Water pump impeller clearance

I finally got around to replacing the water pump seal and drive shaft etc. 

I have all the required tools and the Maintenance Manual Heavy (MMH) states: The spacing WP01 (impeller clearance) is automatically established with insertion jig part no. 877258.

The problem is, I have replaced the seal twice using this tool and both times I can only acheive .007" clearance and not the minimum of .012" wich is a bit tighter than before I touched anything (previously .010")

I still have a small amount of Axial (End) play as before and confirmed this play is the gap between the drive gear and the housing. There was no visible wear in the housing or the gear but also the manual doesn't specify how much gap is acceptable between the gear and housing. I have checked on 3 seperate engines now and they all have a similar amout of Axial/End play as mine so I conclude this is normal.

I am now wondering if the tool (jig part no. 877258) doesn't have the correct dimensions. Given I have done this job twice now and got the same result both times, I think it would be possible to either make a 1mm thick water pump gasket or perhaps replace the washer shim behind the impellar with a thinner one (would probably have to machine one or modify the old one to suit) so that I can obtain the correct clearance. 

I am open to other ideas about what might be causing the lack of clearance and what to do about it if anything.

Murray,

For the purpose of this discussion, I will refer to “forward” as towards the impeller of the water pump (towards the back of the engine), which is the direction the spring in the seal pushes the shaft.

There will be some axial end play as you push firmly back on the impeller and compress the spring in the seal. And as you say, this is the axial clearance of the gear in the housing. The reason this measurement is not specified is that it is not important. The spring in the seal holds the pump shaft to its most forward position within this axial play, and all that is important is that when it is held to the forward extreme the impeller has the required clearance from the pump housing. If there was any wear, it would be between the forward surface of the gear and the housing, in which case the clearance between the impeller and pump housing would be reduced and out of spec. How far it pushes rearward to the point the backside of the gear contacts the housing is not important since it would contact well before the impeller would contact the ignition housing.  If the two measurements in the manual, WP01 and WP02, are within spec, you are good. 

Regarding your problem with not being able to achieve .012” for the WP02 measurement, maybe you are not getting the shaft-seal assembly fully seated.  The interference fit with the shaft and gear is very tight.  Are you chilling the shaft and warming the gear before pressing it?  The WP01 measurement will reveal this, what is that measurement?  Are you tightening the impeller to the specified torque before measuring? Also the WP02 measurement should be made with the water pump housing gasket installed.  Lastly, the wide collar of the pump gear points inward towards the crankcase.  

Thanks Jeff,

I never tried cooling the shaft and warming the gear but that is good practice. That along with, I am a bit afraid to go hard on the hydraulic press leads me to believe I may not be getting the seal fully seated. I probably also got caught up in the fact that it was out of spec before I even touched anything.

The WP01 measurement is slightly out of spec at 9.0mm and the seal is no where near flush with the housing as shown in the drawing. I think the drawing is just wrong though because if it were flush as shown, the WP01 measurement would be around 7.5mm and way out of spec.

I think I will have another go at this and do as you recommend and hopefully get both measurement within specs. I will also measure the distance the rear of the shaft is to being flush with the housing to see if this made any difference after reinstallation of the shaft and before assembling it all back together again.

Murray

The water pump tool, Rotax, sets the seal on the shaft first.  This controls the preload of the spring inside the 2 part seal.  The outer ring of the seal already has a bead of blue Loctite on it.  With the seal assembled on to the shaft you then use the same tool to push the shaft and seal into the housing.  (use a new oil seal inside.  Follow the instructions exactly from the maintenance manual heavy.  You need to push the seal and shaft until the tool bottoms.  The shaft may not come flush on the inside but dont worry that depth control is done with the press tool.  It is very important to bottom the seal into the housing, a firm press to full stop.  Be sure to use a new gasket every time, the gasket sets the clearance.  

I suspect you did not bottom the seal into the housing from the description.  

Cheers