Water pump myth?

[Dearg1275]

I’m building up a 997 small bore and a 1275 S large bore at the same time. I have a deep vane cast water pump and wondered if it would fit the 997 without fouling. I have read many times that you can’t use a deep vaned pump on a small bore engine.

In a rough and ready way I tried one in the 997 block ( no locating studs and just a couple of bolts ) and thought it was catching slightly. I ground the edge of the rotor fractionally and the problem went away. In retrospect I should have put the dowels in and done things properly. I now think my grinding completely un necessary.

As the two blocks were to hand I did some measuring. Firstly to the cylinder jacket and, off course there is more clearance to it on the small bore (0.820” as against 0.620 on the large bore). The vane is 0.540” deep from the fitting face. So what else could foul. Something on the periphery of the pump aperture in the block perhaps. Starting with the big bore the nearest possible obstruction to the rotor was 0.660” and everything on the 997 block was further away I.e. greater clearance. I had a late 850 block sitting close by and that too had greater clearance.

Now this is the early 997 block, not an improved later version.so where did this misunderstanding come from. Is it just early 850 blocks that had a problem taking deep vane pumps or is it all just mythological codswallop?

Back to the 997. I think that the catching I was getting was the rotor touching the edge of the aperture because I didn’t have the dowels in.

D

[woodypup59]

With the (sometimes poor) quality of replacement parts nowadays, I think its always a good idea to trial-fit or dry-fit all parts before final assembly.

Prime example -
When fitting an short engine to a different gearbox - do a trial fit without gaskets to check that the crank / big ends don't foul the gearbox webs. You can then also use lumps of putty / why to check how much clearance you have.

[Andrew1967]

As far as I am aware it was only the early(ish) 850 that you could only use the shallow impeller pumps.

[Spider]

I have a recollection it's also the early 1100 and 998 blocks that it can foul on. I recall doing it once on a 998 block long ago.

There's a reference to the need to grind in the Special Tuning Booklets, one that covers the 848, 997, 998 cars and also in their general 'Overheating' guide.