Hello all,

I picked up a 12A a little while ago for dirt cheap and was hoping to use it for a project if it was in good enough shape, but I've never dealt with anything on any rotary this old and am looking for some insight and opinions.

I got the engine disassembled and cleaned first off, then starting taking measurements and pictures to figure out what was still usable in the motor, and what is possibly sellable if too much of it is shot. Where I'm at now is that the engine definitely has some issues, and isn't going to be a wonderful engine without some serious part replacement/refinishing.

The first issue I ran into was that there was chrome flaking on both housings. I know this is fairly common on these older housings and my reading came up with opinions on both sides of the fence with how much was acceptable and whether or not the housings were usable at all. The peculiar thing about these housings is that the flaking is on the front of the housings, not the rear where the apex triangle piece rides. I could not find anyone who had documented any where in this location without it being on the rear side as well. Both housings have about the same amount along the bottom between the spark plug hole and exhaust port, maybe 5.5" long and no more than 1/8" towards the center of the housing. The rear housing also has some slight flaking on the upper portion(also on the front), but not nearly as bad as the rest. I've included pictures illustrating the wear and would love to know if you guys think these are work using, or are good enough to sell as cores to be refinished.

The next issue that I found was that the apex grooves are slightly 'V'd out. The rear rotor is worse than the front rotor and the worst of the slots on the rear rotor had enough room to fit .006" feeler gauge in the top of the slot, and only .003" in the bottom. Keep in mind that I measured this against the seals that came out of the motor that do have some decently significant wear, so those gaps should shrink with new seals. Are these rotors good enough to see service again? Could I possible square up the slots and run slightly oversized seals that are available?

The irons do have some step wear, although I do not have the proper tools to measure it so I can not say how much. They are all perfectly flat though, as illustrated in the pictures as well. These should be able to be resurfaced and used again.

Everything else in the motor looks pristine, the e-shaft is true and the bearing clearances are still all proper, as well as the teeth on all of the ring gears and stationary gears.

Basically I'm looking to see if this motor can be put back together with some new seal springs(the protrusions on the corner seals especially are below spec), apex seals that aren't nearly as worn, and some new coolant jacket seals. If it can't be done or wouldn't be worth it then I will be parting out everything I have related to this motor.

Thanks a bunch, and here are the pictures!

 

poop

 

The housings look good in this day and age. Where are you gonna get new ones?

The irons look fine. If you resurface them, I'll give you space AIDS.

Apex slots in rotors are decent. Get some easy-break-in seals like Atkins and call it a good rebuild.

On second thought, part eveything out. You'll make Ray Green real happy.

 

I have seen better condition housings for sale, but they sure aren't cheap. I have a friend, who is used to later 13B stuff, that basically won't stop carrying on about how the flaking is going to cause huge issues and I'll be lucky to make 60spi of compression out of it. I figured the apex slots wouldn't be a huge deal, they're not THAT bad.

Why would you say not to resurface the irons? They do in fact have step wear, and it's definitely out of spec if my calibrated fingernail is any good.

 

Been a while since I've posted... In your situation, I would resurface the housings.... Look into flame sprayed plasma applications... There was a member on here that did this a few years ago. It came out damn near perfect.

 

Well, what this basically comes down to, is I have an ideal budget for this build, and if it's going to cost more than 'X', it would be better for me to just part it out. While I agree they don't look awesome and it will be far from perfect if it is rebuilt with the absolute minimum, the point of this motor wouldn't be long term reliability or anything like that. With that being said, in their current state, would you consider them passable? Meaning I can at least make enough compression to get the thing running and put some power down for a little while?

 

A rotary isn't a piston engine. You don't have compression rings that need a crosshatch pattern to file into during break in. When you resurface the irons, you remove the factory nitrided surface. This factory surface was specifically designed to work with the side seals, oil seals and corner seals. When it's removed, the side seals and especially the oil seals wear out super fast and you get an engine that smokes on startup every time and costs more for less engine life.

Take it from someone who's torn down several engines. The ones that had resurfaced side plates were always junk. Nothing was reuseable. On the flip side, the factory assembled engines like yours had lots of good resuable parts and often with the kind of wear you're seeing, just needed a set of new springs, apex seals and oil o-rings only. They go back together and don't smoke on startup other than the first initial start as assembly lube is burning off. They make somewhat low compression at first but quickly break in and get up to 100psi quickly. They also aren't hard to start. I think your friend is just that - a friend. But a rotary expert he is not.

