RB25 water pump on RB20: Pictures.

[mello88]

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...

The Pink line traces the edge of the water passage under the pump. The yellow line outlines the slot size, and the green dot is the bolt hole. Finally the Blue shaded area shows the exposed passageway/leak area.

The slotted hole, the only visual difference in the pumps, is what causes the problem on the RB20. It will bolt up just fine, 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. While it seems like some alignment/adjustment might "fix" the problem, the pump cannot be pushed up or right due to the bolts holding it in one specific place. 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 water 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:



How I found this out... I pressure tested my cooling system the other day, and found a huge leak out of the left side of the slotted bolt hole before I knew about this issue..

Ok, I figured I must have just 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. I hand-tightened the bolts, allowed the RTV to cure, then torqued the bolts to factory spec in the factory order. Next I hooked my pressure tester back up to the heater hose pressure tested the cooling system to 2 bar. It held briefly but almost immediately blew a hole in the RTV in exactly the same spot on the left side of the slot. It actually blew out some RTV I squished into the back left corner of the slotted bolt hole (in a feeble attempt to help cover the gap, before I saw what the issue was)... Which means it wasn't even sealed from the back (block side) at all. I'd rather have it blow out on the engine stand than on the freeway... I guess that's the nice thing about pressure testing.

Well, your mileage may vary, but as far as I can see the RB25 pump is not a viable option for the RB20. Pick up an RB20 specific pump with a single bolt hole from Phase2 or a similar place.

Questions: 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:



Thank you for reading, hope this helps someone make the right choice when buying their water pump! And a final note to anyone currently running an RB25 pump on their RB20.... Your cooling system is running on borrowed time.

Modified by mello88 at 1:01 PM 2/6/2007
Modified by mello88 at 4:14 PM 2/8/2007

[jobestudios]

I wouldnt use RTV sealant to seal that up anyways.

Would it be possible just to find someone with an aluminum welder to fill that hole and then drill out the spot you need?

[mello88]

Thanks for the reply. The FSM says to use RTV, the pump that came on the motor was RTV'd on, and there wasn't a gasket that came with the OEM gasket set. So, what would you suggest I use to seal it with? The RB20 pump should seal fine with RTV since there is more metal in that area... But if you have a better idea please let me know.

I figured you could probably fill the slot with something (RTV, Molten metal, jb weld, etc), but it was just as easy to order a new rb20 specific water pump and sell my rb25 pump. I just wanted to post some pictures of this issue, so hopefully nobody finds this out the hard way. I could see it sealing just enough to hold back water under no pressure, but as soon as you lay into it and build some heat up....... *** PSShhhhhhttt ***

edit: it just dawned on me you must be talking about filling the slot hole, not talking about sealing the pump to the block with RTV... ??
Modified by mello88 at 1:03 PM 2/6/2007

[Eikon]

Great work Mello.

Thanks for taking the time to ducument your research and share it with us. On behalf of NICO as a whole... Thanks!!

[mello88]

Yup... Got a new RB20 water pump from Phase2 on the way now (about $90). And I have that never-ran RB25 pump for sale if someone needs it..

[Sil240]

NICE work!!!!!yeah i did mine with RTV alsoThe nissan gasket kit comes with RTV in it.Your right the fsm says to also.Maybe even the instructions with the pump too.

[wob]

Good information...Now I don't hafta think twice when I see a good deal for an rb25 pump...

I've personally used rtv and its held up nicely

[Yellow4g63]

I have a RB26 water pump on my RB20 .

[mello88]

Yeah I know some of you guys are running the 25/26 and N1 pumps, I would like to see why you don't have the same issue? Is your block that different or did you do something to seal it better?

[jobestudios]

The problem isnt the RTV around where it should be, its the fact that you are trying to hold the pressure of a coolant system with nothing but rtv. IIRC they do not make N1 water pumps for Rb20? So if you wanted a N1 pump, I would say it is possible to cover the side of the hole on the machined face, fill the slotted hole with JBweld, CAREFULLY dremel down the excess, take a drill press to the excact spot to your rb20 hole is and that should work, that is assuming that the coolant passage where the waterpump seats are the same on 20 and 25 and the ONLY difference is that one bolt (can you verify this please?).

Yes it may be a little extra work, but to have an upgraded waterpump would be nice

RTV will hold fine in small cracks, but if you're trying to fill open space, its gonna blow right out.

[mello88]

Joe, yeah thats what I was trying to get at with this thread... There isn't any way to easily get it to seal with just RTV... The gap is just too big. Would require welding or epoxying or some other equally shady solution to make it work. Might as well just pick up the RB20 pump and be done with it.

Also, someone else asked if the only difference between RB25 and RB20 pumps was the slot.... It's not, I found another minor difference:

Last night I was sitting around in the garage working on a few things when I noticed the RB25 pump looked like it had different sized blades than the RB20 pump. I measured both props with my micrometer, and sure enough, the RB25 pump has 11mm wide blades, while the RB20 only has 10mm wide blades. IDK if this was done to increase the RB25 pump's flow rate or what, but it is another minor difference I haven't seen documented anywhere.