Are Sierra thermostats OK?

[dan-in-tn]

I looked into your prices on Amazon. I don’t know what is going on, but BRP doesn’t make that brass thermostat anymore. (Since 1993 I believe) #5005440 is not a brass stat! You are buying those parts from dealers that are giving Amazon a cut of the action. (Not the best way to get the best price, usually!) If you look down at the third option you will see a stainless stat with seal. Happens to be the Sierra #18-3543 I believe. Only the body of the stat is stainless. The pellet on both stats are brass, either coated or not. The stainless stats are far superior to the brass ones in fresh water or salt. MarineEngine.com had the best price on these SS stats that I noticed. I did not look on Ebay.

Dan in TN

[frankr]

Help me to understand something. As you know, motors such as the 6hp under discussion here have two water flow paths through the powerhead.

The first one is through the exhaust cover, bypasses the thermostat, and exits down the back side of the cylinder head, and finally out the exhaust housing. This flow path is not thermostat controlled and is necessary to keep the exhaust system cool.

The second flow path is around the cylinders and outer edge of the cylinder head, then looks at the pellet in the thermostat. If warm enough it goes out with the first path water exit.

So, my question is if there is no bleed hole in the thermostat, isn’t there danger of an air lock around the cylinders? And if the flow is totally shut off by the lack of a bleed hole, how does the water even get to the pellet in the thermostat for sampling?

My inquiring mind needs to know.

[PugetSoundBoater]

quote FrankR:

Help me to understand something. As you know, motors such as the 6hp under discussion here have two water flow paths through the powerhead.

The first one is through the exhaust cover, bypasses the thermostat, and exits down the back side of the cylinder head, and finally out the exhaust housing. This flow path is not thermostat controlled and is necessary to keep the exhaust system cool.

The second flow path is around the cylinders and outer edge of the cylinder head, then looks at the pellet in the thermostat. If warm enough it goes out with the first path water exit.

So, my question is if there is no bleed hole in the thermostat, isn’t there danger of an air lock around the cylinders? And if the flow is totally shut off by the lack of a bleed hole, how does the water even get to the pellet in the thermostat for sampling?

My inquiring mind needs to know.

I have also thought about the same issue ,how the air gets displaced by water at the top of the motor with no bypass hole.The bypass hole is a rather small opening,would rather see this a bit bigger passage to get the water up there more quickly. Thinking about slightly enlarging the hole ,want that air outta there.

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[amuller]

I am not expert on this, but when a carburetted 2-stroke engine is idling, or trolling, there are big opportunities for mixture dropout in the crankcase, etc. From this point of view, keeping the powerhead warm could make a big difference in low speed performance.

Multi-cylinder carburetted 4-stroke engines need manifold heat for similar reasons.

[dan-in-tn]

FrankR & pugentsoundboater. That is the way the water flows as I understand it on the 6hp. I just finished talking with Pappy & we spent some time looking in the service manuel to refresh our memory. We are pretty sure the water comes up on the starboard side of the powerhead thru the round tube up the side of the block. It then passes over the top of the block to the exhaust manifold. Water travels down the slot in the inner exhaust plate to enter the bottom of the cylinder head/cylinder jugs. After warming up the pellet in the thermostat to 145 (optimum) the thermostat opens and dumps the water down the center of the cylinder head out the midsection. The old (brass thermostats) had a cut vee in the top hat. You can see this by looking at a 18-3553 Sierra thermostat on ME. The newer BRP SS stat does not have a vent! We remember a small vent in the block, but could not see it in the manuel. If you were going to modify anything the smaller the hole the better. Any short circuit of the system can cause the motor to run cold thus defeating the whole reason for a thermostat. It has been a long time since I have studied the cooling system on this motor, but I have put many new style stats in them recently. No problems. Remember you just need to bleed the air in the system off not water.

Dan in TN

[frankr]

Ar-r-r-g-g-h. That sounds back-asswards to me. That does it, I have a crusty old salt water 6hp out in the shop that is coming apart this evening for inspection. Stand by for the news

[Casey Lynn]

Nope…not bass ackwards. He was 90% correct. Directly from the flow diagram in the service manuals.
The water enters the block on the forward side of the exhaust and flows up and through the exhaust passages. There is also a small passage that places some water on the top of the thermostat which would then flow down the back of the cylinder head independent of the thermostat. Forget that part for now although this would be why I could not see the relief in the casting I was looking for in lieu of the hole in the thermostat. This small passage would serve the same purpose as a hole in the thermostat. Okay….back on to the path of water flow.
The water, once it has flowed through the exhaust enters the cylinder jackets from the bottom and flows upward, around, and to the bottom of the thermostat. Once the thermostat opens the water then flows through it and then down the back side of the cylinder head through its passages and exits the block.
This particular engine is one of the few OMC engines ever designed to run totally on the thermostat for temperature control. This makes it even more important to have it installed and in place. I was off by 10F on the temp last night. This particular thermo is fully open by 145f. For this reason I would not substitute……….
As the thermostat opens and flows water down the back side of the cylinder head more high flow raw water from the pump is utilized to keep the block full and pulling heat away.
On a fully thermostatically controlled engine the recommendation for a washer with "The right sized hole" could very well spell disaster at high RPM and load.
I have emphasized in the past that if you are not very sure that what you are writing is fact….just don’t write it.
There are folks in here that are new and just learning and we would like them to learn correct service procedures and tips vs. theory and wives tales. Just my .02 but I will darn sure stick by it.

[frankr]

Non-thermostat controlled

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Closeup Non-controlled, Water completely bypasses thermostat.

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Thermostat controlled

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Notch in cap. (does this "leak" enough to purge the air from the cylinder jackets?)

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[Casey Lynn]

Exactly as stated. Like your trimmer string though! Would have been easier to just crack open the manual !

[frankr]

But I still go back to my original question. If the ‘stat does not have a notched poppet, how can the water flow up from the cavity that it sits in, and if it can’t flow, how does it present itself to the pellet? Unless the gap in the cap is supposed to leak enough to let some through. Seems iffy to me. I still think it needs a notched poppet.

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