tips on how to get a good measurement?



Thanks

 

a good measuremnt , well im a heavy diesel mechanic by trade(not that i every really get to do any rebuilding, which sucks) and ive always been a fan of the 3 by 3, take a measurement using micrometers and bore gauage, 3 measurements accoss the bearing surface then rotate a 3rd then the same gain then rotate a 3rd and the same then average it out.



this might not be common practice in the rotary world but in my opinion it gives you a very good idea of how your bearing and running surfaces are looking.

 

That is for rotor bearings. I have yet to loose a main bearing just sanding through the overlay with 600 silicone carbide paper.



It has worked fine since 1980.



Lynn E. Hanover

 

So you sand away all the grey overlay right off the main bearings so your left with an all cooper looking bearing?

 

Buy new bearings. Install them. Measure them with a dial bore gage, or have it done at a machine shop.



Spin the piece up in a lathe and oil up the bearing or supply it with coolant or kerosene. Sand away the grey stuff. Measure the bore again. Look for three on the mains and four on the rotors. The grey stuff may be thicker on some bearings than on others. I doubt that Mazda makes any of the bearings, and there were some very bad bearings made up back in the 90s. Do one and measure. The overlay is about .001 thick. Most of the time. You cannot accurately sand off a specific amount, so all off or none off is the rule.



If there is no intention of running the engine above 9,000 RPM for most of its life, there is little need to add more than Mazda race bearings. That will add just one half thousandth to the total clearance. And that is enough. If you sand all of the bearings to the copper, you may find the stock oil pump a bit under sized for the required 100 PSI. So then you need to do the pump mods and cavity mods, and rotor mods, the pickup mods and so on. If you increase the bearing clearance, the rotor to housing clearance check becomes critical Face of the rotor to the narrow part of the housing. A tiny wood wedge on the opposing apex to load the rotor to the side being measured. Press the rotor gears in tight while the engine is apart.



Lynn E. Hanover