The insert trying to be sucked into the impeller is only the total of the vacuum that exists in the center hole. A single screw has the ability to hold that insert in with more force than the vacuum generated by the center hole.

Step drills are made to order and are cheap if you know where to get them. Also the extended length bushing are also standard.

I would imagine it would be impossible to forget where you are in the process. Not likely you would forget to do the drill.

And yes, this is what I do for a living. I am a machinist by trade and a mechanical engineer in education working in the automotive parts industry.

 

Also, each of those screw heads is a point of cavitation that may cause some problems.

 

Sounds like you should have came up with this fix and made it at pretty much cost for all of us to repair our blocks with.

I am very happy with what Greg Brown has done.

 

69 gaugeman, like Greg said, if you want to come up with a couple dozen free stepdrills and provide the longer bushings- go for it! We did the two-step deal to make it possible for anyone to replace a broken or worn drill or tap from stock at Orchard or any other hardware store. And the forces working to pull the insert out of the block are strong enough to overcome the interference fit on the impeller-to-shaft connection, so one screw is not going to keep it in there...and the screws are flush-mount flatheads, so cavitation is neglible, especially compared to a 3-4mm gap cut by an impeller trying to force its way out the back of the car.....
Greg just wanted to be able to save engines suffering this failure without removing the engine from the car and tearing it down. This tool does exactly that, in a way that almost anyone who can change a waterpump is capable of doing himself.
So, I'm glad you are a machinist- the world needs a lot more machinists- but in this case, your failure analysis is wrong- there is a lot more force on the block/impeller area than you think. Enough to pull an impeller off the shaft while it still has a tight enough connection to shaft to machine away the block- with piece of dull cast iron. Quite a bit of torque required for that...

Steve

 

I think he has done a great job as well. Are you saying you would not want it to be cheaper? and easier to do?

I am not saying that this is not good. If this happened to me, I would have made a repair just like this.

Do I really come across as berating or unfairly criticizing this repair? Because if I do then I apologize. Hopefully those who can see the constructive criticism, will take it for that and see they got some good advice for free. If not <holds palms up and shrugs shoulders>

 

If Greg was doing this for free, I WOULD give a couple of dozen step drills to him.

I was a machinist and now am a mechanical engineer. I know of what I speak.

Respectfully I disagree with your assertion that the amount of force to pull that impeller off the shaft is stronger than two screws. Also, the vast MAJORITY of pumps do not have this problem. This indicates that there is an issue with build quality and not design parameters.

And if you think cavitation is a non issue, I also disagree. I have seen impellers completely eroded to nothing due to cavitation.

I also disagree that a lot of torque is required to machine an aluminum block with a steel impeller. I know a lot about machining of all kinds of material. This block wear does not happen in the space of a few hundred revolutions. It is over a long period of time.

 

I think the damage that happened to Greg Browns engine happened very quick, I know mine happened in short order.

 

Any idea of how long it took? I am curious.

 

Mine happened in less than 3,000 miles, and not to put words in Greg Browns mouth, but I think his happened on the first dyno pull on a new engine.

 

To put it in perspective if the pump wore the block out in 5000 km (what I would consider a very short time) the amount of material removed per revolution of the impeller (aprox) with 4mm of material total removed = 0.0000006933mm.

This is infinitesimally small . To put that in perspective in the auto industry the limit of technology to measure distance is .0001mm.

The size of an aluminum atom is 0.0000000300 therefor you would be rubbing off approximately the thickness of 23 atoms per revolution of the impeller. Not a lot of torque required to do that.

 

If that happened to the point of destroying the block on one dyno pull I would be quite surprised.

I would calculate that that would be physically impossible with the press fit on the shaft. That would take in the nature of 10 -20 horsepower to accomplish. I don't think there is a pump shaft on any 928 that could take that torque.

But I have been wrong before.

 

Greg B's happened in minutes. And I'm not going to argue with you. If you want to put this insert in YOUR car with 2 screws, go ahead. This failure is not typical in that all impellers do not always come off the shaft. They usually fail the bearings, and then the impeller hits the block, resulting in a similar problem. It is fairly obvious that the impellers that DO come off are because of an improper fit...Some of these pumps last 100,000 + miles, some don't. Like any production part.

The block is made of Reynolds 390 high-silicone aluminum- it eats HSS and carbide tools. The cobalt drills we supply are dead after a few holes- they are "consumables". It really needs diamond-coated cutters to get any kind of tool life, but thats out of the question, so we went with easy to replace cutters- you can't get custom stepdrills from Home Depo.
And cavitation in this kind of centrifugal pump is NOT an issue- there are bigger deviations than the screwheads in the pump, block and impeller surfaces as-cast. This is not a high-pressure hydraulic system.

This repair thing is not done for money on my part or Greg's part. He and I spent a good deal of our own money and time designing and building this tool as service to the general 928 communty. It is available for loan to anyone, and the money covers insert, cutter, and shipping costs. Its NOT for making a profit...and its a good deal cheaper than a new block!

So, Rod, I am done defending the design here. Everything was done for a reason, and it works fine so far. We'll just have to disagree about, well, just about everything. You are obviously free to do it any way you want. After 40 years in this business, I've learned there is more than one way to do anything, and what ever way works best for you is what you go with.

Steve

 

Rod, you seem to have missed the other N threads on block machining. Your input several months ago when Greg B was discussing the design would have been good to have. In any case, Greg B did have a new Laso pump eat into a block with only a few minutes of running. IIRC, he didn't complete the first dyno sweep. The issue of fluid dynamic forces causing or not causing the the impeller migration seem almost irrelevant - the impellers eat the blocks and almost always with rebuilt pumps. Why? Don't care. We need a way to fix the blocks. Sure, eight screws is overkill. Two or four might do the trick. Hell, some of us have used metal epoxy to fix the block with success.

If you'd like to contribute and make yourself useful.... there seems to be a large variance across engine blocks and water pumps in the clearance between the block surface and the impeller. It would be both timely and useful if someone with easy access to the right books or tools could calculate the minimum and maximum useful distance between the impeller and block surface. Too far and the pump won't pump. Too close and various bad things happen.

 

Rod:

Thanks for your critique. As Steve says, there are always different ways to approach a problem. I decided, early on, that I was going to retain the insert with 8 screws....no science....it just looked like it would never move or distort with 8 staggered screws. Maybe 2 or 4 screws would have been plenty.....maybe just epoxy would hold it....I didn't want to kind out...it has eight screws and I suggest using epoxy.

Here's some information on the costs of building this tool and repair kit....just for your education.

I personally have right at $800.00 of my own money in this project. No time included. Steve Hattori has a similar amount invested...and much more time. Greg Nettles donated a Snap-0n 1/2 drive air drill, so that people would have something to turn the cutter with. The "token" $200.00 charge covers shipping, inserts, drills, taps, etc. Those costs are about $100.00 per use. That leaves $100.00, per use, to pay back the "investment". Let's see....that will happen....never.

If I was going to charge for use of this tool and repair, with the intention of making money, It would be more like $600.00 per use. Even at that, it would take 6 or more uses to "recover" just the cost of building this....no mention here of our time!

If you'd like to donate some pieces to improve the use of this tool, I'd encourage you to do so. We've donated about as much as we can afford.

 

You left out having to resharpen the cutter, that is not going to be cheap either.

You and Steve are never going to break even, but you both have saved a bunch of blocks that were junk.

Thank you both for that.