Lynn,
I have a '90 13B engine that I am rebuilding (not for aircraft use, yet).  I rebuilt an '87 engine and had the cast iron housings
commercially lapped.  For the '90 engine I would like to know if there is a do-it-yourself way to lap the housings to just
take off enough material to flatten them without getting into the nitride any more than is necessary.
If I recall correctly, the commercial lapping house said that they had to take off a set amount even if
it was more than the minimal amount to flatten the housing.

One other question, what do you do to prepare the rotor housing before a rebuild.  Do you lightly scuff the surface or not?

What do others on the list do for a rebuild?

Thanks,

Jim Brewer
Albemarle, NC

 

I just slide my magnetic based dial indicator across the wear area, where the side seals create the step on the plug side of the irons. If less than .006" then no machining is required.

You could even get away with a bit more wear, with the understanding that this discontinuity will round the outer edge of the new side seals, and reduce sealing (and increase blow-by) to some extent. Not much of a problem in a street engine.

 

This is like the ring ridge in a piston engine's cylinder that appears at the end of the top ring travel.

 

Just glue some 400 grit silicone carbide paper to a DA pad (Orbital sander) and while keeping the iron wet with kerosene or diesel fuel, shine up the running surface. Notice that the carbide paper will cut thousands of little circles in the iron. It hold oil great and the wear rate slows down a bit.

 

In the repair manuals as well as the Mazda factory allow for far more side seal to corner seal clearance than is required, (in my opinion).

 

In a piston engine you have one end gap in the sealing ring. In the rotary there are 4 obvious end gaps, plus the 4 gaps around the corner seal, apex seal junction. So, there are so many leaks that a slow starter motor or low battery can result in a no start. In other words, not enough heat of compression to maintain a vapor state. Worse in cold weather. Worse in engines ported with later intake closing points.

 

My first race engine was a stock junk yard engine with 9MM apex seals. I poured in motor oil and fuel and 1/2 cup of hot coffee to start it. Once warmed up it would start OK. The smoke cloud was impressive. I run on as usual.

 

I trim the side seals to the point that they fit snug between the new corner seals. So long as the seals will pop back up after being pressed into the grooves. The side seal is pushed around the engine by one corner seal. So a notch that is made by the side seal appears in each corner seal. When the engine is fresh the seal grows from heat and fills whatever gap is present so as to make a near perfect seal, and makes a mark on both sides of the corner seal. As the engine wears this groove into the corner seals the gap at one end between the side seal and corner seal opens up and begins to leak.

 

If the corner seals are used, there is a groove and the actual clearance cannot be measured.

So use new corner seals every time.

 

I run a fresh engine for one hour at fast idle (2,200 RPM). Then at the track, one lap under 8,000 RPM, one lap under 8,500 RPM and one lap under 9,000 RPM and then hammer down, shifting at 9,600 RPM.

 

For rotor housings, just minimal grooving below the spark plug holes, and very little chrome flaking along the edges. I run a diamond file along the chrome iron junction, so as to reduce the chrome to iron pressure during operation. Usually on street engines the apex seal corner piece does most of the chrome damage. Also poor lubrication (top oil) and over heating add to the problem.

 

Using sump oil for top lubrication in street engines results in gumming and carbon buildup. Use of a straight weight sump oil. At the least, this removes the long chain polymers of the multi-weight oils. A big improvement. 

 

If there is a bit of chrome peeling, touch up the damaged edges, with a round diamond file.

 

Touch up means just reduce the failed edge of the chrome .0005" or less. Just touch it with the file. Each missing flake is a compression leak in a design that leaks a lot already.

 

Assemble the apex seals with the solid part of the apex seal over the damage chrome. Run the corner piece over the best chrome. You might break the iron side of the corner piece edges with the diamond file. Just touch it to remove the rather sharp edges.

 

That is the big stuff. The rest is just follow the book.   

 

Lynn E. Hanover