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View Full Version : Vertically Split Crankcase: Good Idea or Bad Idea?



Jim Hubert
11-02-2007, 09:59 AM
When I found out the SXV/RXV cases split vertically, I was really disappointed. Then I figured, it will probably be a looooong time before that needs to happen -- maybe I won't even own the bike that long. Bad assumption.

I guess I made the assumption, that being a twin, Aprilia would move away from single-cylinder dirtbike practice of having the crankcase split vertically and go with the multi-cylinder streetbike practice of having them split horizontally.

What's the difference? Maintenance costs for one. With horizontally split cases, you don't need special pullers to disassemble or assemble them. (If you have a dead-blow hammer, you're good to go.) And ball bearing removal/installation is for the most part a non-issue too.

All of engineering is compromises, and vertically-split cases are probably good for something. What immediately comes to mind is that you can probably get away with less material in the cases, which of course, is a good thing if you are trying to build a race bike as light as possible. But I would trade an extra kilogram of aluminum for ease of maintenance any day -- especially in a racebike.

Lots of twins were built with vertically-split cases -- Nortons, Triumphs, BSAs. And we all know what happened to them. ;) I read somewhere -- probably Kevin Cameron -- that the British twins used vertically-split case for ease of machining. All the cylindrical "holes" could be done in a lathe. This was obviously prior to the advent of NC and CNC machining.

Another twin with which I am familiar is the TZ250 roadracer. (Yamaha moved from vertically-split cases to horizontally-split in about 1972 on their production roadracers.) All the V-model TZs have an aluminum beam chassis. The front (bottom) half of the crankcase can be removed with the engine still in the chassis. This allows two reasonably skilled workers to remove and replace the crankshaft in under 2 hours *at the racetrack*. And I even know an ex-pro 250cc racer who said he did it in 1 hour because that was all the time he had! Now, I am not comparing apples with apples (2-stroke versus 4-stroke, race bike versus "streetbike") but obviously maintainability was not a high priority on Aprilia's design criteria. The other evidence I see of this is that the swingarm pivot bolt must be removed in order to remove the engine from the chassis. Great for chassis rigidity. Not so nice for maintainability.

Rebuttals encouraged.

irie
11-02-2007, 12:30 PM
Vertical crankcase splitting also creates cylinder sleeve sealing problems.

Jim Hubert
11-02-2007, 08:42 PM
Hmmm... Not as bad as I had thought. There does not appear to be any special pullers required to disassemble the cases. Quoting the engine service manual: "Separate the crankcases with a mallet."

No special tools needed to assemble them either. Again from the service manual: "Join the crankcases."

Bad news is that there are quite a few special tools required before you get to the point of using the mallet!

twinshocker
11-04-2007, 06:21 AM
Hmmm... Not as bad as I had thought. There does not appear to be any special pullers required to disassemble the cases. Quoting the engine service manual: "Separate the crankcases with a mallet."

No special tools needed to assemble them either. Again from the service manual: "Join the crankcases."

Bad news is that there are quite a few special tools required before you get to the point of using the mallet!



Yes indded!
As the main bearings are plain bearings the crankshaft just falls out of the cases.
The gearboxshafts also come out easily (at least after you realized that also the gearbox drainplug goes through the whole engine and acts as a additional screw.....:bangwall:)

Jim Hubert
11-04-2007, 09:02 AM
...at least after you realized that also the gearbox drainplug goes through the whole engine and acts as a additional screw...

True, but at least the manual mentions that. I'm more worried about the "unknown unknowns"! ;)