How-to: convert your Merc raw water pump to crank-driven Johnson (take 3)


undercover

undercover

Member
Last season, I had an impeller go out on the stb motor (350 MAG MPI). (I never change them until they fail because they're a bit of a pain to get at.) The first outing this season, I was in a rush and left the dock without checking everything over and quickly got the beeps. Turns out that same pump didn't prime, and I burned up the new impeller. I guess I hadn't used it enough for the new impeller to wear into the slight grooves in the bronze pump housing.

Rather than try to clean the caked-on impeller residue out of the pump, I looked into converting to crank driven pumps, and here's what I found:

Pros:

Cheaper pumps ($150 vs $700 for the bronze Merc w/ air fittings)
Easy-to-change impeller (4 screws, pop the front plate off, and there it is - no need to remove the whole housing)
Cheaper serpentine belt ($16 vs $70 for the long Merc belt)

Cons:

Initial conversion is about double the work of an impeller change on the Merc pump
Costs about $250 per engine to convert (still way cheaper than replacing the Merc pump if yours is worn/failed)

You'll need about 3 inches in front of the motor, or 6 if you want to do an impeller change without unbolting the pump

20190709_161658.jpg20190709_161634.jpg
 
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Here's what you need (double recipe for 2 engines):

1) Johnson F6B-9 Hi-volume pump ($150)
2) Raw water pump bracket - I used skidim.com part #49-5200 ($40) since it's adjustable, Marine Power 308711 ($35) will work as well
3) 3/8" billet SBC pulley spacer ($17 ebay)
4) 1/2" OD x 3/8" ID x 3/8" long bushings - qty 3 ($6 ebay)
5) 1 1/4" to 1 1/2" billet hose repair barb ($7 ebay)
6) 3/8" x 1 1/4" x 12" aluminum flat stock ($6 onlinemetals.com, or use stacked 3/8" washers)
7) 3/8-24 fine thread bolts 1.5" long, qty 3 - must have small head like allen head or CV axle-style 12 pt head (ebay $6)
8) about 3 ft of 1 1/4" marine water intake hose ($11)
9) Gates K060763 or equivalent serpentine belt ($16)
10) optional Mercruiser part# 862778 alternator bracket ($21) - I haven't done this yet
 
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A few things to note a pay attention to if attempting this conversion:

- You MUST use the bushings (#4 above) in the water pump flange. The holes in the pump flange are 1/2" for some reason, and without the bushings, the pump can be installed off-center causing a wobble/vibration.
- The pump and bracket is normally installed with the hoses facing port. I twisted the bracket 45 deg (see pics) so that I could clock the pump with the hoses down and to stb.
- The flat stock is if you want to make nice spacers to fill the void left by the original raw water pump, just measure and make spacers using the flat stock or stack washers
- I had to round (file or grind) the corner of the pulley spacer to fit flat inside the crank pulley as it was slightly curved inside
- You can't use regular bolts to hold the pump to the pulley as there won't be enough clearance for a socket.
- Pay attention to which hose is in and which is out. On the Merc pump, one is 1 1/4" and the other is 1 1/2". On the Johnson pump, they are both 1 1/4".
- Keep the idler pulley bracket or try part #10 above to make sure the alternator still has proper support.

Everything else should be pretty obvious to you, but if I left anything out, ask questions while it's still fresh in my mind! I've been running this setup for about 25 hours now, and temps look great with no problems to note.
 
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Anyone try this on a 8.1? Seems a lot cheaper than buying the hardin marine stainless pumps when it's time to rebuild
 
Mix bag, with later motors but can be dealt with

The 8.1/496 and other motors use the later gen pumps which may also have easy drain systems attached. To use the crank ones, you also need to remove part of the drain system which IMO is not a big deal. So instead of pump and pull drain plugs, you just remove hoses or install drain Tee's in the lines
 
I don't see why this wouldn't work on the 8.1/496. You would probably need the bracket for big block crank-driven pumps. The rest should interchange.

My 350 horizons had the air-drain system, but I was never a fan of it. Some of the valves would get stuck, and I found myself pulling the blue plugs anyway. To keep the air system, just plug off the ports that go to the lines to the pump, and the remaining drains will work just fine.

