[cmoyle]

I just sent my pump to Tom for a rebuild with the new thrust bearings, so the jet pump is on my mind. After reading many of the posts about how clearance in the wear ring can cause significant loss of thrust, I began to wonder what the proper clearance between the impeller blades and the wear ring should be, and also how that could be measured. Does wear occur on the blade tips or could manufacturing tolerances allow more clearance than would be optimum for best thrust performance? I was wondering if there was a standard shim kit available for maybe another application, that could fit between the hull and Tom's wear ring to move the cone rearward if excess clearance exists to bring it to spec. Maybe shims could be added to achieve a slight drag, and then the correct amount of shim material removed to create the perfect clearance?

Has anybody experimented along these lines?

[happul3]

In my experience, the standard plastic wear ring (Ive dealt with two so far) after installation is is direct  contact with blades. The first rotation removes some plastic leaving as little clearance as the can be given whatever play there is in rotor bearings. Something similar happens with Toms' aluminum ring + epoxy, I gather. So the shims would not seem useful. This becomes even more moot after some actual use, when additional ring material is removed.

As for the power loss, I often wondered myself just how important the clearance is. Yes, one hears a lot about improved performance after wear ring change. But some of that is just a natural human psychology - a sort of placebo effect.  When I personally replaced wear ring I could not get any measurable increase in top speed, but perhaps the ring I replaced was not worn enough. Also, one can find some papers where people studied efficiencies of ducted fans as function of clearance. But that's not quite the pump we have...

[cmoyle]

Interesting. I have no experience with the stock plastic rings, but when I do the epoxy on my wear rings from Tom I use the blade to match the aluminum surface just filling grooves. I can reinstall and feel that there is no contact resistance at all from impeller to ring contact. That makes me wonder how much the clearance could vary from boat to boat with long term wear on the ends of the impeller blades or slight differences in where the impeller actually sits on the shaft due to possible manufacturing tolerance or machining. Maybe I'm being too cautious when I apply the epoxy, and could allow a thicker layer to be "machined" out by the impeller rotation. The shim concept was considering moving the "cone" of the wear ring tighter onto the impeller if indeed there was excess clearance to be dealt with.
I may have to find the equivalent of the old plasti-guage we used years ago for bearing clearances in engine rebuilding. 

[Painlesstom]

Wear ring clearance is everything when it comes to speed, .030 clearance around the circumference will easily drop 3-4 mph off your top end, more depending on occupant and conditions. The closer you can get it without touching, the better. It really is a big deal. Your engine will run less rpm too when it's properly loaded. The whole point of my wear ring is so it can be resurfaced with JB Marine Epoxy. Wear rings wear, that's their job, shimming isn't the way to deal with that. I made this video years ago so people would know how to check their clearance, here it is again starting at that point. Oh, and if you have my latest pump rebuild with the thrust bearing, you must pull on the shaft so the thrust bearing is fully seated while you check your clearance.

[cmoyle]

Thanks for jumping in Tom. I've been rebuilding your ring according to the video for several months now due to our shallow gravel bed rivers!
 Since you are doing the rebuild on my unit now with the thrust bearings, which direction are you referring to about pulling the shaft when checking clearance, toward the wear ring or the nozzle?

I guess that nobody is understanding my musings that are based on "blue-printing" performance auto engines years ago.

My assumption is that, using a brand new one of your wear rings, if the clearance between the impeller fins and the wear ring would be actually measured on multiple boats with the pump installed in position, there might be different amounts of clearance show up due to the casting and machining process of the impeller, the actual machined placement of the impeller on the shaft, or the machining of the forward wear ring housing during production by Mokai - or - that (over time) the tips of the impeller blades might wear down a bit (besides the wear ring). Either of these hypothetical situations could allow excess clearance between the outer cone of the new wear ring and the impeller blades when everything is installed into position.
Maybe Mokai's production methods are precise enough for this to be a moot issue. I was just aware of huge increases in those auto engines from bringing all the specs into the most precise tolerance, and wondered if anything other than the expected wear on the wear ring could be a potential for excess clearance. That is what brought to mind the concept of shims being used to move the outer cone (wear ring) closer to the inner cone (impeller blades) if excess clearance existed. Probably moot as I said, but just was something that I was wondering about.

[Painlesstom]

It has less to do with manufacturing tolerances (a tiny bit, yes) and more about impellers wear too and none of them are the same size. It doesn't matter what the size is, it matters what the clearance is between impeller and wear ring; and you check that as shown the video I shared. You can only pull the shaft one way, away from the housing. If you go toward the housing then you are pushing. When the pump is making thrust the force is pushing the impeller toward the motor and that is the position you want to see when checking clearance.

[cmoyle]

Thanks for the clarification Tom. I wasn't sure if you were referring to the "wiggle test" you showed first in the video that would have me pulling the shaft up toward the wear ring, or the installed "turn to check clearance" test that came after that in the video. As you've stated, either would be a pulling action and the latter would require the engine being removed.

[Painlesstom]

Yes, you pull the shaft up while doing the "wiggle test". If you pushed down and checked clearance there would be much more, furthermore if you built the ring up to have a good clearance while pushing down, the impeller would crash into the wear ring as soon as it started to generate thrust and moved forward.

[Craig5251]

I can tell you that resurfacing the ring makes a HUGE DIFFERENCE for me! I’m a bit big for my 2012, so going against the current with a worn ring is very slow going. That’s how I can tell it needs it.
Afterwards I gain 3-4 mph! My top speed down stream is 10mph, so that’s huge.