Now I recently overheated my engine and I plan to pop the engine open and rebuild it


On the off chance the rotor housings are too warped (beyond .06" variation I believe) is it possible to just machine the surface of the housing to make it nice and flat again ?

Or should they be warped, should I just bite the bullet and source new housings?

 

I doubt it, but if it was possible I would say the rotors would need to be machined also. You would be loosing displacement. Doesn't it say in your book with specifications?

 

i'd use it up to 0.06 or maybe a little more. if you think about what we are measuring, we are measuring the width of the housing, and when it shrinks, where does the metal go? what we are actually measuring is the sagging of the sliding surface, the narrower the rotor housing is, the less straight the sliding surface of the housing is.

so machining the housing flat fixes what you measure, but it doesn't fix the thing we are actually measuring

 

FSM says if its out of tolerance to replace.

I mean I figure if I'm milling a miniscule amount just to remove high areas and get it back in to tolerance the change in displacement would be negligible. I just need to figure out if the milling machines at my school can even get a tolerance of 0.06mm....

 

.06mm (.0024") is the entire tolerance. You'd want a machine that can do .0005" resolution at least. It's tough to take off much less than that just by doing your initial touch. So, it's pretty risky.

The displacement change isn't the concern, it's the fact that you're decreasing the width of the housing (also spec'd in FSM). That width directly affects the amount of travel for the side seal. Given the application, it's a pretty tight tolerance with all the heat expansion and pressures involved.

Although that's not the biggest issue. Consider the fixturing for machining. Since it's assembled using the dowel pins, you'd have to use those as your datum. If you just use one of the uneven sides, you end up with a housing that looks like this (//) instead of this (||)

 

in severely overheated engines the spark plug side of the housing actually shrinks, if it's beyond .08mm from the rest of the housing after cleaning and blocking it just look for replacements.

in most cases i focus on minimum thickness, the rest of the housing variations can be cleaned up(but you will never find a housing much thicker than the 80mm standard). going beyond that spec you will begin to close the gap between the irons and the rotors.

 

Would a compression test tell you if your housing were warped? I never had mine all the way up but it's seen 3/4 a few times...

 

Yes, the area of the housing around the spark plug holes will SHRINK for whatever reason if the housings are truly warped. Believe it or not it actually takes a LOT of heat for this to happen. I have seen engines that were definitely overheated significantly and repeatedly, and yet the housings were not warped and the irons were flat. Usually if you see a ton of carbon buildup around the spark plug holes, that is a dead giveaway that there is a lot of warpage to check for. If the area is clean and shiny, you are probably good to go.

To reuse a warped housing you would not only have to machine both sides down enough to cover for the shrinkage, BUT you would have to machine the rotor sides by an equal amount...otherwise the iron walls move closer to the rotor sides by that amount, which would be bad news. I have actually heard stories of old rotary shops doing this, though I don't know how well it worked or how true it is.

The other half of the concern with rotor housing warpage is the chrome sealing surface itself. If you saw a lot of carbon buildup there, then that means that some areas of the chrome surface swelled outward into the seal path (usually the area directly adjacent to the plug hole) while other areas are now low spots that the seal is not touching (thus the carbon buildup). IF the seal lifts off of the housing surface in this part of the engine, compression from the trailing face can be blown out under the open seal, and you will always be down on compression and have a shitty running engine. So you would need some way to restore flatness of the chrome surface in this area, depending on how badly it is distorted.

So by now you are looking at paying for "resurfacing" plus round trip shipping. So there you will be at about 3-400 bucks, give or take. And you are looking at paying for both sides of each housing, and both sides of each rotor to be "shaved", and depending on who you find that is even willing to consider doing this (most local machine shops will be afraid to touch it and I have honestly never heard of anyone having this work done at a rotary shop in recent times so I would expect many of them would not want to do it either) it might cost a hundred or 2 for that work. So now you have 4-600 total investment in doing this, for used rotor housings with a questionable history and which will still have a questionable future with these modifications having been done. Seems like the money and time could be better spent with just finding some nicer housings without the issues.

The other issue you could run into would be related to endplay...you could have issues getting endplay within spec because you will have changed the total length of the engine slightly.

 

honestly i don't recall the last time i ever pitched a set of rotor housings with good chrome that gave me an issue. 79.90mm is about the minimum i have seen and the difference in clearance wouldn't cause any pinching problems.

this equates to 0.0039" less clearance with the .1mm change.

 

Well, I think the theory is that the shrinkage will unshroud an area of the coolant seal slightly more than it would be otherwise, and could allow extra combustion gases to push into that area and degrade the seal prematurely. But I guess if you used your coolant seal savers or some RTV sealant there, it would offset the miniscule extra clearance.

Still, if you have shrinkage issues in that area, you will almost certainly have much more significant issues with the chrome around the plug holes, and that is what I would worry about.