Inspection

Start by knowing your tools! The video below shows general usage for a dial caliper. If you don't have one of these, don't continue. In a race application you might be fine, but in an aircraft you HAVE to know that your parts are all within tolerances, or you could become a messy example to future builders!

A dial caliper and a good set of feeler gauges are the most important tools to have handy. A digital caliper is also very helpful, as is a good magnifying glass.

Before you start, you may want to study some photos of other engines and what different types of damage look like. Here are some links that might help you out:

• Rotary Resurrection. In the Tech section of this site you can also find wear photos and troubleshooting guides with lots of helpful photos.

Inspection of the housings basically consists of three steps.

First, inspect the inner wear surfaces for signs of gouges, cracking, or unusual wear. The FSM calls out “acceptable” wear: almost all used housings will have some wear. But if you see gouges, you may need new housings… and in that case you should also look at the matching rotors and apex seals to try to determine the cause.

Next, examine the side mating surfaces, including the flats and the O-ring grooves. You're not likely to find wear here - what you're looking for is shrinkage. (If you don't find any, you're doing it wrong.) The 'top' of each housing is the intake zone, and is the coolest spot on the block. Starting from that region, measure the housing width every inch or so from top to bottom, around the housing. You'll find it's narrowest near the exhaust port - make sure you're within the specs called out by the FSM. Also, make sure the O-ring areas aren't warped in any way, indicating the engine was severely overheated.

Finally, inspect the housings for cracks and external damage, particularly around the ports and spark plug holes, and in and around the water passages.

Housings are the most expensive pieces to replace in the engine. At the time of this writing, each housing cost $800 new from Mazdatrix. Rotors are $700, side housings $465, and the eccentric shaft is $475 - and if you have damage in your housings, you may also need some or all of those, too.

Since you can get an entire used engine for like $1800-$1900, if one or both housings and/or rotors are damaged, you may want to just get a second core. There's no universal rule of thumb here, though. My engine was carbon-locked, which is common, and had a failed side oil seal in the rear rotor. The rotor itself was fine - it was just the O-ring which had let go, probably due to an overheat.

That may seem like a waste, but the chances are actually pretty good that you can get a usable set of parts if you have two cores. And you can use those extras: you can practice porting work on the “bad” housings, try out different ways of cutting down the water pump, etc.

There are quite a few aspects to examine on the rotors: there are six side seal grooves on each rotor, three apex seal grooves, six corner seal holes, three faces, two sides, a gear, and a bearing. Take your time.

If you end up combining parts from multiple engines, or you have to replace a rotor, be aware that the rotors, eccentric shaft, and counterweights form a balanced rotating mass, and if you disrupt that balance your engine will be rough, at best, and could fail quickly, at worst. The video below shows you how to check rotor weights to be sure they're in spec.

To evaluate the eccentric shaft you need to determine how much runout it has. That requires a dial caliper and a pair of V-blocks.

In case you're curious, here's the “story” I found when I cracked my core.

First, all of my accessories looked to be in good shape. There was some minor corrosion but not much - so much of the rotary is aluminum that there's little material to rust on it. I briefly examined my alternator, water pump, and other external items, then set most of them aside for reuse.

My intake was thoroughly gunked up with fuel deposits. You could varnish a bartop with what I found in there. I wasn't planning on reusing it, so this wasn't an issue for me - but it did suggest that I wouldn't want to reuse the fuel pressure regulator (since it would probably be full of garbage) and that I should plan on cleaning the injectors.

My turbo manifold has a small crack in the center divider. This suggests an overheat, which I later confirmed with other evidence. However, there was no external indication of warpage or cracking in the core itself, there was no damage to the turbos, and all other components in this area looked fine. If the engine was overheated, it certainly didn't experience what racers can do to a core.

The front and rear main oil seals looked fine. They were total garbage, of course - they were dried out, and crumbled when I removed them. But they weren't LEAKING, which is an important indicator because a leaking main oil seal usually indicates some sort of internal failure.

The pilot bearing in the end of the eccentric shaft was toast. The end of it was crushed, and it didn't move at all when I tried to spin it. When I removed it, I noted that all of the damage appeared to be right at the outside end of it, and there was no scoring or other damage to the eccentric shaft itself. I believe the most likely explanation is that the core was removed from the engine very roughly, and when the transmission was pulled off, its alignment pin was allowed to pull down at an angle, crushing the pilot bearing.

This theory is supported by the fact that the engine came from the importer without a transmission. That's rare - they almost always come together, so I suspect the transmission was sold separately, and removed in a quickly/roughly by the importer. That's fine - it's a cheap part, and the eccentric shaft was in good condition.

When I cracked my core I found that all of the internal seals were carbon-locked. The rear rotor was the worse of the two, which is apparently not uncommon. The rear housing and its internals were jet-black with thick, caked-on carbon and oil deposits. It's common to have carbon buildup on the rotor faces - but not on the SIDES of components. Those should generally be 'relatively' clean.

The side seals were particularly difficult to remove from the rotors. The experts advise using broken pieces of side seals to lift the others out. Well, that didn't work for me. I had to repeatedly soak them in solvent (B12 Chemtool worked the best), then use a screwdriver that I had ground down to fit in the side seal groove to work underneath each side seal and pry it up. As it was, they came up in half-inch pieces, indicating severe carbon buildup.

The housings, rotors, bearings, and eccentric shaft showed no other signs of damage - no cracking, shrinkage, scratches/gouges, or other signs of wear. In fact, even the apex seals looked like they were in excellent condition - they would easily have been candidates for reuse in an automotive application.

Fig. 1: Varnish deposits plus corrosion from old fuel and moisture left in the intake.

Fig. 2: Severe oil and carbon buildup. Side seals are flush and impossible to move.

Based on what I found, I believe the following occurred:

1. The engine was run hot, and the oil changed less frequently than it should have been.
2. Although the overheating wasn't enough to warp the housings, it WAS just enough to cause the oil control O-rings to fail in the rear rotor, allowing oil to leak into the rear rotor housing.
3. As excess oil was burned in the combustion cycle, carbon built up on the rotors faster than usual. This extra carbon caused the seals to stick, allowing blowby gas to make the problem worse by cooking even more oil.
4. The compression loss from the stuck apex seals would have made the engine run rough, then not at all - so the engine was pulled for replacement.

As stories go, this one is great. The compression loss obviously occurred before significant damage was done to any other components. The side seals broke as I removed them, so I could reuse them - but I would have replaced them anyway. Basically, I was able to reuse everything 'expensive'. I wish you the same luck!