Exhaust Bolt Holes Dilemma

[londonboards]

I'm working on my GS1100 Road Runner Project and I now have a dilemma with my exhaust bolt holes. They (when measured with a caliper spike) are all the same depth but on some I can turn a bolt in 6 times only and on others 14 times.



or



So I thought I should try a little tapping and all this stuff came out after about 4 or 5 turns:



The bolt does fit in the tapped hole nicely now (up to the 4 or 5 turns) but should I continue tapping down? I am afraid I might be doing irreparable damage. Seems like there is a lot of metal coming out.

Or is there a tap that is different from the M8 x 1.25 that I am using that should be used just to clean up the hole?

Greetings

[Steve]

M8 x 1.0? (OK, it's been corrected.)

I thought they were 1.25 or 1.50 pitch.

Have you tried spraying carb cleaner, brake cleaner, WD-40, anything in there to try to flush out the crud?

.

[TopTechRacing]

Pretty sure that into aluminum it will be a 1.50 pitch. I would start looking for a helicoil kit for the ones that are in bad shape.

[londonboards]

Steve - absolutely correct: they are 1.25 and it was my typo. I used 1.25 tap.

I have corrected the original post.

I will try spraying out. But where is all the crud coming from?

Greetings

[Nessism]

The threads are deep into those holes, but the bolt doesn't go full depth. Cleaning them with a tap is the right approach, just make sure you have the proper tap. I believe you need 8mm x 1.25, but please measure to be sure.

EDIT: post above showed up while typing.

The crud is corrosion deep into the hole. The bolt only uses the first few threads, not full length, so crud build up at the bottom. Make sure you use anti-seize on the bolts and go easy when tightening the bolts to avoid stripping.

[londonboards]

You can see in this pic of the bolts used by the PO (all different sizes and with various numbers of washers) that some of the bolts were only biting by a couple of threads (see 2nd bolt from left). I have bought longer bolts. Originals were 45 mm long and I have some 50s and 55s. I would like to get a bit more bite.

[chuck hahn]

Arent stock ones something like 35MM from shoulder to the end? To keep them from bottoming and punching into the head itself??? And yes on flooding the hole with a spray bottle full of water or something to rinse the chips and crud out and lots and lots of air to blow as much out as you can get out.

[Steve]

You also don't want to drill any deeper than what the holes are now.

Some of those holes are just a couple of millimeters away from oil passages.

Don't forget your safety glasses when spraying fluids in the hole and blowing air.

.

[chuck hahn]

Which was the reason I posted about what the stock bolt length was. Remember some time ago someone posted they drilled and then had oil running out one or two of the bolts holes.

Had a 750 with a bad crank so it became a parts bike. When I took the pipes off oil came out of the bolt hole on cylinder 4 I believe it was. Anyhow, when I measured the bolt it was 40MM from shoulder to the tip and it was an allen head like he shows in his pictures. So I assume the head has been punched and its no good.

Would you agree Steve?

[Steve]

Not real sure, Chuck. I have no personal experience with those engines, just what I have read here.

The existing holes should not be any problem, only if he does any drilling to install Heli-Coils.

Another thing to watch for is to make SURE there is no fluid left in the hole when inserting a bolt.
If the thread seal well, it could build up some incredible pressure behind it and possibly force a crack.

.

[chuck hahn]

Installing a Helicoil or Timesert isnt an issue as long as he does NOT go deeper than what the depth measures with the probe. In fact, I would stay 3 MM minimum short of the stock depth just to insure I didn't punch thru anything.

[hugo151]

best bet with all suzukis that use bolts to hold the headers is change them to studs & use spring washers & brass nuts instead, i,ve changed them on all 3 of my engines, after problems like yours. you need to clean up & make good the threads you've got left, tap them a bit deeper if required, then thread lock some good hi-tensile stud material in, or a cut down full thread set screw to required length.don't use cheap general threaded bar ! brass nuts can undo themselves so leave enough length for 2 nuts locked together.

[Kiwi Canuck]

Suzuki lists the correct length bolt as 35mm but if you have the newer aluminum collars they are thicker than the older steel ones.

I ordered Suzuki OEM bolts and they didn't even reach the threads.

My finned collars are 26mm thick plus the 3-4 mm space as they do not tighten flush against the head, plus the washers, that leaves only a few thread in the head if that.
I needed to use minimum 45mm bolts to get a decent bite, can't remember if I used 45's or 50's but 55 is too long.
Check the thickness of your finned collars, then measure the depth of your holes and use a bolt that fits but doesn't bottom out.

Re-taping the hole is fine, your taps look like they are taking out a little material, because you're using a but as long as the bolt is still firm and not loose in there then no worries.

Just found this, the article recommends that when tapping into aluminum you should have threads 1.5-2 times the depth of the bolt diameter, so that would be 12-16mm of threads.

Nut and tapped hole strength - How much thread engagement is needed?

If a screw / bolt fails because the threads strip, it can be hard to detect both during installation and later because the threads will still have some grip on the screw. If the bolt breaks, however, it will be completely loose, be easy to detect and remove, and usually fail during installation when additional torsional loads are present (torsional loads usually dissipate within minutes after tightening if you're wondering why we didn't take them into account before). Because of this, fasteners are designed to fail in the bolt, not the threads, so most nuts are more than adequate--just make sure you use a similar grade of nut compared to the screw.
How much thread engagement is needed in a tapped hole, then? According to "Fundamentals of Machine Component Design", 3rd addition, by Juvinall and Marshek, p. 413, if the bolt and nut are of similar material the thread stripping stength will equal the bolt tensile strength when the nut is .47 * diameter. Standard nuts are 7/8 of a diameter, for comparison.
Interestingly, more than a third of the load is held by the first thread in a nut according to this. As the bolt tightens, its threads stretch and the nut's threads compress, which reduces force on the far threads.
The .47*dia calculation above takes this imbalance into account, but it will certainly be different for other material combinations. This offers some formula (also found in machinery's handbook) for calculating the shear area of threads, but it's uncertain how one would apply that formula given the imbalanced thread load. The Machinery's handbook suggests at least 3 threads of engagement. We recommend 1 diameter depth for steel and 1.5-2 diameters for aluminum. The referenced formulas may at least provide a rough estimate for sheet metal, where thread engagement is limited. Unbrako's Engineering guide has several charts showing experimental testing of various sized holes. According to their guide, formulas have performed poorly at predicting thread strength.

[tkent02]

best bet with all suzukis that use bolts to hold the headers is change them to studs & use spring washers & brass nuts instead, i,ve changed them on all 3 of my engines, after problems like yours. you need to clean up & make good the threads you've got left, tap them a bit deeper if required, then thread lock some good hi-tensile stud material in, or a cut down full thread set screw to required length.don't use cheap general threaded bar ! brass nuts can undo themselves so leave enough length for 2 nuts locked together.

What he said. Save what's left of the threads.