Just when I thought I was getting close... POW.

After assembling the top end of my 74 GTMX/80, I broke one of the four cylinder studs when torqueing down the head.  I was shooting for 15 ft-lb (180 in-lb), which is the lower end of Yamaha's 15-18 ft-lb torque spec.  I did tighten in a crosswise pattern in 50 in-lb increments and was on the last pull from 150-180 when the stud snapped.



I'm trying to decide between filing down the top of the exposed portion, flat before carefully trying to drill out, then EZ out,  the broken stud... OR trying to weld a 6mm nut to the exposed portion of the broken stud and work it out with a 10mm wrench or socket... similar to the technique shown in the following short YT video.

www.youtube.com/shorts/Uyp4C-pfhDE

Has anyone successfully removed a broken cylinder stud with this welding method?

Before trying the weld method on my engine, I tried a couple of test welds of a nut onto a short 6mm bolt but so far, my test results have not been satisfactory!  The first attempt twisted right off with minimal force.  My second try was a little better but the welded nut came loose with a little more than 25 in-lb of torque.





I think my problem was not getting enough heat or penetration into the bolt; though there was plenty of heat and penetration in the nut which was red hot on the second try.  The difficulty here may be caused by the zinc plating on my test bolts and nuts. Or perhaps trying to do the weld initially, with the nut in place; rather than getting some weld on the top end of the bolt first and then finishing up with the nut in place.  Any suggestions on this are welcome.

I'm also curious about the Yamaha GT80/MX cylinder studs.  Are these studs symmetrical?  I mean is the amount (length) of threads the same on both ends of the stud?

And finally, I'm wondering if I should replace the other three studs to be safe?  UGHH!
Thanks!  ZT



 

In that type of situation I've welded a piece of flat with a hole onto it. Done it in all sort of situations including my 360 motor which broke trying to remove the stud to swap to another case. Mine was flush but I see yours is protruding so that method should work easily. I only have an old arc welder but modern tig would be best & more controllable guess. Was the torque setting the recommended setting--ie reading the right model, only small diameter studs. Possibly been way overtightened before by someone & you drew the short straw.

“I'm also curious about the Yamaha GT80/MX cylinder studs.  Are these studs symmetrical?  I mean is the amount (length) of threads the same on both ends of the stud?”

I’m rebuilding a JT60 right now and the studs are not symmetrical. The threaded portion that screws into the cases is longer than the threaded portion that gets the head nuts. But again, I am referring to the JT60.

Thanks much RT325!  I appreciate your reply.

"In that type of situation I've welded a piece of flat with a hole onto it. Done it in all sort of situations including my 360 motor which broke trying to remove the stud to swap to another case. Mine was flush but I see yours is protruding so that method should work easily. I only have an old arc welder but modern tig would be best & more controllable guess. Was the torque setting the recommended setting--ie reading the right model, only small diameter studs. Possibly been way overtightened before by someone & you drew the short straw."

I think the flat plate is a great idea and it would eliminate the problem with the zinc plating on a 6mm nut.  But I'm trying to visualize the plate... it can't be very long (for leverage) to clear the raised cover screw boss that's only about 5/8" outboard of the broken stud's center.  (See pix above)

Can you describe the shape and size of the plates you've used?  And I have  no idea of the torque required to remove these studs.  I assume they are an interference fit.  Can they be removed without a lot of heat?

I have a HF MIG machine.  I verified the torque setting (15-18 ft-lb) in the Yamaha service manual for the 1973 GT1/MX.  My GTMX is a 1974.

It's anyone's guess how many times these studs have been tightened and presumably stretched.  But would you say the risk of breaking a stud during removal is reason enough NOT to preemptively try to replace the other three studs?

And thanks to you too, Schu!  This is good info for me.

"I’m rebuilding a JT60 right now and the studs are not symmetrical. The threaded portion that screws into the cases is longer than the threaded portion that gets the head nuts. But again, I am referring to the JT60."

I'm trying to visualize how much of my stud is still in the case... and how tight it might be!

I'm not too familiar with Yamaha models other than my GTMX, but If your JT60 is actually a 1971 JT1 or '72 JT2, it looks like the cylinder studs are both in fact the same Yamaha part number as my 74 GTMX... 90116-06026-00.

So, if you haven't yet installed your new studs, would you mind measuring the total length of the new stud?  Or the length of the threads on the bottom?

I don't see anything in the service manual on removing or installing the studs.  Do you normally just use two "jammed" nuts on the top threads to accomplish both tasks?  Do you normally use heat for both tasks?

Do you know off-hand if the cylinder head torque for your JT60 is 15-18 ft-lb?

I do appreciate your help!
ZT

The overall length is 116mm and the amount of thread on the case end is 20mm.
Mine pretty much unscrewed by hand. I think I had to use pliers on one but it turned out with ease.

Hopefully yours will unscrew once you come up with a method. It looks to me like you could use a sharp prick punch and lightly rap it with a hammer to turn the broken stud.

My JT service manual only gives you a generic torque spec per fastener size and I doubt that would apply to a long 6mm cylinder stud. Your torque value of 15-18 ft/lbs seems way too high to me. Hopefully someone will set us both straight on what the cylinder nuts should be torqued to.
 

I just checked service manual and indeed states 15-18 ft-lbs. However, the bottom of the DT1 manual also has this:

Thanks again for your help, Schu!

Thanks PABDT! That's quite a difference for these 6mm studs... more than double... 7 vs 15+ ft-lb.

So, how tight do you mini-enduro guys pull these cylinder head nuts, out here in the real world?

I know that I'm a bit of a Philistine/ignoramus (delete as appropriate!) but, by and large, I just tighten any bolt until I think it's tight enough. If I'm fitting a cylinder head and it blows after I've tightened it (carefully at first), I tighten again until it doesn't blow.

Zinc plating can easily be removed with muriatic (swimming pool) acid or even vinegar.  Just drop the nut in and the plating will come off.  

When I was about 11 my buddy had a YG1T 80 and broke a stud when he finished decarbonizing the head and piston.  (We premixed with regular motor oil, always looked like we were fogging the area for mosquitos when we rode LOL)

I talked to my Dad and we took his bike to the truck shop.  We could not find a stud and metric was almost "unobtainium" back then.  One of the old time mechanics selected a piece of stick arc welding rod slightly larger than the old stud.  He had me carefully clean the flux off the stud.  We tapped out the hole in the case with USS thread and used a die to cut threads on the welding rod. Mechanic showed me how to make a gasket.  It worked great!  

I made up an "invoice" and made $5 for the repair.  First paying job I ever had!  

I found this post online from 3 years ago where someone was asking the same question. I think I’m going with 8 ft/lbs on mine whenever I get to that point.

imgur.com/a/mEXbN6C