Thread: GPR crankshaft out of "phase"
06-16-2007, 11:37 PM #1
GPR crankshaft out of "phase"
After many days of banging my head on this boat I finally found the problem.
Even though I am going to need a new crank I am very happy I finally found out what was wrong. Thanks to Lowell for sticking with me when my attitude toward Yamaha's slipped away.....Still not a Yamaha fan but man they are fast.....when they run. Which is not much for me!!!
From the measuring I did today the phases are several degrees from where they should be!!!!
06-18-2007, 08:32 AM #2
- Join Date
- May 2007
- Buzzards Bay, Massachusetts
well good to hear you found the problem...personally I have never had any problems with Yamaha products...in fact they have proven to be far more reliable than most other brands over the years of working at a marina and motorcycle/"toy" dealership.
let us know how you fare with it fixed
06-18-2007, 08:34 AM #3
How does something like this happen? More details please.
06-18-2007, 10:18 AM #4
Most 2 stroke cranks journals are pressed together in large presses using precision guides.
The cranks are then adjusted for runout during and after the assembly phase.
This is a pic of your gp12/13r crank taken apart.
Here is a 2 cylinder version, both assembled and taken apart.
On a working engine, crank phase can be lost by a few things such as water ingestion (common on an aftermarket gpr) crankshaft journal failure, or even something such as sticking a piston at 7000rpms.
When you hear the term "welded crank" on a two stroke, it infers that a crank that has been properly phased and trued, has had the journals welded (or pinned internally) to prevent the phase slip.
Trueing a crank infers that the runout is properly set, as well as all the other critical crank parameters.
On the GPR there are a number of common failures:
-Rear Rod bent or rear journal out of phase. I'm 99.99% convinced over the years this is from water ingestion.
-Front crank Journal snout snaps off. Here is a backside of a flywheel with the snout snapped off.
-Mid Crank bearing failure. 2002 models suffered a rash of mid-cylinder bearing failures. Unknown what the root cause was, but in almost all cases if the ski was under warranty, Yami would replace it that year.
Last edited by Hydrotoys; 06-18-2007 at 10:41 AM.
06-18-2007, 10:32 AM #5
Some top shops actually make their own journals.
Some use both computerized and manual optical mapping to check thread tolerances to a very, very finite point.
Of course, some cranks like stroked banshees have such a large market that some absolutly beautiful work is created by these shops.
Other work, such as stroking, balancing, lightning, and hiding the truth from race officials, are done at shops, quietly, and for large sums of cash. They are absolutely incredible pieces, done simply, yet with precision.
Polaris 1200 crank, trued, welded, ready to go.
Last edited by Hydrotoys; 06-18-2007 at 10:43 AM.
06-18-2007, 10:47 AM #6
WOW, great post mark!!
Now I know what was done to my crank when I got it "trued and Welded".
06-18-2007, 10:50 AM #7
Where did this crank come from and what were the symptoms ?
Last edited by JetSkiDaddy; 06-18-2007 at 10:58 AM.
06-18-2007, 12:06 PM #8
It was a OEM Yamaha crank with about 25 hours on it.
It had no power whatsoever, barely enough to plane the boat and would only top out at 6300 rpms....
Looks like the pump came unloaded because of a wake and the rest is history. I guess the shock cause the pins to slip.....I was only going about 25 mph....
I learned my lesson. Never ever use a Yamaha crank even for a scooter without having it trued and welded!!!!
06-18-2007, 12:13 PM #9
It must have been a bad crank or something. I dont think this is a common thing expecially with 25 hours and going as slow as you where. I bet there was something wrong with it. I have abused the shit out of my ski's and this has never been an issue.
06-18-2007, 02:31 PM #10
There was something wrong with it. Also the Yamaha crank is notorious for breaking the snout.....Not crazy about this engine, anyone else.....?
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