Honda hornet cb900 exhaust stud size. - Wrist Twisters
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post #1 of 9 Old 06-18-2019, 11:05 PM Thread Starter
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Honda hornet cb900 exhaust stud size.

Anyone know what the OEM stud sizes are. I'm a little unsure about the external thread pitch.
The thread going into the head is M8 with a course thread, 1.25 pitch. The external thread through the headers is M7 and a fine thread. Something like 1.00 pitch. Although one source mentioned .8 pitch.
I'd like to use something corrosion resistant, stainless or titanium. But would like to replicate the stock setup.

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post #2 of 9 Old 06-20-2019, 12:52 PM
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Quote:
Originally Posted by Islandboy View Post
Anyone know what the OEM stud sizes are. I'm a little unsure about the external thread pitch.
The thread going into the head is M8 with a course thread, 1.25 pitch. The external thread through the headers is M7 and a fine thread. Something like 1.00 pitch. Although one source mentioned .8 pitch.
I'd like to use something corrosion resistant, stainless or titanium. But would like to replicate the stock setup.
A
M7 Coarse is 1.0 Pitch
M7 Fine is 0.75 Pitch

B
Now it gets really interesting.
Do you want thread tolerance class 6g or 4g6h?
You can get both in either of the M7 Pitches.
(my guess is that the same holds for the M8 1.25)
6g means the same tolerance applies for the Major (nominal) Diameter, and the Pitch Diameter.
The Pitch Diameter is between the OD and the Thread Root.
6g allows for plating.
4g tolerance of the Pitch Diameter means a tighter toleranced threading indicative of a more specialized and critical application.

I have a feeling you will have a practical challenge finding multi thread size studs for a price you won't suck wind when you hear the price, maybe due to a minimum charge from a fastener supply house.

My guess is that the OEM stud is simple 6g tolerance at both ends.

As for Titanium Fasteners, as a rule they should not be used where anything over Grade 8.8 (roughly same as SAE Grade 5) has been applied.

High temperature anti-seize compound may be an easier route.

I think my interest in fastening started when I converted the cam tower bolts in my '73 SOHC CB750 over to SAE Grade 8 1/4-20 allen head cap screws. Then torqued the bolts to the bolt spec, totally oblivious to the fact that the cast aluminum cylinder head would be able to retain the necessary Helicoil inserts due to the bolt torque level. A rather sickening education that one was!
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post #3 of 9 Old 06-20-2019, 04:33 PM Thread Starter
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Thanks mcromo44 great information.
Greatly appreciated. I learnt something about thread tolerance grades today. Don't think I'll get a choice there.
And yes I have done a little wind sucking at prices. Especially the titanium.
I've narrowed my choices down to two.

http://www.ebay.co.uk/itm/162762829512
These titanium ones I like. The nut is identical to look at as the OEM ones. Fricken pricey but I figured I'll only do it once. Also the bikes a keeper.

http://www.wemoto.com.au/parts/pictu...m8m7studkitx8/
I probably would've gone for these, 1/3 price but I don't like the nut.

Both sets go from M8 to M7. Pitch also changes from 1.25 to 1.00.
I plan on using a copper based high temp anti seize lubricant.
I'll check the torque values and see if they are for dry threads.
Fingers crossed my OEM studs come out easy. 17yrs old but look clean and no rust. I will soak in PB and have a gas torch handy.
Now you must tell me how you fixed that CB750 head. Sickening...I felt ill just reading that. We often learn a lot from mistakes.

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post #4 of 9 Old 06-20-2019, 06:01 PM
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Quote:
Originally Posted by Islandboy View Post
Thanks mcromo44 great information.
Greatly appreciated. I learnt something about thread tolerance grades today. Don't think I'll get a choice there.
And yes I have done a little wind sucking at prices. Especially the titanium.
I've narrowed my choices down to two.

http://www.ebay.co.uk/itm/162762829512
These titanium ones I like. The nut is identical to look at as the OEM ones. Fricken pricey but I figured I'll only do it once. Also the bikes a keeper.

8x Stainless Steel Exhaust Stud M8x1.25 Reduces To M7x1.00 With 8x M7x1.00mm Stainless Steel Nuts
I probably would've gone for these, 1/3 price but I don't like the nut.

Both sets go from M8 to M7. Pitch also changes from 1.25 to 1.00.
I plan on using a copper based high temp anti seize lubricant.
I'll check the torque values and see if they are for dry threads.
Fingers crossed my OEM studs come out easy. 17yrs old but look clean and no rust. I will soak in PB and have a gas torch handy.
Now you must tell me how you fixed that CB750 head. Sickening...I felt ill just reading that. We often learn a lot from mistakes.
The Service Manual torque values are for dry threads, assumed to be new or as new re cleanliness and thread condition.

