Why Apex Seals Fail
 

Most people think that that their apex seal failures are caused by detonation. More specifically, they think the detonation was caused by some combination of poor fuel, advanced ignition, and high boost. This can and does happen but is not the root cause of the seal breakage.

One of my engines, which I thought would run a very long time because of reliability mods and low boost (14psi), lost an apex seal at about 30K miles. The only good news was that upon close examination I discovered a second apex seal in the set that was cracked but had not yet broken. (Sorry the magnification isn't better.)

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I was surprised to find crack was emanating from the bottom side not at the exposed working face!
It makes perfect sense. The main part of the seal is supported unevenly at three points. The forth spring contact point is on the wedge portion of the seal. When the seal rocks over the high point in the housing, specifically, the spark plug openings, the tip is under pressure from the wedge.

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So you can try a little experiment to test this hypothesis. Install a .0015” feeler gauge at the center of the apex seal simulating the raised spark plug area. The seal will be rocked-over .003“ and the wedge will put pressure on the pointed end of the apex seal. Note the exaggerated condition depicted at the top of the picture.

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But we still haven’t gotten to the root cause. The real problem is the raised spark plug area on the housing. There are at least three approaches to this problem:

(1)- Use cold heat range plugs like NGK R7420-10 or 10.5. (This is easiest part of the solution.)

(2)-The harder second part is to cool the housing better.
A better pump is a good start. Second gen. cast impeller type pumps are a lot better or the Remedy pump for 3rd gens.

(3)-The most difficult part would be to port the housing in the spark plug area to minimize distortion.

But what is the best way?
The factory racing peripheral housing have a trough cast into the inner water jacket in this area but the are for NA applications.
Attachment 710705

Notice that the thru-bolt bosses have been severed.

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This is what a stock housing flows like. Notice we have too much flow on #6 passage below the spark plug and not enough on #5 passage.

Any thoughts?
Barry

this is pretty interesting. i like you idea. i can't wait to come back home and read some more ...

I have been thinking about this also and I turned to the Ro80 rotor housings for answers. What NSU did was to make the entire boss out of copper with a press fit into a bore. This boss was then tapped for a spark plug.

My thought has been what if we were to use a holesaw and cut the boss free of the outer rotor housing wall and cut the webbing free as well. This way as the sparkplug boss heats up and expands it will be able to expand out instead of push into the inner housing wall. We could even machine it to where we could have a threaded nut with a pair of o-rings that can work back and forth along the spark plug boss' outer surface. All we really need is an o-ring material that would stand up to the heat.

Well, I would try and do better machining than a hole saw but I get the jist of your idea. If the nice water tight press fit was in the "front" of the jacket only (the area at the tip of the plug) A slip fit and o-rings at the back would still allow for thermal expansion differences and put the o-rings in a more temperature friendly place. Some sort of positive safety against unwanted/accidental removal during operation such as a snap ring would also be prudent.

Why not aluminum instead of copper? I know copper will get rid of heat better (aluminum is no slacker at thermal conductivity either), but a a nice bored hole, getting rid of a lot of material and a THINNER due to the fact it is machined and not cast with draft, sleeve could still offer improvements. Thermal expansion differences between copper and aluminum plus electrolysis effect, cause other problems.Just some thoughts as a machinists.

You guys came up with a very similar solution to mine. If we use o-rings in the water jacket area they will survive the relatively low water temps.

TonyD89 can a surface be machined smooth enough so the o-rings would seal well?

Barry

I am using the 6725 plugs currently and have some 7420s coming in from Japan-land.

Another easy change (depending on your climate) is to run more water/less coolant. Most of us tool around with 50/50 because it's easy, the default.

Yeah I got a set on the way as well, for the price, I hope they last a long time :lol: I'll be running the 7420 10.5's in the leading and some 6725 11.5's in the trailing.

Not sure how much those will like to idle and cruise around.

I'm going with 10s and 10.5s for 20 psi all day, every day :)

Most certainly. Things like reamers and boring tools will do the trick and are nice for guys with a Bridgeport milling machine or a good drill press. Simple and easy. Myself, I would follow what I'm accustomed to and circular interpolate the hole in a CNC milling machine.:gwink:

Good to see we are all thinking on or about the same wavelength.

Um.

"Cold" spark plugs don't actually run colder, they just have a shorter path of heat transfer to the threads.

I don't think that it would actually result in colder temps at the spark plug hole.

Anybody with a chassis dyno, various spark plugs, and some shim-under spark plug temp probes feel like doing an experiment?

Not sure but I'm guessing my lx92 coils and crane fireball hi-6's shouldn't have a problem firing these guys :) I'm going to be running around 20 - 22psi all day as well with water injection pre turbo so the guys that have proven this setup all say r7420 10.5's are the way to go. I just figured I'd split up the plugs I'm getting and the ones I've already got so I can have two sets of every day plugs since both sets cost me about 300 bucks :sad:

You have a bad tune,this is why the housings are distorting from to much heat.you can do alot of mods to cover it up but is just simply what you will be doing,just covering up a problem.

Bad tune is the main cause of broken apex seals.

Brent

You have been given more than enough "keys"....... So please use them and let yourself out of your "box".

-J

Tony, what would it cost to circular interpolate the hole in a CNC milling machine and cut two housings?

Peejay, this is something I have wondered about.

Is it heat at the plugs tip a result of:

(1) The heat of the combustion transmitted to surface area of insulator heating the plug.

(2) The electrical current firing the plug.

(3) A combination of (1) and (2).

Brent,

Kenichi Yamamoto might disagree with you. Most people treat it as an inherent problem and we are trying to solve/minimize it. Your help would be great.

Thanks,
Barry


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I guess the Mazda factory tuend ECU must be off too since the carbon tracks can be seen on housing from stock/unmodified motors.

Jack

I have seen this problem many times especialy in high output track engines,I did many mods like you are doing to help the problem,I even ran external water lines from the radiater to the housings which made a big difference especialy on the rear rotors of the 20b.
I have since changed my way of tuning and have found no need for these mods any more.
I dont see any carbon build up on housings.
Now with a happy tune up water temps dont go past 78deg C and not only on 450+hp 13b's but also 600hp 20b's.
even with 800+hp drag setups I dont see this anymore.
Also I have never done it like the last pic but would be worried that the housing would move even more under load.

Brent

Could you explain about happy tune/ unhappy tune?

Brent,

That sounds great. It appears that we are trying to achieve exactly the same goals.... moderately high but reliable power. We need your expertise and experience. What are we missing?

Thanks,
Barry

... and the room goes silent. This is one of the biggest problems in the racing community. I KNOW the puerto ricans and the aussies have tuning down pat however it's a secret. So us "yanks" are left sort of in the dark to figure it out all over again. where is the rotary love :) All these secrets do nothing but hold back the potential of these rotary motors. Oh well props to you guys for figuring out these tuning secrets.

use your head. if we accept that the housing is running too hot, what controls the housing temps?

CBR says "water temps dont go past 78deg C"

do you think lowering the water temp by more than 60 degrees F changes the rotor housing temp?