To drain the crank-driven pump, you would have to loosen the 4 screws on the impeller cover. Or, if you're crafty, put a drain fitting on the lower (intake?) hose. I'd leave the water in the pump, though, so you're not spinning it dry on each startup. The key is to get the seawater out of the manifolds/elbows/block (if rwc).

p.s. I believe the Merc pumps are the same between the 350 MAG and the 496's.
 
undercover said:
Here's what you need (double recipe for 2 engines):

1) Johnson F6B-9 Hi-volume pump ($150)
2) Raw water pump bracket - I used skidim.com part #49-5200 ($40) since it's adjustable, Marine Power 308711 ($35) will work as well
3) 3/8" billet SBC pulley spacer ($17 ebay)
4) 1/2" OD x 3/8" ID x 3/8" long bushings - qty 3 ($6 ebay)
5) 1 1/4" to 1 1/2" billet hose repair barb ($7 ebay)
6) 3/8" x 1 1/4" x 12" aluminum flat stock ($6 onlinemetals.com, or use stacked 3/8" washers)
7) 3/8-24 fine thread bolts 1.5" long, qty 3 - must have small head like allen head or CV axle-style 12 pt head (ebay $6)
8) about 3 ft of 1 1/4" marine water intake hose ($11)
9) Gates K060763 or equivalent serpentine belt ($16)
10) optional Mercruiser part# 862778 alternator bracket ($21) - I haven't done this yet
Click to expand...


Hey, I know this is an old thread, but I?m doing the same conversion with a closed cooled set up on my 311. I?m curious about the spacer. eBay shows a spacer for that price range with slots around the circumference. Got a picture by chance?

https://www.ebay.com/itm/BBC-SBC-Bi...p2349624.m46890.l6249&mkrid=711-127632-2357-0

Thanks for posting this up. Very helpful!
 
The one you linked is exactly the one I used. Iirc, I grinded (beveled) the outside edge so it fits flat inside the engine pulley. The engine pulley is sort of rounded on the inside edge, if that makes sense.
 
Yep, I understand it well. I replaced all of the pulleys after trying to clean them up from twenty years of use. I'll order the spacers today.

My understanding with this pump is that the flange uses the larger holes to match notches in the older GM stamped pulleys but our serpentine pulleys do not have the notches, and the pump only centers one way. Any other thoughts on alignment?

Thanks again for responding! Much appreciated!

Tom
 
Yeah, I think that's it. The aluminum spacer is required to put the pump out further so it doesn't scrape on the (deeper) serp pulley. The holes are bigger in the pump flange requiring the bushings. There shouldn't be much play when you bolt it up, but if there's a little wobble, just play with it too get spot on.
 
Does Volvo also offer a vdrive version like merc does? This would put the pumps way in the back, and might be part of the reason merc kept the pump in the same location they always have.
Merc also packaged it with the special pump drain system and is in a great spot to get the hose from the bravo transom assembly. These are the two immediate things that pop out. I don't think their first requirement was to make a pump that was very expensive to replace, though I'm sure they didn't mind that. Every previous water pump was in the same post also. I do wish the stuck with the 7.4 style cheap easy to rebuild pump. Either way, I just went with the hardin marine.
 
My 311 TRS boat was converted from 420 engines to Gen VI 502 MPI's. The pumps are much lower in the accessory drive compared to the 330/365/420 so it was already fun trying to get the standard Merc pump out. My new engines are carbureted with aluminum heads so closed cooling was a must-have requirement. With it all put together, there's no way to work with the pump without removing everything to get it out (heat exchanger, circulation hoses, coolant) hence my need to do the conversion.

My pumps came in yesterday, the spacer is due here today, and I'm picking up bushings at lunch time. Hoping to put it together tonight.

One thing that I need to figure out is the belt. Removing the Merc pump requires going straight to the alternator without a lot of contact on the circulation pump. I make need to figure out a different route. I don't want to leave the old pump in the accessory drive. Anyone with a big block have a solution for this?
 
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