So Close!...
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@SpookDog Crazy your gasket problems as done loads of engines and every head gasket from cheap white ones in those cheapo full gasket sets to athena gaskets and never had a problem. On barrels always use blowtorch on studs and 40 years working on 2 stroke engines and never snapped a stud yet. Remove studs then piece of glass with 240 wet & dry on barrel and head and 100% flat, I use my cooker hob as it glass really strong and it
doesn,t move about, just do mine when the wife is out.lol -
That’s the stuff! Old mirrors can be good as well
I always use heat for stud removal as well. The only time I didn’t I snapped a stud, it had only been in for 6 weeks and not even tight! In hindsight it was too new and shiny (came with a head and barrel I brought) and didn’t have the dimple on the top. Was a cheap fleabay copy I think...
I’ll have to try 240 drift. I can’t remember what I used before. A nice even Matt finish and a check with a steel rule is my usual MO. Head and barrel. The barrel base is more awkward but I use a square glass drinks coaster...
My latest iteration is still hold up. It’s been nearly a 1000 miles now
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I know, but I can’t figure it out! Not to blow my own trumpet, but I don’t think I’m doing anything wrong with my methodology or workmanship. It’s consistently lasting for weeks after fitting (about 1000 miles this time) If it was my idiosy it would fail straight away, not last 100’s of miles of normal use!
I’ve tried 3 or more different head & barrel variations. So it’s not the barrel or head to blame!
Tried different makes of gasket.
Ditto base gaskets for the barrel.
Different coolant, pure water combinations. Different 2stroke oils, ect. Just in case one of them was reacting with the gasket coating!
Different carbs. Different jetting combinations!...The only thing that is constant is the exhaust. I’m thinking this because there is always a rattling~knocking noise when I’m idling along at very low (but not closed) throttle in any gear. I thought it was piston slap at one point in time, but it’s done it with various barrel and pistons. It might be pre/miss~firing maybe? (I heard about a similar rough/flat spot that cleared up with an exhaust change) It kinda ties into a problem I’ve always had with a low rev stumble/flat-spot that happens when I crack the throttle slightly, that I always took to be a jetting issue that I could never resolve...
I’m really fuckin clutching at straws here, I know! I just can’t imagine what else could be causing this. I’m even thinking about building another bottom end
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What is your squish gap?
Is it easy to fit the cylinder head engine to frame mount bolts, or are they under some strain?
Are you using some form of stock jetting?
What are your exact symptoms of head gasket failing? Pushing water out the expansion bottle? Temps raising into red?
Condition of radiator/cap? -
@SpookDog have you checked the radiator pressure cap? Is it faulty or is it the correct one? They have different pressure settings and they are vital to keeping the correct temperature. My dad once spent over £500 on parts he didn't need for his over heating car then found out it was a failed pressure cap causing it.
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@Calum it sure does get overlooked. I think most people fail to realise the importance of this part and the problems it can cause. Obviously the correct term is pressure cap, as it regulates the coolant pressure which in turn regulates the coolant temperature.
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The head to frame mount is good.
The radiator cap is good. Temp is now excellent until the radiator stars emptying out of the expansion tank
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Squish varies from head to head (I’m on my fourth different head&barrel) but none are altered from factory.
On all the gaskets that have failed it’s been the black coating. It goes soft at certain spots. Every time but once it has always failed between the head and the gasket. The seal between the gasket and barrel has been tight and dry, which I think is weird.
It’s always between a coolant channel and the bore, never coolant to outside edge.
I’m going to get a new piece of plate glass and some fresh wet&dry from 180/240 up to 600 or so.There has to be a physical cause for this, but I can’t figure it out yet. I just can’t see in my mind how the bottom end can be a factor in this.
I’d love to have a standard exhaust to try using. Also a different stator assembly, just in case a previous owner has bodged an advance on the ignition somehow?
There is a very pronounced rough spot at just cracked open throttle. The engine knocks and sputters when trying to maintain a consistent speed at this amount of throttle. I don’t know if this is relevant, but it’s been around as long as my head problems 🤪Thanks for everyone’s patience with this nightmare!
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Reason I ask about the head mount, is my Mick abbey modified head. Needed the bracket holes enlarging, because of how much had been skimmed off the head.
If you've been skimming the heads and barrels, the squish gap will have closed up. It may be a insignificant amount. But with how many times you've been doing it, it would be worth checking.
With how old these bikes are now, you can't guarantee things are to standard specs. Skimming your head and barrel will raise the compression ratio.
Also after reboring a cylinder and fitting oversize pistons, you increase the cc slightly also affecting the compression ratio.
All this will have a slight effect on jetting, add in newer ethanol fuels burn slightly leaner as well.
Not convinced these are the reasons for your issue's but worth ruling out.
Could the piston actually be making contact with the head?
I would pressure test your radiator with compressed air (don't need alot of psi) while submerging it in water, to check for leaks. -
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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