Topic: 750 K8 exhaust - threads appear to be buggered

[fxef79]

Previously I had pulled out most (2 wouldn't budge) of the exhaust studs on the K8 engine I just swapped out of an original frame into my wife's trike.  Now I'm putting back on the exhaust, and was going to use the bolts that held it on, rather than put the studs back in.  But one of the holes for the #2 exhaust, can't get the bolt to thread into it.  Looking inside, the threads look pretty bad.  I was able to get one of the original studs to thread in, but it just doesn't feel right.

I think I should go ahead and fix these threads.  I think what I'm also going to do is go ahead and just replace all the studs with new ones and use studs instead of bolts, like her original frankenstein K2/K8 engine had on it.

That said, if I'm going to put in studs, and this won't be something I mess with again (won't be going in/out with bolts anymore, what's the "hive mind" say for helicoil vs. timesert?  Is it necessary to go to the extra expense and complexity of timeserts?

[jonda500]

I vote for helicoils - I have had good luck with them, never used timeserts though...
John

[Alan F.]

I'd think helicoil. Does someone experienced have tips on helicoiling a blind hole?

[fxef79]

I can't find the thread spec anywhere, just "M8".

I believe they're M8x1.25. Is that right?

[Alan F.]

You can always take it to a hardware store and find a nut that fits without doubt, but I think you're correct.

[754]

If its 1.5 it will be very very close to 3/8 coarse bolt... hold one against the threads..,
Its the no pitch guage trick

[fxef79]

If its 1.5 it will be very very close to 3/8 coarse bolt... hold one against the threads..,
Its the no pitch guage trick

I swear I have a pitch gauge around somewhere - just haven't been able to find it.

[PeWe]

M8 - pitch 1.25mm.
I should refresh with an M8 tap carefully entered to refresh only, not make wrong threads if not correctly aligned.
I have fixed one with a cheap helicoil type thread kit. Too expensive to buy a time sert kit for one thread.

[flybackwards]

M8 X 1.25 should be the thread on the exhaust studs - that's what is on my 77.

 I heli coiled some blind holes for camtower mounts on the 77 head. Absolutely need to control the depth of your oversize drilling prior to using the heli coil tap. Measure the depth of the hole with maybe a small screwdriver and a sharpie mark flush with the surface on the shaft. Put some tape on your drill bit to mark the depth and make that mark at minimum 1/8" short of the bottom of the hole. The heli coil will be shorter than your existing hole and you don't want to drill into whats beyond the hole! I found the existing hole was enough to guide the drill by hand without precison alignment. That being said the holes for the cam tower bolts are 90 degrees to the flat side of the head - the exhaust stud holes are on welrd angles.

If you have fancy tools you can use depth stops and measure with a caliper but the result will be mostly the same.

Be gentle with the drill and with the heli coil tap - don't bottom the tap hard against the bottom of the hole and break something - these heads seem to be a quite soft alloy which is why they strip so easy.