What does the cooling bypass pipe - bypass ?
- Thread starter aSTerix
- Start date
The cooling system consists basically of the pump, the radiator and the cooling channels within the cylinder/head castings. Coolant is pumped around these to pickup heat from the combustion and release it to the air via the radiator. The thermostat is in this circuit to control the coolant temperature. When the coolant is cold, the thermostat remains closed so coolant does not travel via the radiator; instead it is circulated through the bypass hose directly back to the coolant channels in the engine, so that the coolant and engine come up to operating temperature more quickly. Once the coolant in the engine block is sufficiently hot, the thermostat starts to progressively open to allow more of the flow to the radiator, which then will prevent the temperature rising further provided there is a cooling airflow.
Aha ! Now I can understand why my bike was able to function with a totally blocked bypass pipe, it didnt have a thermostat fitted . Wonder what would have happened with a blocked bypass pipe and a thermostat fitted , I guess no flow at all until the coolant got very hot (probably very quickly )
I think the OP was asking about the small 5.5mm diameter "bypass tube" (#4 on picture below). I don't think anybody has ever figured out what it does, or at least I've never seen any explanation for it here over the years.
According to the Honda manual coolant flow diagram, nothing gets bypassed when the stat is closed, the only "out" path from the stat housing is when the stat finally opens and coolant flows to the radiator. The bypass tube flows in the same direction as the main coolant hoses from the engine V. So, unless the diagram in the manual is wrong, its still a bit of a mystery as to what the bypass tube does.
According to the Honda manual coolant flow diagram, nothing gets bypassed when the stat is closed, the only "out" path from the stat housing is when the stat finally opens and coolant flows to the radiator. The bypass tube flows in the same direction as the main coolant hoses from the engine V. So, unless the diagram in the manual is wrong, its still a bit of a mystery as to what the bypass tube does.
ST1100Y
Site Supporter
I'd say the bypass provides minimum flow for the waterpump in order to prevent cavitation while the thermostat is still closed...
The thermostat plate again has a seep-hole to ensure some circulation, so it can actually detect the raise of temperature during warming up...
The thermostat plate again has a seep-hole to ensure some circulation, so it can actually detect the raise of temperature during warming up...
BINGO!
On the 1300, the bypass hose exits the high pressure side of the thermostat housing and connects to the suction side of the pump. The pump drives the flow to the cylinder heads and from there it is back to the thermostat. So the bypass is absolutely the only way for coolant to be circulated during warm up except for any incidental leakage around the thermostat. I suspect the 1100 is the same arrangement but the diagram in the manual shows an incorrect flow direction in the bypass hose.
The ones describing it as a way for some circulation to take place are correct. The T-stat keeps cold coolant from leaving the engine, but it should still circulate so the engine heats up more evenly, without hot-spots developing nearest the heads.
Back when I was a teenager, I drove my parents' 1967 Chevy Caprice with a 396 engine. There was a bypass hose on top of the pump, to the engine below the T-stat, a rubber elbow about 1/2" ID. It burst once, so I became very familiar with it.
Back when I was a teenager, I drove my parents' 1967 Chevy Caprice with a 396 engine. There was a bypass hose on top of the pump, to the engine below the T-stat, a rubber elbow about 1/2" ID. It burst once, so I became very familiar with it.
My bypass hose just split and it was pretty brittle (right where it comes out of the Tstat housing. It looks like you have to tear everything right down to the water pump to replace it, right?
Smudgemo
Intermodal Man of Mystery
No. I took a look at some of my photos of work that I did on my projects, I'm and not sure, but you'll need pull the shelter and the air cleaner housing to get at things. Then you may have enough room to get in there and slip on a new hose on the nipple. You'd be pulling back the rubber mat to access it. Maybe you can get at it with a hemostat or long pliers if you can't get your fingers in there. Hopefully you can get at the thermostat side with just the access cover off. Mine was stripped down, so I don't know how feasible it is, but you can always pull off more plastic as needed.
But maybe it's a sign that it's time to do all the hoses and gaskets. It's not terribly complicated, but you'd then want a few new rubber parts for the carbs/fuel while you're in there.
But maybe it's a sign that it's time to do all the hoses and gaskets. It's not terribly complicated, but you'd then want a few new rubber parts for the carbs/fuel while you're in there.
Smudgemo
Intermodal Man of Mystery
You can see it here just behind the left timing cover on the main water pipe. Access seems kinda iffy with the carbs in place, but you can start pulling things apart and stop when you get enough room. If you have to pull the carbs to get at it, then for sure I'd do all the hoses/rubber if they are aged.
Attachments
agree with the answer of 'no' above, it doesn't attach to the water pump at all. It attaches to a vertical metal pipe underneath the carbs. I don't know if anyone has replaced it with the carbs in place. If you aren't sure of the condition of the hoses down under the carbs in the V, then its worth pulling the carbs and replacing everything at once and the bypass hose will be a simple connection while the carbs are out.