My guess is that with some PB or similar, the original studs will be not bad to get out - just be sure that the studs are well cleaned and dried at your grab point. (unless you plan to double nut and jam and then use the nuts for a grab by a wrench - unless you are so lucky as to have the two nuts end up perfectly aligned with each other)

I don't screw up often, but when I do, it's a real doozie!
I managed to get around the CB750 head problem with a "get away with it" that wasn't truly a proper fix.
I managed to get the Helicoils out.
The drillings for the original Helicoil inserts were deep enough to allow for deeper threading, adequate for a deep sited replacement Helicoil insert.
I backed off on the torque, and used thread locking compound.
I can only imagine what I might find the next time I take it apart for an inspection.......
That head had mega hours put in it re port work and special multi angle valve job, winter '74/75 engine warmover with cam, mild head work, and '75 SS pistons for a bit more compression. Carbs played with as well, plus air box inlets opened up.
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post #5 of 9 Old 06-21-2019, 07:44 AM
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Stainless steel is not recommended when threaded into aluminum due to a much greater galvanic potential difference (G.P., measured in volts using a calomel electrode as reference) between aluminum (G.P. between -0.83 to -1.05 volts) and stainless (G.P. -0.01 to -0.102), especially when heat is applied to the joint in the presence of an electrolyte such as salt water. And yes, simply living near the ocean is sufficient to cause galvanic transfer of material from aluminum to stainless, especially when there is a considerable amount of heat involved: every time there is a heat cycle the threaded hole the fastener is screwed into "breathes" slightly, drawing in salt laden humidity which then dries during subsequent heat cycles leaving salt embedded in the joint. Progressive deposition gradually increases the electrolytic activity until there is sufficient transfer of elemental aluminum to the fastener to weaken the aluminum enough for joint failure, usually when the engine is cooling off after a goodly ride. Application of a copper based anti seize compound will slow the process, but not stop it.

Unfortunately Ti is in the same boat (as it were) as stainless as its G.P. is close to zero as well. Then there's the price!!

Frankly, carbon steel is preferred due to its much closer G.P. (-.58 to -.64) regardless of local conditions. It may surface corrode more than stainless or Ti, but the galvanic index is a much closer match and can be considered as the preferred material especially if you plan on keeping it for quite a while.

Rob
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post #6 of 9 Old 06-21-2019, 11:19 AM
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Quote:
Originally Posted by robtharalson View Post
Stainless steel is not recommended when threaded into aluminum due to a much greater galvanic potential difference (G.P., measured in volts using a calomel electrode as reference) between aluminum (G.P. between -0.83 to -1.05 volts) and stainless (G.P. -0.01 to -0.102), especially when heat is applied to the joint in the presence of an electrolyte such as salt water. And yes, simply living near the ocean is sufficient to cause galvanic transfer of material from aluminum to stainless, especially when there is a considerable amount of heat involved: every time there is a heat cycle the threaded hole the fastener is screwed into "breathes" slightly, drawing in salt laden humidity which then dries during subsequent heat cycles leaving salt embedded in the joint. Progressive deposition gradually increases the electrolytic activity until there is sufficient transfer of elemental aluminum to the fastener to weaken the aluminum enough for joint failure, usually when the engine is cooling off after a goodly ride. Application of a copper based anti seize compound will slow the process, but not stop it.

Unfortunately Ti is in the same boat (as it were) as stainless as its G.P. is close to zero as well. Then there's the price!!

Frankly, carbon steel is preferred due to its much closer G.P. (-.58 to -.64) regardless of local conditions. It may surface corrode more than stainless or Ti, but the galvanic index is a much closer match and can be considered as the preferred material especially if you plan on keeping it for quite a while.

Rob
Great info, I never even thought of that!
Especially given Islandboy's proximity to salty moist air 24/7.
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post #7 of 9 Old 06-21-2019, 03:48 PM Thread Starter
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Thanks Rob. I appreciate the advice and will go ahead and order some new OEM studs and nuts.
What you said makes perfect sense. I live on a salty island. I've seen the rot on my aluminium boat around stainless fixings.
Your advice was timely too. I tried to order the titanium studs yesterday but my Internet was down. Was going to try again today but now I've read your post.
I'll hit Partzilla up for some studs and nuts.
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post #8 of 9 Old 07-15-2019, 04:13 AM Thread Starter
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Got the old studs out. Got to admit it was a little scary. The bike is 17yrs old. The studs did have a good hold in the head.
One backed out when I took the nuts off.
It came out real easy, no galling. I had soaked it all in PB blaster days before.
The rest of the studs needed heat and gentle tapping with a small hammer.
Using two of the exhaust nuts locked on I could grip with a small 10mm spanner.
Only after I put heat into the head surrounding each stud did the stud let go. Using a LPG torch I heated up that protruding part of the head sticking out with the stud in it. Enough heat to boil PB out of thread. Quickly put on and lock two exhaust nuts. Gentle pressure on a small 10mm spanner while gently tapping the nut. After 4 to 5 taps all the studs let go.
All wound out very easily. No noise.
The thread inside the head and stud end looked like new. The stud threads inside the external nut all showed some signs of corrosion and damage. Mild damage and they probably could be reused but I'll put in new ones and worry about it in another 17yrs.
I intend on using a nickel based high temp anti seize when I put in the new studs.
Got me some new nuts too.
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post #9 of 9 Old 07-15-2019, 10:56 AM
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some of those bolts sure clean up nicely
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