The way this whole "resurface your irons" mentallity got started was back in the 70s. Not on the early irons from twin dizzy engines as those had some surface hardened sections, but I'm talking about non-nitrided 74-78 irons which could be resurfaced several times before they were too short to reuse. These had a harder iron than the 79 and later nitrided irons which you can feel with a dremel (early irons take longer to port). For the general public, these early engines formed a first impression which as you and I know, is hard to change. Because of this, the stigma of resurfacing will be around forever.

Did you know that by 1979 when nitriding came along, it made resurfacing obsolete? Resufacing was obsoleted even further in 1986 when Mazda moved the coolant seals to the irons. If you resurface these later irons more than like one time, the coolant seal grooves get so shallow they don't work anymore.

Remember it's not a boinger (piston engine). The smooth hard factory nitrided surface is there for a reason. Once it's gone, your engine is junk.

As for your specific side plates, I'm sure you can reuse them as they are. The side seals and corner seals are already broken in to the irons so might as well use them - just change out the springs. Or if you like spending A LOT of money, follow others' advice.

 

Great info, Jeff.

I don't have anything else to contribute to this thread other than to say those are some FANTASTIC shots of engine internals. Bravo.

 

Thanks you very much for the detailed response and solid advice. What you describe replacing is exactly what I was looking at doing to this engine, I'm glad that I was on the right track and that this motor seems to be usable. The housings still scare me a bit even after your reassurance, but I guess at this point it's not that much out of pocket to get this thing together and see what happens. It will be receiving some pretty serious porting work, as this would end up in a car that would see only track duty for the rest of its days and the goal was to make a dirt cheap toy out of it, which I think is doable. I wish I had some new 3mm apex seals to measure the slots against just for peace of mind since my numbers don't mean much when the sides of the seals aren't even parallel, maybe I'll snap some pictures of those too just for the hell of it.

If you wouldn't mind another question, I'm quite curious as to why the chrome flaking is present on the front side of the housings, instead of the rear where the apex corner piece is and the wear typically occurs. Do you have any ideas as to the cause of that? The motor had nitrous jets plumbed into the intake manifold when I found it, so it has definitely been abused at some point, maybe that has something to do with it.

Thank you very much, this was actually my first time trying to get good pictures of engine parts. A buddy lent me his good camera and we spent a few minutes fixing the color mistakes after the fact and they did turn out pretty good, makes me want to get some proper equipment myself!

 

The minor chrome flaking on your housings looks fine. I'd totally use them in a build. And because you can't get new 12A housings anymore, it makes the amount of wear you have seem like even less. Does that make sense? The only other options are expensive to buy brand new ones from someone's hoard, or get some 13B housings machined down to 12A size (70mm) which is grossly expensive if you ask me. Like 700 bucks each or was that for both? Doesn't matter because your housings look awesome. I've built engines with that much and even more chrome wear and they run fine.

How's this for an example. The white and silver engine (seen in a recent thread) had a line of worn and some missing chrome all the way around both housings. It started up yesterday for the first time within 4 or 5 seconds of cranking and didn't smoke. It was just a test fire with no coolant in it to make sure it would run and all that. Like 40 seconds or so. Today I'll poor some coolant in and give it a full heat cycle. And to think I was a little worried about the minor chrome flaking on the housings. lol

As for your question, the reason the chrome is missing more on the front edge of both housings has to do with how the engine sits in the engine bay. There is a 6 to 10 degree tilt. When the engine isn't running, any oil tends to settle at the rear edge of the housings due to gravity. If you examine the side plates, you'll see there is more step wear on the front iron and the intermediate rear facing iron. Then you'll see there is less step wear on the rear iron and the forward facing side of the intermediate iron. Again, due to gravty. The solution? More OMP oil. Or premix. I tend to jack open the OMP with a couple more washers under it. I don't care for premix but others swear by it.

 

yeah those rotor housings would have looked great when you COULD get new rotor housings, be happy there is ANY chrome on them

 

Thanks for all the great info guys, if everything pans out this engine will be back together somewhat soon and I'll make sure to keep you guys updated on the project.

My plan at this point will be to get a engine gasket kit, new oil o-rings, atkins apex seals and springs, FD corner seal springs, and see where that gets me. The edge wear on one or two of the oil control rings themselves was a bit large per the factory manual, but I don't see that being a huge issue as it will only spell burning a tad more oil. Any other suggestions on things to replace? Keep in mind this is as budget-minded of a build as possible.