So Close!...
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Cheers guys, I appreciate all the help, food for thought!
God knows I need it!! 
The radiator (& cap) was behaving like it should. Holding fluid right up to the neck every time I checked it, and believe me I checked it a lot! Expanding and contracting like it should. It wouldn’t vent through the expansion tank if the cap wasn’t venting pressure. It also wouldn’t have had a full rad if it wasn’t contracting and returning the coolant when it cooled. Also the outer seal on the caps is good otherwise it’d leak from the cap and wouldn’t make the Schweppes noise when I open it hot.
I really appreciate any ideas, I just don’t think the caps to blame. It won’t stop me putting on a new one if I can find a genuine new one though
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@SpookDog but the cap isn't supposed to vent pressure. It's supposed to hold in the pressure. The expansion tank is only used in cases of over heating. If as you say the cap is venting pressure then something is not right there. The coolant is pressurised as it boils at a much lower temperature when it's under pressure. I even more so think it's the cap now. Try a new cap and make sure it's the correct pressure rating.
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I don’t think that’s right bud. It expands and contracts to aid cooling. No manufacturers would waste money on an overheating tank. An overflow pipe maybe...
There are two rubber seals. One to make the radiator airtight, the other inner one allows expanding fluid to pass back and forth...
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@markus-w said in So Close!...:
@SpookDog but the cap isn't supposed to vent pressure. It's supposed to hold in the pressure. The expansion tank is only used in cases of over heating. If as you say the cap is venting pressure then something is not right there. The coolant is pressurised as it boils at a much lower temperature when it's under pressure. I even more so think it's the cap now. Try a new cap and make sure it's the correct pressure rating.
PV=nRTCoolant boils at a higher temperature under pressure.As atmospheric pressure decreases, water boils at lower temperatures. At sea level, water boils at 212 °F. With each 500-feet increase in elevation, the boiling point of water is lowered by just under 1 °F. At 7,500 feet, for example, water boils at about 198 °F.Therefore as the pressure inside the coolant system increases, so does the boiling temperature.
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@SpookDog I know that bud but they are set so they open at a certain pressure and different caps are set to different pressures due to different strength springs (dt125r cap should open at 12.8psi with a tolerance of +/-1.5psi) and if it's old and worn and opening too soon/late then it won't regulate the temperature correctly. Springs tend to weaken as they age. It's surely a coolant issue you have. I may well be wrong but given the trouble you've had a £15 cap is surely worth a try?
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@markus-w
Hey! Don’t get me wrong, I appreciate all and any ideas. I’ve got 3 old, original caps. I’m definitely gonna find an New original one just to be sure that I cross i’s and dot the t. But I’m not sold on it being the cause.
I’m doubting my own workmanship more at the moment... -
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One thing that I’ve never done is re~torque the head after riding the bike up to temp. I’ve just put a ring spanner on the head nuts and given them a tweak. It was surprisingly easy to tighten them (Very surprisingly easy!!) Looking back it was never hard to undo them when tearing down the top end. I haven’t seen anything about re~torquing in the Haynes. Only ever seen it recommended with car head gaskets that are those silver multi layered ‘compressible’ stuff...
Does everyone else retorque after running up to temp?...
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@SpookDog haha my suggestion to you, hopefully it works , I know it is really a car thing but in theory heated metal expands especially alloys, be nice to know what the torque wrench says with it heated vs what It said unheated although hard to get one in a built bike
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@MH-Bikes-N-Bits using a 1/4" drive torque wrench (£20-£30 from a cycle shop) and 1/4" drive universal joint and 12mm socket you can get to all the head nuts with the engine in situ, even the one at the front which is close to the frame.
Alternatively you can remove all the engine mount bolts apart from the swingarm pivot and rotate the whole engine around it a few degrees to improve access (I did this once to replace the O-ring in the steel water pipe without disturbing the waterpump cover as I had reason to believe one of the cover bolts was seized)
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That’s been circling around my noggin for a while. There was a thread about someone getting it done at PJME but I never saw any follow up. Interested in knowing the price. I’ve got a 4fu barrel I wouldn’t mind getting sleeved by them as well...
@ mikes bits
I did remember someone saying about re-torquing the head Is it after it’s been run to temp then cooled down? I wouldn’t do it while hot!...@ hotshot
I’ve never done it before, but the nuts were not super tight, so it got me wondering...I remove the bottom and front mount bolts, loosen the swing arm bolt and lift the front of the engine with a big screwdriver. Once you’ve taken off the thermostat cover you can get in there nicely with a 1/4 drive torque wrench
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@SpookDog said in So Close!...:
One thing that I’ve never done is re~torque the head after riding the bike up to temp. I’ve just put a ring spanner on the head nuts and given them a tweak. It was surprisingly easy to tighten them (Very surprisingly easy!!) Looking back it was never hard to undo them when tearing down the top end. I haven’t seen anything about re~torquing in the Haynes. Only ever seen it recommended with car head gaskets that are those silver multi layered ‘compressible’ stuff...
Does everyone else retorque after running up to temp?...
After about little run in on the first km's I check every bolt and re-torque the head and cylinder.Almost every time was ok,one time one headbolt need it a little more but nothing serious.
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@MadGyver
Cheers geez, it’s all good to know. I didn’t think it was absolutely necessary, but there you go. No two situations are exactly the same...@anyone
I’ve gotten myself a new piece of plate glass (6mm was thickest I can get locally) and loads of fresh w&d from 80grit on through to 600. I’m gonna have one last go and give the head (especially) and barrel a good lapping. If it fails after this I’m gonna get a good 3mb head cnc grooved for orings (is it one or two on a standard head? I still want to know why it’s good for about a 1000 miles before it fails. I would imagine it’d fail sooner if it was poor finishing of the mating surfaces? But, hey ho! One problem at a time! I will get both surfaces skimmed if I can find a good local engineer that hasn’t lost practice cause of Covid
I still want to build another bottom end when I can afford genuine seals and bearings all through. I’ll never buy non genuine fleabay again, that’s for sure. I prefer my Crap Shoots to happen in a Casino!
...Good luck to the people’s with problems, love to all else!
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Just took of the head and the gasket failed at the back of the head, passing pressure to the big coolant opening on the gasket (the others are just 4mm holes) at the mounting point. It’s failed there a few times. I don’t have an original bolt at the front engine mount point, just an Allen bolt. I’m wondering if it’s putting stress on the head mount. Or if it’s just the easy/biggest target?...
