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Discussion Starter · #1 ·
I have a crank case from a Rotax Type 343 I was thinking of using. There are some marring marks and grooves on the top fo the top half of the crank, where the bottom of the cylinders sit. It is around the stud holes, and they are gummed up pretty good. I smoothed off the high spots, but there are still deep sort of circular grooves around some of the stud holes - deep, maybe 1/16" in a few places.

Is there a sealant I can apply that will fill that when I assemble? The gasket there is thin paper, and I can not see it making a good seal as is.
 

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I'll try to take a picture of it. I was going over it and thinking there's no way a thin paper gasket will make a good seal there.

I already took down the high spots, but those grooves are pretty bad.

It isn't the crank case seal between the two halves, it is the top of the upper crank half where the cylinder sits on it. I thought the Anerobic sealer was just for metal to metal. The gasket set I have comes with a thin paper gasket for the cylinder to crank connection.
 

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I am old school - Use Permatex Aviation form-a-gasket on all my paper gaskets between the cylinder and crankcase. Whether precut gasket from supplier - or ones I make myself from old brown paper grocery bags.

Works well, sometimes too well because you will have to scrape the sealer and paper off the surfaces next time you open up the engine

Can't tell just how deep those grooves are from the photo - but have used on some pretty mismatched cylinders that you could see gaps in places along the edges.
 

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There is another possibility.

Get the entire surface machined down and use a thicker base gasket to maintain proper height.
 
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Discussion Starter · #5 ·
There is another possibility.

Get the entire surface machined down and use a thicker base gasket to maintain proper height.
very interesting. I had wondered if I had it machined smooth (or at least better), if I could just double up on head gaskets to make up the difference. if I added some gasket sealant onto each side of the paper gasket at the bottom of the cylinder, that would give me some heigh back. Then if the compression was too high I could just go back and add in a 2nd head gasket to add more space on top.

Oh, this isn't the 343 I've been working on since last year. It is another one I got out of a parts sled. I got it knowing the motor was done from the previous owner, but it is interesting to see what salvageable parts are in there. Clutch was the best I have out of all of these I've seen; white spring again, which was interesting, since I think they should be light blue, but I've seen the white spring more than once now. Carb was actually rotted into dust from mouse activity, that mostly ruined the pistons and cylinders, so - I've thinking the case might be of use, and the crank actually doesn't seem to bad, but would need new bearings for sure. Cylinders might be good, could just need to be bored over for a fresh start, heads are good. Recoil, exhaust, all other parts that attach to motor seem good.

I should really put this aside and get on assebling the 343 that I think is good, that I took apart last year, but new stuff is fun to go through. The clutch from this one will go on that other 343, it is just better - condition wise, perfect rollers, cleaner, less wear/corrosion.
 

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very interesting. I had wondered if I had it machined smooth (or at least better), if I could just double up on head gaskets to make up the difference. if I added some gasket sealant onto each side of the paper gasket at the bottom of the cylinder, that would give me some heigh back. Then if the compression was too high I could just go back and add in a 2nd head gasket to add more space on top.
My version of that would be to double up or use a thicker base gasket.

Reason is that when you machine the case - you will be dropping the ports that much lower in relation to the piston. Lower ports = lower performance.

Raising the cylinder back to where it needs to be with some sort of thicker base gasket arrangement will keep both ports and compression the same as stock.
 

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There is another possibility.

Get the entire surface machined down and use a thicker base gasket to maintain proper height.
This IMO would be the only way to do a proper and permanent repair on that case. If the grooves are 1/16" deep I wouldn't trust a gasket sealer or anything else to keeping air from seeping in or out of the case.

The machine operator should measure the deck height before and after machining the surface as well as keeping track of how much has to be taken off the deck to get it flat. You would have to add the amount removed to the thickness of the original gasket to get the correct port height restored.

Automotive parts stores sell gasket material in various thicknesses. You should be able to come close to the thickness you need and cut out new gaskets with an "Xacto" knife.

Lynn
 
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Discussion Starter · #8 ·
I have another set of cylinders and crank that is better than this one. I was cleaning up the surfaces, but there still appears to be some staining or oxidation on the mating surfaces. I found a plate glass that is like 18" by 6" that is for knife sharpening I think, but supposed to be .001 tolerance from end to end - and I have a quality straight edge on the way to also check for flatness in spec. Is it reasonable to just put the case halves on the plate glass on say some 2020 wet dry paper with some oil, to surface the mating surfaces? Then I can just check it with the straight edge and a feeler gauge - and should be good to go, yes?
 

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You might be able to clean the deck with your method but if that doesn't work you could try this:

https://www.zoro.com/scotch-brite-bristle-disc-1-in-dia-58-in-trim-80g-61500129913/i/G3165592/feature-product?gclid=Cj0KCQiA0ZHwBRCRARIsAK0Tr-rRtl9gICrWVdQjG_EMSBDjoUCVDBqb72Cc8VQS2_7SoGZsjnfR5noaAr-OEALw_wcB

I would use the 80 grit in an electric drill at a slow speed. These bristle cleaners will clean but not remove metal or aluminum. You will need the adapter to use the bristle brush which runs about $15 at most automotive parts stores which have a paint department.

Another suggestion would be to soak the case in a deep pan filled with water and a 1/2 cup of detergent like "Pine-Sol' or "Simple Green" to loosen the corrosion. Start out with hot water and totally immerse the case.Let it sit overnight or longer. Dry the case before using the brush.

If you use the glass plate and sand paper do not apply a lot of pressure while sanding the case. Apply even pressure on both ends so the case doesn't become tapered from one end to the other.

Lynn
 

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I'll try to take a picture of it. I was going over it and thinking there's no way a thin paper gasket will make a good seal there.

I already took down the high spots, but those grooves are pretty bad.

It isn't the crank case seal between the two halves, it is the top of the upper crank half where the cylinder sits on it. I thought the Anerobic sealer was just for metal to metal. The gasket set I have comes with a thin paper gasket for the cylinder to crank connection.
You got a few different Options. But first take a Straight Edge to see if it's Warped, level across.

1. You can take a piece of Glass with Rubbing compound or some 400 grit wet dry sandpaper and try to Smooth it out! Like this with your Case Top.



2. You can try JB Weld and a Razor Blade to fill the Voids and then Smooth. Something like this.


3. You can Use them Alumiweld Sticks to fill in Voids and File them Smooth.


4. Take it to a Machinist and have it Trued up, but then you need to use Thicker Base Gasket.

5. Or Use a Good Sealer as sightation said, or a combo JB Weld to fill in Voids with a Good Sealer.

.
 
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