odd head stud issue. help!

[andrewc]

im writing to ask for your guys opinion on what my head stud issue im having could be caused by. the motor i have is a 72. ive built it with "Forgetrue" 80mm pistons,(must be close to 10:1 compression, they have a pretty big dome on them) 3 angle valve job and recut valves, (stock valves), new valve seals, new rings, re-honed cylinders, mild port wok in the head, and a stock cam. i got the motor not knowing it had the sleeves and the big pistons in it. first time i went to fire it, i found it had a blown head gasket. i took the top end off and found it was blown from the left side back (outer rear stud on the shifting side) stud which as you know has one of the oil returns encasing it. i bought a new big bore base gasket from mikes, a cometic head gasket, and did all the head work at that time. put it back together and it started to stretch that one (and only that one) stud which caused the head gasket to blow in the same spot again, ultimately letting oil and exhaust up and out of the top nut on the valve cover. (it made a horrible sound that sounded like a bad exhaust leak and i could feel air coming from under the acorn nut) i tried re-torquing it, a regular shouldered nut (thought maybe the nut was bottoming out inside before torquing properly on the brass washer ), and it ultimately snapped as i was in one of my re-torque cycles, due to it stretching. so i figured it was a weak head stud that over time caused the studs to get weak and that one gave out first. (but still, it was the only one that gave me an issue) i then found 8 NOS studs and replaced all 8 with the NOS ones. i just fired up the bike today after getting it all back together, and everything seemed great. i made it maybe 5 miles from my shop and it started leaking again, i could see the acorn nut shaking and could feel exhaust coming out again. (as well as see oil and exhaust staining around the washer/nut on the valve cover) it doesn't seem as the bolt has broken because it will re-torque, but im almost positive that this caused the head gasket to blow again. the head gasket ive been using (ive gone through 2 now) is a cometic 80.5mm, and the base gasket i made from aluminum. do you know why this one stud would be causing issues over and over again? i dont have any issues with the other 7, just the outer left backside. (closest to carb on shifter side) i put the head on a surface table and it was flat, no warping or play. have you ever heard of this before? im starting to think this motor is jinxed or has some sort of voodoo on it haha. i apologize for the short novel, i just wanted to explain my situation in detail. any help, tips, advice, or solutions are greatly appreciate. im almost at the point of just buying another stock motor and swapping my charging system and hand made ignition and wiping my hands of the whole "hopped-up" engine nightmare. thank you in advance.

 

[angus67]

maybe the threads are fubar in the case, chase a threader down it, see if it feels crappy, or smooth.

 

[TwoManyXS1Bs]

There were some posts here recently about folks finding a case crack in the area of your stud. Most consider them to be normal casting flaws, but maybe not in your case. Try forum searches "crack", "case", "flaw", and "casting". Hopefully you'll find the posts and pictures...

 

[andrewc]

maybe the threads are fubar in the case, chase a threader down it, see if it feels crappy, or smooth.

i chased each threaded hole in the cases before threading in the new studs. i cleaned them afterwards and double nutted each stud as i tightened them into the bottom end. so in theory, they should all be fine.

the only thing i can think that may be causing this issue is, i remember now that the sleeves seemed as if they were pressed a tad bit too far down into the cylinders, which may be causing enough of a "step" or void for the gasket and fire ring to seal. it still just seems odd to me that this is only happening at one stud, every time. i gave in last night and won an ebay auction for some clean stock sized cylinders and stock pistons from another 72. so when they arrive, ill blast, clean, re-hone and re-ring the pistons and see if my problem goes away. i have officially given up on my "hot rod" motor. maybe down the road, ill plane the top of the 80mm cylinders and swap them to another motor. or maybe ill just sell them and the pistons to someone who may want to use them for a race bike.

 

[TwoManyXS1Bs]

Hey Andrew, just thinking out loud here, but suppose that due to the recessed sleeve, possibly more pronounced in that area, that the fire-ring and gasket start to leak in that area, hot gasses heat the stud, it expands, more leakage, more expansion, ...etc.

 

[andrewc]

Hey Andrew, just thinking out loud here, but suppose that due to the recessed sleeve, possibly more pronounced in that area, that the fire-ring and gasket start to leak in that area, hot gasses heat the stud, it expands, more leakage, more expansion, ...etc.

yeah thats what im thinking. i also had to ream a bit of the head gasket around the 2 dowels, (which one is at the stud im having an issue with) so that probably had a bit to do with it. my "new" cylinders should be here this coming week, i hope that solves the issue. i guess if i still have a problem with that stud, it will tell me that the cases are the culprit.

 

[malloym]

did you ever get this issue sorted. I'm have the exact same issue with my motor, except my right inner stud. rephrased 750cc motor just blew the headgasket. 2 inches of the fire ring got blown out. the head nut kept coming loose.

 

[jay760]

I had it once where the engineering shop, didn't seat the liners properly, so after about 40 miles and the engine was hot one sleeve dropped leaving a ridge, had to get the barrels skimmed and ended up with a very high compression 750 engine.

 

[malloym]

I'm starting to think this is my issue. I have a very slight ridge from the cylinder to the jug. it's very small, but enough to grab a finger nail with. the jug is going back to the machine shop.

 

[TwoManyXS1Bs]

After machining, dry-fit the head and cylinder with the dowels installed, check for gap caused by too-long dowels. The dowels can be shortened slightly if necessary...