There are three weak points in the cylinder head fasteners.  Two of them I see frequently on this forum, exhaust stud holes strip and head cover holes strip.  The third I have not seen mentioned on the forum, but I have personal experience with the 8mm stud that secures the head to the cylinder (just below the exhaust port).

I have come up with a fix for each of those weak points.  This post provides the details.

What I am about to describe worked good for me.  If you decide to try this stuff on your own, you assume responsibility for the outcome.  If you don’t have the skills, don’t do it.  If you don’t understand something, STOP and get help.  Get a manual.  Read up.  Comply with ALL the safety requirements outlined in the manual.  Make sure you know what you are doing before attempting any of this stuff.

Stripped threads can result from cross threading or overloading.

Get the threads cross threaded and continue to turn the fastener and voila, you chew up the threads on the weaker material.  The weaker material in this case is the cast aluminum cylinder head.  Bad news because that head costs a helluvalot more that the cheesy bolt.

Overloading the threads is generally connected to thread engagement.  Thread engagement determines the total shear stress area of the thread system.  More engagement equals more shear stress area.  More shear stress area equals more resistance to shearing.  When using a steel fastener in an aluminum component, you generally want engagement on the order of two to three fastener diameters.  So, for an 8mm stud installed in an aluminum component, you should expect at least 16mm thread engagement, 24mm would be better.  Let’s face it, wouldn’t you prefer that the cheap stud snap off rather than the threads pull out of the super-expensive head.

When you are juggling with the head pipe, flange, and fake studs, it’s very easy to cross thread the fastener into the cylinder head.  It’s also easy to get the collar on the head pipe cocked such that you can hardly achieve any thread engagement.  It’s not cross threaded but its only in the hole maybe two or three turns.  Then, thinking everything is cool you start turning the fake stud with a wrench.  Under these circumstances, you either wreck the internal threads by forcing the misaligned bolt into the hole, or you overload the internal threads because of the insufficient engagement.  Voila, wrecked head.

Yep, I know it’s not as pretty.  But its better, lots better.  Its stronger.  Its easier to install the head pipe.  It can hack the mission.  Use stainless steel nuts if you want to dress it up a bit.

Any of you ever seen one of these?  That would be your typical head cover bolt.  You know, the long ones that secure the right-hand side of the head cover.  There are three of them (115mm, 125mm & 130mm).  I’m always seeing reports about these fasteners on the forum.  They love to yank out the internal aluminum threads.  You can see this one was in there real deep, and the internal threads still sheared.  IMO, the aluminum fatigues over time.  The repeated thermal cycles take their toll.  It’s not a crossed thread issue.  Pretty hard to cross thread a bolt that’s being aligned by a hole that’s over 100mm deep.  I’ve had two of these fail, and I don’t overtighten fasteners.

Then you install a ¼-20 UNC Grade 8 bolt with a grade 8 nut.  Major league strong.   On the front-right and center-right bolts, there’s no risk of putting aluminum chips into the engine.  Just take appropriate precautions to keep things clean.  For instance, you wouldn’t wanna do this with the clutch cover off, or the oil filler cap removed.

The right-rear head cover bolt (130mm) is another story.  Unfortunately, I believe it’s the one that fails most frequently.  The right-rear bolt goes through an oil passage.  Any sort of drilling will put metal chips into that oil passage.  If you wanna fix that one you need to remove the cylinder head to block the oil passage and clean things out after the repair is finished.  I use plumbers’ putty to seal off the oil passage in the cylinder.  Then temporarily reinstall the head and head cover to use as a guide for the drilling operation.  Then remove the head cover and head to clean things up.  This can be done with the engine still in the frame, but the head and head cover must be removed for cleanliness plugging and cleaning.

Because the bolt hole is pressurized with oil when the engine is running, the bolt-head must be sealed, and the nut on the bottom must also be sealed.
 
A standard Suzuki sealing washer works fine under the bolt-head.  I apply a dab of silicone sealant too.

You can’t use a standard nut on the bottom.  It will leak.  You need to use a closed nut, like an acorn nut.

To provide a decent sealing surface for the acorn nut, you need to apply a spotface.  Hand grinding won’t do.  One of these cheap valve seat cutter tools works well.  The cutter kit comes in handy for other stuff too.  You only need the cutters, the handle, shaft and guide won’t be needed.

Once you have drilled the hole through the cylinder, you need to relieve the cooling fin just below the location that the acorn nut will bear on.  That will provide room for the spot facing tool, and also the nut.