Rotax Rebuild

Share your thoughts, photos and general help to all builders

Postby gdewsbury » Thu Feb 02, 2006 10:24 pm

Karl
The crank with a larger back end is the normal from Wildfire. Looks like they may have had a problem on the flywheel end. I bought new cranks when one was damaged about 5 years ago. You'll notice there is no key way on the prop end either, unless someone had it put in after. One of the bearing numbers you indicated looks a bit odd unless misread. The 6205 and 6304 are what wildfire supplies. The 5304 number refers to a double row bearing. These numbers translate to any manufacturer that makes Single Row Deep Groove ball bearings. The LU designation is for the seal on yours. Any reputable bearing supplier can get these at a reasonable price. NTN, FYH or SKF are all good brand names. As Shanon says they should be replaced any time you're doing a rebuild since it's cheap insurance. $10-15 each. Once I've gone that far I replace shaft seals as well. They're even cheaper insurance for under $10. Don't think you want to leave the bearing seals in since they will stop the flow of fresh oil mix through the bearing. Suppliers in my area don't supply with out the seals because it would cost them more than it's worth. Just pop them out and clean out the grease. Grease in bearings actualy reduces the RPM & Life ratings compared to oil.

Cheers
Glen
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Postby lazairiii » Fri Feb 03, 2006 1:40 am

Karl,

Please take a pic of the parts essentially lined up in order of how they go on. Maybe lay the crank in the middle, and the spacers, bearings and seals in a row to each respective side of the cranc shaft in the order they go on, and the casings on each end. This will give us a good perspective of the order in which these parts all fit together.

You are really learning a lot here and you will no doubt have some sweet running engines.

George
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Postby flyalaz » Fri Feb 03, 2006 7:14 pm

Hi Guys,

Thanks for the info Glen. I have new seals to match new bearings, and new gaskets for all the mating surfaces. Someone drilled a keway for the prop hub, using a drill rod instead of a woodruff key. It's possible I misread the numbers because they were quite faded. I picked up the standard bearings, and hope to see at least the same performance or better.
George, as requested, I laid out all the parts as per the rotax manual, for the purpose of these pictures, the shims I still have to measure to see how many and how thick I will be needing, but this is as per spec. If this shot isn't clear enough, I have a close-up I can post later.

Cheers,

Karl
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Postby flyalaz » Fri Feb 03, 2006 7:27 pm

Cleaned cases with a little touch-up and solvent bath to do!
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Postby flyalaz » Fri Feb 03, 2006 7:31 pm

And a clean cylinder too! Just need to do a solvent clean to get rid of the residue.

No blasting done to inside surface of sleeve,
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Postby flyalaz » Fri Feb 03, 2006 10:43 pm

Hi Shannon,

Thanks. I hope the pic makes it a little clearer. The flywheel though, is not the PTO, so why exactly the rear is beefed up and not the front beats me! I don't know what kind of problem at a no load area would make Rotax beef it up .195" more in diameter? Maybe this particular crank was repaired sometime in the past and a newer rear lobe was installed. The top end of the motor is still in good condition, with a decarbon job still to do on the piston (that's why I didn't show it!). So it will stay in for now. I can always do that job at a later date.
If I get this right, it means that:
The "new" crankshafts from Rotax look like this, eg. mag at .983" dia, PTO at .788" dia.
The "old" crankshaft is equal in diameter at .788"dia, is considered a "large" crank, and

the "real old" crankshaft is...????? (I don't think I have one), and please correct me if I have something wrong. I looked at the available pictures showing the small and large cranks, and couldn't tell the difference. Am I blind? Something about a groove on the shaft? Does someone else have or can make a drawing to show this difference?

Blue skies,

Karl
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Postby Guest » Fri Feb 03, 2006 11:22 pm

Hi Karl,

Your right. I don't know what I was thinking. The PTO end is the PTO end.

Your larger than normal on the mag side crank is a "small crank" on the PTO end. This is evidenced by the abrupt taper down in the shaft just before the prop hub. A large PTO end crank has a shallow ring groove around the shaft immediately forward of the oil seal then tapers down evenly into the prop hub.

Shannon
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Postby Guest » Fri Feb 03, 2006 11:37 pm

Nice photo of the parts layed out Karl.
I'm supprised that someone put a drill roll in the front end. Not sure what the original purpose of the engines were but a key way of any sort is a week point but may have been needed for timing. I have been running with out since the new crank was put in. Pulling the tapers together is actually one of the better ways to hold a hub on. Straight keys are used a lot because it's cheaper to machine and if weight is not a concern then make the shaft thicker and straight. I replaced the original crank because of damage cause by pieces of the key galling the shaft and hub when the hub came loose. Engine was at full throtle when a piece of the piston skirt came off and when into the combustion chamber. Caused a hell of a shock to the engine and me. With out the key the shaft probably would have been OK. Key on the back end makes timing the fly wheel a lot easier.
Has anyone had a crank pulled apart to replace lower needle bearings? Been runing the engines for about 5 years and thinking this should be checked as well but it's not some thing I can do at home.
Cheers
Glen
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Postby gdewsbury » Fri Feb 03, 2006 11:47 pm

forgot to log in on the last on.

Karl
The new shaft still has a groove like the one I threw out but it is machined a lot better to remove abrupt changes and sharp edges. Doesn't look much different at first glance. Suttleties in the machining like the size of a corner fillet can make a hugh difference in strength of a shaft.
Glen
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Postby flyalaz » Sat Feb 04, 2006 12:51 am

O.K,

I guess that's settled then. The previously completed engine is 100% small shaft.
This engine is a hybrid- large mag / small pto. Now we all know?????

If any of you guys can post a pic of the "large" shaft, both mag and pto sides, just for visual reference, all question of what people have can really be laid to rest. I couldn't make heads or tails from the available photos, and I think one good, clear picture would be a nice addition to the archives. A side by side comparison would be the best.

Personally, I don't understand why there isn't a radius on the shafts. I learned to always round off corners, chamfer edges and so on. Having such an abrubt change in profile creates a huge stress riser that I'm surprised that more of them don't fail.

The Rotax dealer at St. Lazarre airport told me that I can get my old crank repaired somewhere North of here, I think Thetford Mines or Shawinigan, for somewhere around half the price of a new crank. If I do that I will have another hybrid engine like this one but reversed! I think I'll save my pennies and get a completly new crank.

Next step- reassembly! Who needs more pics?

Karl
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