Identifying Bad Rotor Housings


We consider there to be four "types" of rotor housings:

1) New - require only quick inspection to be sure there was no bad casting or shipping damage and they are ready for assembly or porting.

2) Used in EXCELLENT Condition - very rare to find, require lots of cleaning and inspection.

3) Used, not sure if they are good or bad - these are the most common and require lots of cleaning and a DETAILED inspection before using.

4) Really trashed used ones - very common and require no cleaning and almost no inspection - should go direct to scrap pile.

Benefits of NEW - Very little time spent, virtually no question they will work great, least amount of break-in time, will outlast any of the others, will make more power than used housings (better sealing), etc. etc. The ONLY drawback is the cost.

This page is to help a little with categories two and three --- deciding if those used ones are acceptable.

We could include literally hundreds of pictures, but they would mostly be variations of the key pictures shown below.


Start with an easy one - definite scrap piler - this one ate an apex seal.
Also for the scrap pile - massive chrome flaking.
The image does not really show how rough this one is. A good example of just flat worn out! This housing ran for a LONG time, and is now done.
This is a questionable one. Here at Mazdatrix, we would not use this one - it would be sold for $35 in our "as-is-no-warranty" "you-inspect-it-you-keep-it" area for "walk-in" customers (and they all sell). If used, it would "work", but will always be blowing by the flaked chrome and will require a long break-in time before it even gets marginal.
The chrome flaking, as above, is the hardest judgment call for the home engine builder. There is no "solid" rule for when to discard the housing - it becomes YOUR choice. Minor flaking (less than 1/8" wide, for maybe 2-3" total length around the housing) can usually be used with good success. Flaking in the 1/4" range becomes highly questionable - you need to really think about what you want out of the engine.
Here is the real enemy of rotor housings - corrosion! If the anti-freeze is not changed frequently enough (or left out!), the water starts eating away the aluminum of the housing. This one is junk because the corrosion has gotten into the water seal area.
If you see ANY corrosion then look the housing over VERY carefully! This image shows bad pitting into the exhaust port.
On this 86-92 housing, the corrosion is too close to the water seal - it is way too risky for us to use.
This shot looks down into the water jacket around the spark plugs. ALWAYS check this area very close on ALL used housings. Even minor pitting can be reason to scrap the housing. We have been fooled on two engines by minor pitting here! (within a two week period!).The housing was in great shape, but we missed a small corrosion pin-hole that let water into the engine. We caught one on the engine stand, but the other was installed in the car before we found the problem. The one on the stand cost a gasket set and time, the one in the car was gasket set + install parts + full remove and re-install the engine! All for no charge to the customer - better believe we now check this area VERY close!
The bottom line is: if you are using used rotor housings - check them VERY close! This page does not even include the factory manual thickness measuring check. We do that AFTER we have done the visual inspection. It is rare to find one that will pass the visual, and then fail the thickness test (but NOT unheard of - so also measure them). We know new housings are very expensive, but we also can always tell the power difference between a "new housing" engine versus a "used housing" one on our test drives after installation.

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