So yesterday I’m doing final assembly on my little red gt80.

all systems go.  Seems like compression a bit soft despite new piston, machined out cylinder.  Hmmm.  Do I have my head torqued down okay?  Was the bottom gasket I butchered and tried to cover up with some blue gasket stuff the issue?  As I attempt to recheck the torque, I wind up snapping a stud.  The blankety blank wrench didn’t go off.  

either 50’year old studs are weak and easily broken, or buying a torque wrench from Amazon was dumb.  

so we wait, again, for four new studs and a washers and a new gasket .  And hoping the stud comes out.  I am sending it out to a fellow to finish up as these two days of fall break from school were really my last opportunity to do serious work on her.  

anybody ever had this issue or ?
what might me some other causes of low compression after rebuild?  Exhaust not on tight?  Duly noted my compression gauge also was probably made by teenagers in china, so there’s that!  
*not a complaint against teenagers in China, a complaint about poorly and cheaply designed tools and my cheap a*% propensity to buy such crap.

 

I use the beam style torque wrench for head/cylinder torqueing. You usually go 2 or 3 rounds working up to the final torque. I never thought the click style would fail. Never broke a stud, but I did have the case crack on an AT1 years ago. And I was using a beam style torque wrench.

Maybe try torquing your engine to frame bolts to see if it clicks. Were did the stud break at hopefully not flush with the case also did you rebore your cylinder?

I've been saying this for years...  compression gauge readings are meaningless unless you have experience with your gauge on your exact engine or one exactly like it.  This is especially true on the smaller engines. 

I'm sure that's the reason that none of the Yamaha manuals for these bikes have any compression reading specs. 

The gauges with hoses are usually the worst...  add a few cc's of hose and internal gauge volume to the combustion chamber volume of a little engine that only has a few cc's of combustion chamber to start with, and the reading is going to be low. 

I found this out the hard way on my DT50's....  they started and ran fine but compression was below 50 psi on a gauge!  Lowest reading was with my expensive Snap On gauge.  I rebuilt the top end on one of them because that was just too low...  they shouldn't even run...  Compression reading on my gauges did not improve at all. 

Allegedly there are special factory two stroke compression gauges that are designed for these engines...  the one Kawasaki sells can be bought still.   I say "allegedly" because the last time I checked it cost well over $200!  So I've never verified that they work any better.

What you can do is check compression with your gauge using your method (cranking speed, etc. makes a difference too) and use that to compare with readings in the future.  So if it reads 80 psi and then after a year of riding reads 60 psi with same gauge, it might be time to rebuild. 

The larger engines are not as affected but there still can be massive variations between gauges.  And the later 360/400 had an automatic compression release which complicates things even more.  Someone once posted a video here on how to modify a Harbor Freight gauge to get a more accurate reading.

Studs on a GT80 aren't very big & don't run a lot of torque. Somewhere i have an inch lb torque wrench that i've not used for years but is good for studs/nuts that size to get a good feel coming up to clicking.
Compression should have a good positive feel on the kick lever especially by hand. Might need a run for a minute to settle things but should be pretty good & positive from the start.