TOPIC: 1965 Mercury 500 vs 1961 Mercury 400

I would appreciate some advice:
I have a Thompson Thomboy 14ft wood boat. There is no HP rating on it as it is from the early 50's. Other owners of the same boat have 35hp motors on them. I have been searching for a short shaft motor for this boat for the last year with little success. I now have found 2, a 1961 Mercury owned by a Mercury collector. He has had it for a long time (20 years?) and never ran it. Says he wouldn't have kept it if it wasn't a good motor. No compression test, no controls. It is a clean motor. No prop.
I spent an hour talking to that gentleman, and he is a very nice guy. I get the feeling he isn't in the business of screwing people out of $400 for an old motor, but tons of unknowns with this one. That motor is about 10 minutes from me.
The other motor I found is a 1965 500. It is complete, and the seller said he hooked it up to a battery and it started right up. He says the wiring harness is shot. That one has the prop and controls.
He is at the same price, but offering to buy it back if I get it and find out it's junk. He did a compression test on the easy three cylinders, all at 115, added some oil and got 120.
My gut tells me to get the 500 (more hp, can hear it run, has the prop, etc)
I don't know enough about these old Mercury motors to know what I'm doing, and wanted to find out if one is generally better than the other, anything to watch for/stay away from etc.
By the way, both of these motors weigh less than the 35 hp I've seen on the other Thomboy, and I'm told by the wood boat shop near me that the transom will easily take either of these Mercury motors.
Thanks for your help,
Dave

Dave, the 500 has a better-designed lower unit that is much easier to service than the '61 400. Besides having flow-thru exhaust (goes thru the hub of the prop), which is more efficient and will give better top-end and economy than the 400.

Be sure to pull the lower unit drain plug on any outboard you inspect. If the oil is milky or full of water, you're in for a lot of work and $$. A motor that's not run in 20 years is definitely gonna need some maintenance, at minimum new impeller and go thru the carbs and fuel pump. So there's some more expense. Likely it'll have no spark after sitting so long, so you'll have to service the magneto as well.

Controls will be rather expensive for the '61, this is offset a bit by the 500 needing a new internal harness. There are a number of members here with parts so you may be able to find a decent deal on a good usable harness.

CDI Electronics makes a new harness that cross-references to a '65 50hp. P/N 414-2770. I found on eBay a good used one for $113 and a new one for $133.

The 400 has more of a "classic" look than the later motor. So there's that. Depends on what you're looking for. But you have to consider the cost of servicing either one; if you're not going to do the work yourself, it'll add up quick.

HTH & G'luck with whichever you pick...........ed

I agree the 500 is a better deal and parts are more available and easier to work on. the mag is the same.

I bought the 500. I was able to hear it run, and it sounds really good. The motor is very clean. The wiring in the motor looks new, as do the gas lines. The wiring harness that is shot is the external one that plugs into the motor. He gave me a better (used) one. I’ll replace the lower unit gear oil and impeller. I’ll work on getting it all set up this weekend and see how it goes. Thank you for the advice.

Sounds like you lucked out on that one! Good news that you got a decent wiring harness. These are really great motors when you get one that's in good nick. Have fun on the water and let us know how it goes......ed

you can make a new external harness if you need.

So I ordered a new impeller kit, and it came yesterday. I started working on it today. First thing I did was drain the lower unit oil. Which wasn't oil...lots of water. I drained that out and flushed with some lower unit oil, drained, and filled. I wanted to make sure I got all of the water out, so I filled until the oil came of the fill level plug, plug that, and filled until it came out of the vent (top) plug. I put the vent plug back in, and noticed black oil dripped from the prop hub. Then I noticed the new gear oil coming out of the water drain holes. That was running pretty quick.
I assume I need a new prop spindle seal. But why the oil out of the water drain holes? Is that because I overfilled it?
I suspect the lower unit is cracked internally from freezing (I am in WI) and that's why its leaking.
I do have a money back guarantee on the motor, so if it is cracked, I'm ok. But I don't want to condemn it because it just needs maintenance. I appreciate the advice...

sounds like you need a reseal job on the lower. prop shaft seal water pump drive shaft seal and shift shaft seal.

What Dave said. Most likely your driveshaft seals are leaking. Very common on older models to see the steel shafts erode in the area where the seals ride. If this was a freshwater motor, it may just be a matter of pulling it down and replacing the seals, which are pressed into the water pump base. You'll need a few gaskets and O-ring, besides the seals, to do this job.
Also a good time to renew the impeller.

If you find the seal area on the shaft is eaten up, you can repair it using a stainless steel "Speedi Sleeve". Also marketed as a "Redi-Sleeve" and a few others brands, IIRC. What you do is clean up the area real good, then the repair sleeve is tapped down the shaft to cover the bad area. The sleeve has a flared section which is perforated so it can be torn off after the sleeve is driven in place. The kit has a aluminum-tube driver that seats against the flared part of the sleeve. We can go into this in more detail if you end up needing to do the work.

You should also check your shift shaft seal for leakage. The shift shaft seal/bushing assembly is slotted and can be unscrewed with a special tool. Or make your own out of a piece of steel pipe. Don't pull out the shift shaft at all when working on this, that will open up another can of worms!

If you've never pulled one of these down before, I'd highly recommend studying this online manual before proceeding:

boatinfo.no/lib/mercury/manuals/mercury_1965-1989_40-115.html#/4

Last thought, yeah it has a guarantee and you can take it back, but it's a runner and this is a very common problem on old Mercs. So if you did return it, it's likely the next one you find might be in even worse shape and require more work. Worst case you could probably find another lower unit, lots of them out there. But I doubt the casing is cracked, I've never seen one cracked at the top; if they crack from freeze-damage, it's at the rear end of the gearcase, the open end that's the weakest point in the casting. Also water in the drain cavity up front, can freeze and bust out the case. But that's not an oil-containing part. The fact oil is dripping out the water intake ports is a clear sign of bad driveshaft seals.

HTH..........ed

I have a good lower unit for it if needed.