Hi All,
I know that this subject has been beaten to death and that opinions vary, but for the sake of building the tightest engine possible I feel that I must create yet another thread that compiles some important info and then poses a question or two.

I should start by saying I have read nearly every thread on side seal clearance across all of the popular boards and ultimately the message is the same, but I feel like the actual measuring process is missing or unclear in most cases. I also see that in a lot of videos, a well known builder will just grind it, smack it in and knows it is good by feel. I'm not quite there yet!


We know:
-.002"-.003" seems to be a fairly universal target for a "tight" turbo engine (which is what I am hoping to build).
-.002”-.006” is Mazda's recommended range
-.016" is "maximum" Mazda spec but that is very sloppy and will be problematic for compression and keeping fuel out of the oil.
- Zero clearance -or- less than .001” is occasionally suggested, but more commonly in naturally aspirated applications and this also requires an extended break in. There have been numerous failures affecting corner seals, rotors, and irons using zero clearance so this is not recommended unless you know what you're doing and are patient with break-in.
- A mounted point in a drill press, aligned using a modified "dead" rotor seems to be the best way to grind the round profile into the side seal.



My question:
* IN DETAIL - how do you measure your clearances? (I suspect there will be some differences here)

I personally started by test fitting my seals into a dry rotor and found it very difficult to obtain a consistent result. My process was to have the corner seals sitting up high in the rotor and measuring the side seal with a very small amount stuck down into the rotor. I figured they would measure looser this way and tighter once pushed down fully which would give some extra "wiggle room" in the process.

After tossing a couple seals that went beyond .006", I felt that this was an impossible task or that I was doing something wrong. I took a break and when I went back to it, I tried lightly oiling the groove to see if it would help. I was instantly able to grind seals to a tolerance of .001" without issue! I got all the seals between .002"-.003" while raised up and double checked when they were pushed down. To my surprise, they didn't get any tighter being pushed down into the groove or when the neighboring seals were installed. I guess that means the corner seal pocket is pretty tight on my rotors - right on!

Anyways, at this point I got thinking about the dynamic nature of the seals. I thought about the compression/combustion pushing against the seals which would seat them against the inner edge of the groove. I then wedged a few .0015" shims on the outside of the side seal and remeasured. This may have tightened up the gap slightly, or may be distorting my measurements due to the fact that the seal cannot slide laterally in the groove.

My concern at this point was that I am not accurately measuring the clearance and my seals may be too tight under operating conditions.

Is there a reason why the RX-8 process involves jamming the seal against the inner edge of the groove but the RX-7 process does not? I realize that .0015” of clearance should not allow a noticeable change in measured length… Am I just overthinking this?




All my measurements were obtained by measuring drag (not a go / no-go method). If the feeler gauge slides freely, that’s what I consider “clearance”. Next size up on the feeler gauge (.001" thicker) will fit but with noticeable drag. ex: if .002” slides smoothly but a .003” binds (bends if pushed), I would consider this .002”.

For peace of mind, can some of the more experienced builders explain exactly their process to grind and measure the seals - ex: dry/oiled, shimmed, how you "read" the feeler gauge, etc. Even though this is my first true "engine build", I would really like a professional result from my efforts.



Thanks in advance!
Alex

 

RX8 sideseals are keystone shaped with a vertical side and an angled side, so you jam them against the vertical side to get your measurements.


In the RX7s I have only had luck getting consistant measurements with side and cornerseal springs and all seals installed.

So, leave the finished side seals in place, go on to next and then double check all seals once they are done.

I would ONLY go as tight as the smallest Mazda measurement on your 1st build.
Triple check clearances by moving all the seals/springs down/pop back up to make sure there is no binding from an angle cut sideseal end (too long down in the groove).

 

Thanks! I have been flipping the seals over to make sure the ends are square. Minimal difference in spec, my jig must be successful!




Does anyone else fit their seals lightly oiled? I found all parts of the clearance process easier with a light lubrication.

 

After watching the "port evolution" videos, I stumbled across these. Quite interesting!

 

on the of the really old SAE papers used seal lengths. if you just took all the piston rings from your favorite engine, and then the rotary, the rotary is longer. more seal area = more leaks, PLUS the rotary has more TIME to leak, because the strokes are 270 degrees instead of 180...

but then the piston engine is lazy, and spends 3 out of the 4 strokes not doing any work....

 

...and when you are a lazy fool and chose to look the other way on side seal dynamic fit, you pay the fool's price: Spic Racer GT40R - Page 17 - RX7Club.com - Mazda RX7 Forum