1990 vs 1996 9.9hp 4 stroke cylinder head/ gasket interchange?

[retroroy]

I have a minty clean 1990 9.9hp Yamaha four stroke motor, in a long shaft model, recoil start. That someone destroyed the cylinder head on. The guy broke off the sparke plugs, then proceeded to drill out the heads and install two 18mm Motorcraft spark plugs so his outboard and truck could share spark plugs. To make it short, he drilled into the back of both valves doing so and it never ran again. It sat in his basement like that since April of 1990, two weeks after he bought it new.
FF to 2026, I got the motor off his widow for $20. Its still got the hang tags on it from the dealer.
The dealer tells me no parts are available for it. I have the owners manual and it calls for the following,
FT9.9ELD 1990
6G8-11181-A0-00 head gskt
6G8-11111-02-94 head
Now, I also have a 1996 Yamaha/Mariner that was hit by a car. Its cylinder head is perfect, it was hit when it was new. It severed the mid above the lower mounts. It was given to me by a local dealer who closed down about 10 years ago, it was hit while on the lot when new.
I pulled the power head figuring one cay I'd find a good lower for it.

I see no way to cross over the part numbers from Mercury to Yamaha, but the aftermarket listings show confilicting information. Apparently the power head won't swap but the head may?
Again I'm being told that the head on the 1990 Yamaha is a 1988-1994 year span,
I have a 2001 here, and that head is very different, but is the 1996 mariner and the 1990 Yamaha the same head? The dealer says no, aftermarket listings show two different gaskets. I have both in hand,
6G8-11181-A0-00
6G8-11181-A1-00
Both gaskets in hand look identical, they're the same thickness and have the same size holes.
Five dealers told me that the two motors are not at all compatible but they look identical other than color. One dealer swore up and down that the power head was Mercury built.
What years were the same when it came to the power heads with Yamaha?
The Mariner head will bolt up, the head gaskets look identical. The pistons look identical, as does the entire rest of the motor. (Even the hoods will swap, they appear to use the same pan) but the lower units are obviously different.

 

[99yam40]

heck if you have both ,why not try it to see if it works OK

 

[matt167]

if the bolt holes are the same, gaskets line up and everything and everything is in the same spots than functionally identical. The only things that are going to possibly be different are exhaust and intake might possibly be different, but you have both and valve sizes/ combustion chamber dimensions might be different. I would only use premium fuel as you won’t know the compression ratio unless you calculate it, and the quench and burn profile might not be perfect. Premium fuel should negate both possibilities

 

[retroroy]

I have both gaskets, but one is $19, the other one is $107. They both look identical.
When I asked someone from Yamaha, they said No way, completely different motors, but they look identical other than the paint color. The newer motor is black, the older one blue/gray.
I had put them both aside a while ago after being told that the motor families for the head swap were '88-94, 95-99, and 01-06.
I can see that in 01 the hemi head design went away and the valves were inline not canted from either side. Just about every part for the 1990 motor is NLA, as are many parts for the 1996.

The valves are the same size and have the same lift. I can fluid check the combustion chambers but the gaping holes drilled in the old head kind of ruin that idea. They literally drilled the hole so big they drilled into the valves.
I seriously have my doubts if the motor every had been run at all. the prop, skeg, and all the paint is perfect, the same on both motors. The only difference I see so far is that the older motor uses a 12mm head on the headbolts and the newer one has 10mm bolt heads, both use 8mm bolts. both carbs are the same, same intake, fuel pump, wiring harness, tiller handle etc. just paint and bolt heads are different.
I can't figure why multiple sources said they were different. If I hadn't asked before ending up with the parts motor, I'd have just assumed that the motor family ran from 1985-99. There's a few videos of guys doing head gaskets on Youtube, the power head looks the same up to '99.
There was a third headgasket that was listed with a completely different style p/n that they all had in stock and swore was what the later motor should have but I can find no sign of any such motor in the wild.

Going over the spec sheets on both motors this is what I found.

1990 = 9.8:1 comp. ratio and a 59x59mm bore/stroke at 323cc
6G8-11181-A0-00 head gasket
1996 = 9.3:1 comp. ratio and a 59x42.4mm bore/stroke at 232cc
6A1-11181-A1-00 head gasket
old p/n's:
646-11181-01-00, 6A1-11181-A1-00, 6A1-11181-A1-00 The gasket specs got thinner from the original gasket p/n by roughly .002"

It looks to me like the newer motor is the same bore and stroke as the 8hp, while the older motor had a longer stroke shared with the 15hp.

The 1996 has a larger carburetor bore by 3mm
The 1996 has the same cam specs as a 1990 F15hp.
Both appear to share the same valves and combustion chamber size.
It sort of appears that by using the newer 1996 head and carb, I may end up with a 1990 15hp if there are no exhaust differences, but it appears that both are the same that way but without pulling the older power head I won't know for sure how the lower outlets is made.
There doesn't appear to be any way to tell if the head casting itself is the same or not, but it appears that it is. The parts lists show only a complete head for 1996, but shows both a complete or a bare head for the 1990.
None of which are still good numbers. The markings on each head are totally different, not even close number wise. Nothing at all that says Yamaha or any mfg codes. Just cylinder markings and cam timing marks.
It also looks like every last number on the 1990 was superceded more than once.
The crank and pistons are the only two parts that appear different but they show different head gaskets
Both use the same bore, same exhaust ports, cooling ports, thickness, and fire ring diameter
I did some digging and think I have the P/N system figured out
For an example.
6G8-11181-A0-00
6GB is likely the model it fits or was designed to fit
11181 appears to be the descriptor, as in what kind of part it is,
A0 seems to be the revision or design number
00 is likely color, size, etc.
If this is correct, the first three numbers don't denote anything other than it fits a particular motor, the next three likely just say its a gasket, or maybe head gasket, the next two seem to be a version number, with every change increasing by '1'. the last is likely color, shape, or size if it applies.
It seems to hold true with marine and bike parts.

I have to think about it a bit but I'm leaning toward using the complete newer head for the added lift, if it don't create an issue, and I don't see why it would since the same lift is used on the 15hp motor, plus the larger carb, and then set it up otherwise as a 15hp and it may end up better than it was before.

The only other alternative is to find another early 9.9hp head from a 323cc 9.9hp but if it works, it looks like it could be a good combination.

 

[boscoe99]

Even though the back of the valves are damaged is the damage such that the valves are no longer usable?

 

[boscoe99]

The original head gasket was part number 6G8-11181-A0-00. Replaced by part number 6G8-11181-A1-00.

In stock and available from any Yamaha USA dealer.

 

[boscoe99]

What is the serial number of the Mariner motor?

Look for any casting numbers/lettes that may be on the cylinder head. If you find any, what are they?

 

[boscoe99]

Yamaha made motors or power plants for Mercury/Mariner for a number of years.

See below. What do you think?