Thermostat Housing Upgrade (???) Following EMI Thunder Exhaust Upgrade (1983 Merc 305 898R)

[WenghisKhan]

I've been lurking around for the past two years throughout a restoration of a 1983 Glassmaster Supreme (1983 Mercruiser Sterndrive 898R, 305 2bbl. Engine s/n: 6238904. 'R' Drive s/n: 6262040. Center rise 3" 15 degree exhaust, 140 degree thermostat).

BLUF: Question/sanity check request...Has anyone had successfully upgraded from the stock thermostat housing with check balls (p/n 87290A4) to the newer "Design III" (p/n 860256A3), for any reason? Relevant photos attached. I upgraded the exhaust from the stock center rise (3" 15 degree, wet joint) to the EMI Thunder (EMI-410, dry joint, 4" 7 degree?). For the coolant plumbing, the Y adapter supplied with the EMI exhaust just won't work. I'm swapping out the old check ball thermostat housing (p/n 87290A4) with the updated "Design III" housing specific for Alpha drives 1987 and onward (p/n 860256A3). Has anyone tried this, had success or failure? I'll swap over the 140 degree thermostat, however I did purchase the 160 degree thermostat to confirm fitment.

Backstory: Grandfather's boat bought new in 1982 and used exclusively in fresh water at Lake Anna (Va). Passed away in 2011, lost property/boat. I randomly came across the boat at a Marina in 2020 and purchased it. It was in great condition, but I have been restoring it.

I purchased the EMI-410 exhaust when I didn't know what I was doing. I assumed I'd be able to make it work...essentially converting from the Merc wet joint 3" 15 degree to the Merc 4" 7 degree. I could not find an intermediate elbow (riser to exhaust Y-pipe) to work with the stock 3" round Y-pipe, so I pulled the engine again and updated the Y-Pipe to the 4" oval version (p/n 807130A4).

EMI supplied 1" Y adapter (2 into 1) to mod the coolant flow. Stock design flowed hot water from thermostat to the manifolds, bypass/excess water through check valves to riser elbows. The Y adapter combines these and flows directly to the manifolds. The adapter just won't work, and will look like ass, so I invested in a used "Design III" thermostat housing (p/n 860256A3) with sleeve and intend on using the 140 degree thermostat. Like I said earlier, I purchased a 160 degree thermostat as that is what is called for with the new housing, however I am just using it to check size/fitment against the 140...hopefully confirming they have the same dimensions.

I've compared the schematics between the 898R and the 1987 Merc 200 that uses the updated housing. Most part numbers involved in the connections are the same (PS cooler, water pump), some are different, and new hoses were ordered. I don't see a problem with this set up, but haven't been able to confirm anyone who actually tried it. The old design bypasses the thermostat via the check balls to supply the risers with water, and also to bleed off excess pressure. The new design utilizes cutouts in the thermostat sleeve to do this, in combination with smaller diameter ports to restrict water and keep the engine coolant passages full (to some degree).

Since it's a carbureted engine (and the manual calls for it), I'm sticking with the 140 degree thermo. I expect to see temps in the 140-160 range at the thermostat housing, and the manifolds should be within 15 degrees of each other (all via laser thermometer).

Any input/experience would be appreciated. Attached photos have the overall design change schematic from EMI (stock plumbing on left, Y adapter mod on right), photo of the Y adapter in the real world and how it doesn't match up without using excessive amounts of hose, and photos of the original housing with the check balls (upper right), with photos of the housing I purchased (middle) and a stock photo of a new unit (bottom right).

Side note....I have tried to find a different Y (or "wye") adapter / T-fitting. It would need to be 1" all around, with one of the Y legs coming off at a 30 degree angle to match up with the thermostat outputs (basically a 'T' with a 30 degree leg vs 90 degree). I haven't been able to locate anything of the sort, which was my first option before replacing the housing.

Thanks again, I will continue to lurk. Thanks to everyone for the help you have provided over the years without even knowing it.

 

[alldodge]

Shifting to design 3 housing should be straight forward without using the Y-pipe supplied. It might be needed if the old Stat is being used but I would switch

 

[Scott Danforth]

agree on swapping T-stat housing.

regarding the T-stat - generally
140 degree thermostat is for raw water cooled carb motors (keeps salt-water erosion at bay)
160 degree thermostat is for raw water cooled EFI motors (because EFI motors dont like temps below 160)
160 degree thermostat is for closed cooled carb motors
180 degree thermostat is for closed cooled EFI motors.

if you are in fresh water only, you can run the 160 degree T-stat with a carb.

 

[WenghisKhan]

Ladies and gents....yes, you too can go from this (left pic) to this (right pic). Why would you want to do this? No idea. I purchased the EMI Thunder Exhaust almost two years ago before the fiberglass work began, so once I realized what was involved it wasn't feasible to try and return it. TBH, the Merc dry joint manifolds are almost as expensive as the EMI...but the EMI look better, are considerably lighter, and easier to install. The key is to upgrade to dry joint vs wet joint, however you see fit.

If anyone is looking at upgrading to dry joint and/or the 1987+ thermostat housing design, here is a quick rundown of the "new" parts you will need that wont transfer over. Alternatively, you can cross reference schematics with your set up (1983 898R for me) against the "updated" setup (1987 200 2BBL) like I did.
Exhaust System: You are changing your 3" exhaust riser to 4". There is no off the shelf option to connect it to your 3" Y-pipe. Unless you are able to locate or fab a 4" to 3" ~70 degree elbow, you will need to replace the Y-pipe to the 4" oval type.

• Exhaust elbows. 11086T (port), 14343T (stbd). I used stbd for both sides as it just seemed to line up better.​
• Y-Pipe. 807130A4. I used 3 of the 4 bolts to fasten it to my transom plate. Your results may vary depending on the plate iteration you have. You do NOT have to pull the engine from the boat to replace the Y-pipe...just lift it with a hoist and push it forward a bit.​
• Exhaust shutters. 807166A1.​

Cooling System: The dry joint exhaust no longer requires coolant hose direct from the thermostat, so that hose is redirected to the manifold. EMI provides a Y-fitting to do this...it ain't gonna work, it will look like complete ****. Updating the housing is the cleanest option.

• Thermostat Housing. 860256A3. It should say "ALPHA" on top b/w the two bolts.
• Thermostat....140 or 160 degrees, use whatever is on your boat or what the manual calls for. They are the same size and interchangeable (I kept it at 140).
• Thermostat Sleeve. 23-806922.
• Thermostat Gasket (cork). 8M0204687. This gasket had a different part number than what was listed for the older thermostat housing, but it is exactly the same.
• Housing Bolts. 8M0113179. Bolts the housing to the intake manifold, 1.5" shorter than what is used in the old housing.
• Hoses. 32-8066361 (housing to stbd manifold). 32-8066351 (housing to port manifold). 32-806856 (housing to PS cooler). I would recommend against trying to use bulk 1" heater hose to connect the housing to the manifolds. The Merc molded hoses have 1" flares at the ends but are about 3/4" inside diameter. From what I understand, this is part of the design to restrict water to the block.

That's about it. To the one other person out there trying to do this, hope this helps.