Topic: 750 K0 stripped exhaust flange bolt hole. What to do?

[Bikebuff]

Hello All,

Last night working on my '70 cb750 K0 and I noticed on the #1 cylinder exhaust flange that one of the bolts had 5 washers on it.  All the other exhaust flanges did not have washers on them so I decides to take a look to investigate.  It appears that the first 3 threads or so in the head are stripped but the remaining are still there.  I tried to tighten down bolt down without the washers but I got maybe halfway tightened and I couldn't turn it much more so I backed off, not wanting to strip additional threads.  Now what?  I've read about helicoils and timeserts, and even tapping a hole and putting a stud in instead, but I'm not sure what the best course of action is.  Is it possible to tap the hole the same size because there are several threads in the head that are still good?  It does not appear that a helicoil or any other modification has been done, but the other flange hole I tried-the bolt screwed in without much tension.   

This is a project bike, so the more I do, the more crap I find wrong with the thing.  I'd love to smack the prior owners upside their heads at their 'ingenuity' at solving mechanical problems.

Serenity Now!!!!


There, much better.  Thanks for letting me blow off my frustration.  I'd love to ride this bike someday before the next millenium.

[754]

Clean the threads with a tap, dont force it when you hit bottom, take a clean bolt from one of the others,  wipe or blow off the threads, now thread in by hand  till bottom and back off a 1/2 turn, and leave the pipe spigot off.

Now try wiggling the bolt, and compare how it feels to the others, if it is a lot looser there is usually almost no thread left, and you will need helicoil.

 Not sure what they had in there, should be a taper head screw.

[Bikebuff]

The screw with the washers on it is the same as all the others.  What I don't understand is how the same screw with the washers taken off won't tighten even halfway.  I suppose its possible that it may have been tapped before, but from what I could tell after cleaning it out it doesn't seem to be the case.  But then again, trying a different screw from another exhaust flange wouldn't tighten halfway either.  So...I'll give your idea try.

[Bikebuff]

Bump.  Any other suggestions or thoughts?

[754]

Check the hole depth, they may have put liquid metal, or something in to try to get the bolt to hold. You may get it out turning a drill by hand..

[Lumbee]

...get a tap and tap the sucker before you do anything else.  should be either a 10mm or 12mm by 1.25.  take one of the bolts to a hardware store and they can tell you the thread size/pitch, and hook you up with a tap...once its tapped that should square you away...

[Bikebuff]

I'll do that.  I need to get a tap set though.  I think I read someone getting one at Harbor Freight for like $20.  Would you recommend some kind of machine oil/WD-40 on the tap when I tap it?

Thanks!  Someday I'll ride this thing...

[Dennis]

I'll do that.  I need to get a tap set though.  I think I read someone getting one at Harbor Freight for like $20.  Would you recommend some kind of machine oil/WD-40 on the tap when I tap it?

Thanks!  Someday I'll ride this thing...

You can buy the one size that you need at a good hardware store on maybe your local home center. The reason I'm suggesting this is that you can get much better quality than the $20 "Harbor Fright" stuff. Don't get me wrong, sometimes their stuff is good enough to do the job but I prefer good quality taps. And if you buy locally (should only be about $4 to $6 for one size) you don't have to wait for it to come from wherever, you can go home and get to work.
Yes you should lube the tap, there are specialty tapping lubes but WD should be OK for aluminum.

[Tower]

Compare the bolt sizes to make sure the problem bolt is not longer than it should be.
If the bolt is the right size, but won't seat properly without washers, then there is something at the bottom of the bolt hole, preventing it from seating.  Anything might be down that hole, including the remains of an older frozen bolt.  You'll need a drill press and jigs, so that the bit cuts straight down the hole and into the broken bolt, and to a depth that can be controlled.  Removal by a professional machine shop might seem expensive, but might also end up being a low cost bit of insurance.  If the exposed threads (above the obstruction) are still sound, then you've no more to do - just put in a replacement bolt.

However, the PO may have damaged or stripped the threads when trying to tighten with too few of the exposed threads holding.  If so, removing the obstruction may not be enough.  The entire bolt hole may need to be resized (and heli-coiled) and an oversized bolt may be needed.  Or again, get professional machine shop to do it for that added insurance.

[Clyde]

I take it you are referring to the screw that holds the flange to the head. This is a 8mm  countersunk screw 22mm long.
It sounds like someone has cross threaded the hole and damaged the first few threads. It is easy to do especially with aftermarket pipes. I did it to one of mine when I fitted SS 4-4 pipes
Buy an intermediate tap for 8mm x 1.25mm and clean the thread out. Use plenty of oil and remove the tap and clean it every couple of turns.
You may need to also get a plug/bottom tap to ensure you get maximum depth.
I would then buy a 8 by 25mm countersunk screw (a straight slot is more readily available than a phillips head) and replace the 22mm. This will give you enough of a bite to ensure the muffler flange does not leak. In fact it would be as good as new.
Regds Clyde

[MRieck]

 Taps are terrible to use on existing holes....you need a thread chaser...period. Check the depth with a vernier and put a Helicoil in there. I've done it at least 50 times on those holes