I saw a few old threads asking if an RB25 pump will fit on the RB20, but nobody really answered one way or the other... Except to say the pumps look very similar (rb25 bolt hole #1 is slotted and rb20 hole #1 isn't). Bad news for us RB20 guys -- the RB25 pump will NOT work properly. Check it out...
Blue area shows the overlap/leak area. Pink line is the edge of the water passage, yellow is the slot size and green is the bolt hole.
The slotted hole, the only visual difference in the pumps, is what causes the problem on the RB20. It will bolt up perfectly, but because of the metal removed from the pump to form the slot, part of the water passageway will be exposed! This little gap is enough to cause the RTV not to seal very well in that area, or if it does maybe seal a thin coat, it will quickly blow out.
If you look in first picture, in the the lower left corner of the slotted bolt hole (behind the bits of RTV), you'll be able to see the water passage way. The second picture shows the end of a large zip tie shoved into the gap, it's a pretty good size hole (hard to see in pics)... Anyway in both pics, the pump is aligned and hand tightened on a few bolts. From there, you cannot really adjust the pump up or to the right any more (as it would seem to improve sealing by the slot). This may just be a case of slight variances in the block casting, but I think there is a reason RB25/RB26 use the slotted hole and RB20 doesn't. I think the RB25 has a slightly different casting on the block under and to the left of the #1 bolt hole.
The RB20 pump of course has a single non slotted bolt hole, which does a much better job of sealing off the passageway, and provides more metal to metal surface area for the RTV to seal:
I pressure tested my cooling system the other day and found a huge leak out of the slotted bolt hole before I knew about this issue.. Ok, I figured I must have missed an area with the RTV, so I removed the crank pulley, timing covers, timing belt, etc to re-install the pump. I took extra care to evenly cover all of the pump's mating surface with RTV this time around. After hand-tightening the bolts on the pump and letting the new RTV cure, I torqued the pump bolts down the rest of the way. Next I pressure tested the cooling system again, this time to 2 bar. It held briefly but quickly blew a hole in the RTV in exactly the same place in the slotted bolt hole as last time! It actually blew out some RTV I squished into the back left corner of the slotted bolt hole (to help cover the gap)... Which means it wasn't sealed from the back (block side) at all. I'd rather have it blow out on the engine stand than on the freeway....
Your mileage may vary, but as far as I can see the RB25 pump is not a viable option for the RB20. Does the N1 water pump have the slotted hole? Did anyone else catch this sealing problem on their RB20? Also If someone has a picture of their RB25 block without the water pump, I would like to see if there are any differences under and to the left of the #1 bolt hole:
