Author Topic: Need to fix a spark plug hole thread. Need correct specs (Read 10828 times)

juntjoo · « **on:** March 07, 2019, 11:38:32 AM »

I don't know if there is a standard for motorcycles or something but I've gathered they're smaller than for car/truck plugs so I'm just trying to find the right general thread repair kit. I'm going for a heli-coil kit cuz their particular tool allows for lateral clearance for deep narrow circumstances where the other brand(s) have these fat handles. And I found a used one on Amazon for a good price but I wanna make sure.

I'm measuring close to m12 but as you can see it's not quite exact at least with this little "pocket caliper" but it measures perfect for other taps I tested with it. The thread measurement is exact at 1.25 though.

Anyone know the exact measurements? Thanks

danyo · « **Reply #1 on:** March 07, 2019, 11:44:04 AM »

12mm x 1.25 pitch

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evanphi · « **Reply #2 on:** March 07, 2019, 11:45:21 AM »

Yes the spark plugs are M12x1.25

Let me know how the repair goes. I've had a spark plug hole I've been avoiding repairing...

Are you doing it with the head still on?

juntjoo · « **Reply #3 on:** March 07, 2019, 12:16:45 PM »

Quote from: evanphi on March 07, 2019, 11:45:21 AM

Yes the spark plugs are M12x1.25

Let me know how the repair goes. I've had a spark plug hole I've been avoiding repairing...

Are you doing it with the head still on?

Well I plan to hence my choice of the heli-coil brand tool. I'll update later

juntjoo · « **Reply #4 on:** March 07, 2019, 12:28:45 PM »

Thanks guys

markb · « **Reply #5 on:** March 07, 2019, 02:36:34 PM »

Quote from: evanphi on March 07, 2019, 11:45:21 AM

Are you doing it with the head still on?

How do you avoid getting metal chips in the cylinder?

calj737 · « **Reply #6 on:** March 07, 2019, 02:43:14 PM »

Quote from: markb on March 07, 2019, 02:36:34 PM

Quote from: evanphi on March 07, 2019, 11:45:21 AM
Are you doing it with the head still on?
How do you avoid getting metal chips in the cylinder?

Accident waiting to happen...

jakec · « **Reply #7 on:** March 07, 2019, 02:56:59 PM »

I've successfully used the "reverse tap" tool that you can find on amazon. Works if only the threads closer to the outside of the head are bad, but the inner threads are good. Not sure if that's what you're going for.

As for keeping metal out of the cylinder, you grease the threads and hope that catches it, and then compressed air/kick it over with no plug and hope to get the rest. With the reverse tap you're pulling the metal shavings toward you so it's easy.

jaytee-nz · « **Reply #8 on:** March 07, 2019, 03:16:09 PM »

Lot of "hoping" going on there !

markb · « **Reply #9 on:** March 07, 2019, 03:36:15 PM »

Quote from: jakec on March 07, 2019, 02:56:59 PM

I've successfully used the "reverse tap" tool that you can find on amazon.

Sounds plausible. I'd never heard of that. But if it is a bike I'm going to keep, the head's coming off.

calj737 · « **Reply #10 on:** March 07, 2019, 03:44:19 PM »

Quote from: jakec on March 07, 2019, 02:56:59 PM

As for keeping metal out of the cylinder, you grease the threads and hope that catches it, and then compressed air/kick it over with no plug and hope to get the rest. With the reverse tap you're pulling the metal shavings toward you so it's easy.

Don’t cycle the motor; evacuate the cylinder without the piston moving. Small shards of aluminum will immediately damage a ring if they get between a dry cylinder and the piston. That “grease” you’re applying to the reverse tap will contaminate the shards and make evacuating the shards more difficult.

Pull the head.

sixthwisconsin · « **Reply #11 on:** March 07, 2019, 05:30:08 PM »

Precision # 88036 for spark plugs.
https://www.threadtoolsupply.com/m12x125-34-spark-plug-thread-inserts.html

754 · « **Reply #12 on:** March 07, 2019, 05:48:04 PM »

Aluminum chips won't hurt anything, besides open the exhaust, and blow them out..
The only stressy parts doing it on a bike are getting at it, but I have seen cb 750 middle plug, done in the frame
And catching the tang of you use helicoil..
Make sure you get the right length.

bryanj · « **Reply #13 on:** March 07, 2019, 10:12:40 PM »

Lots of grease on drill and tap to catch swarf but the hard part is removing the drive tang without dropping it in the cylinder and getting at the inner cylinders

754 · « **Reply #14 on:** March 07, 2019, 11:07:12 PM »

This is why I always try to start the first few threads of the inner plugs, with a hose slipped over the porcelain.
Keeps from stripping plugs.

juntjoo · « **Reply #15 on:** March 14, 2019, 03:12:52 PM »

Well I drilled the hole off center. $:-\$ Won't even be able to get the plug in there with the socket as it runs right into the head cover. My near sight and health these days isn't up for such precision work I guess. That's honestly my situation atm. Lot of stress. I did recently install my head successfully though so I am sure I can bolt on a new one. Just gotta get that timing chain on and off again right? Besides, I've stripped like half the head cover bolt holes(in the head). Would be a good time to start fresh with a bunch of lessons learned, basically in working with aluminum.

So... which engines/models use the same head as my cb650sc? Cb650?

onepieceatatime · « **Reply #16 on:** March 14, 2019, 03:21:22 PM »

You need the head from an '81 CB650 or CB650C, or from an '82 CB650 or CB650SC. the '79 and '80 heads are different.

juntjoo · « **Reply #17 on:** March 14, 2019, 03:26:37 PM »

Quote from: onepieceatatime on March 14, 2019, 03:21:22 PM

You need the head from an '81 CB650 or CB650C, or from an '82 CB650 or CB650SC. the '79 and '80 heads are different.

Thanks

onepieceatatime · « **Reply #18 on:** March 14, 2019, 03:30:55 PM »

Quote from: juntjoo on March 14, 2019, 03:26:37 PM

Thanks

No problem!
I have a couple of engines in my garage, that the heads should work for you, but not for my bikes. I'll see if I can get a chance to pull the head from one this weekend, if you are interested.

juntjoo · « **Reply #19 on:** March 14, 2019, 03:41:49 PM »

Quote from: onepieceatatime on March 14, 2019, 03:30:55 PM

Quote from: juntjoo on March 14, 2019, 03:26:37 PM
Thanks

No problem!
I have a couple of engines in my garage, that the heads should work for you, but not for my bikes. I'll see if I can get a chance to pull the head from one this weekend, if you are interested.

As long as they've not been beaten up like I've beaten mine. Are they yours? There's a lot on ebay right now I can order. Are the fins and all threads good? How many miles?

onepieceatatime · « **Reply #20 on:** March 14, 2019, 03:50:55 PM »

Quote from: juntjoo on March 14, 2019, 03:41:49 PM

As long as they've not been beaten up like I've beaten mine. Are they yours? There's a lot on ebay right now I can order. Are the fins and all threads good? How many miles?

Yes, they are mine. I am at work, so I cannot confirm, but off the top of my head, the fins and threads are all good. I do not recall the miles either, but can check the odometers when I am home. I bought a few pares bikes while I was fixing up my '80s, one was an '81 and another was an '82, so the heads are not compatible with my carbs.

Scott S · « **Reply #21 on:** March 14, 2019, 04:38:08 PM »

Since the carbs will be off to pull the head, be a good time to get fresh rubbers and go back to stock.

I'd also do a leak down test on the new head. Put it on the workbench upside down and fill the combustion chambers with Marvel Mystery Oil. Let it sit a few hours and see if any leak by.
I have a valve spring compressor, so if I was in your position I'd pull, clean and lap the valves and install new valve stem seals, just as a matter of course.

juntjoo · « **Reply #22 on:** March 14, 2019, 05:17:08 PM »

Quote from: Scott S on March 14, 2019, 04:38:08 PM

Since the carbs will be off to pull the head, be a good time to get fresh rubbers and go back to stock.

I'd also do a leak down test on the new head. Put it on the workbench upside down and fill the combustion chambers with Marvel Mystery Oil. Let it sit a few hours and see if any leak by.
I have a valve spring compressor, so if I was in your position I'd pull, clean and lap the valves and install new valve stem seals, just as a matter of course.

Hmm.. Much to consider... I'll have to shop for a bit then. There are a lot of them out there. $50-200

754 · « **Reply #23 on:** March 14, 2019, 07:29:34 PM »

I have a brand new 650 sohc head, is yours dohc ?
Your head is still fixable but you need a mill and a lathe.
But paying someone would probably cost too much..

Just a quick rundown.
You need to know plug angle, you may be able to clock it off the original plug seat.
Once clocked, aka dialed in you bore the hole bigger to a tap size, then slightly bigger o' very top..a small relief.
Then you tap and make an aluminum insert or two, you want it to just thread in , no slop, a small lip up top, bored for tap size of your plug. Dont part it off, but part it so around 100 thou wall is left. Then you can cut the insert off, leaving a bit extra to grip it while running it in a final time, use loctite.
Then you mill off the excess up top to your groove, tap the plug threads, then flatten the top where your plug seals and chamfer the threads.
Then clean up the end in the chamber, usually clean up the end of thread to prevent hotspots.

juntjoo · « **Reply #24 on:** March 14, 2019, 09:33:04 PM »

Quote from: 754 on March 14, 2019, 07:29:34 PM

I have a brand new 659bsohc head, is yours dohc ?
Your head is still fixable but you need a mill and a lathe.
But paying someone would probably cost too much..

Just a quick rundown.
You need to know plug angle, you may be able to clock it off the original plug seat.
Once clocked, aka dialed in you bore the hole bigger to a tap size, then slightly bigger o' very top..a small relief.
Then you tap and make an aluminum insert or two, you want it to just thread in , no slop, a small lip up top, bored for tap size of your plug. Dont part it off, but part it so around 100 thou wall is left. Then you can cut the insert off, leaving a bit extra to grip it while running it in a final time, use loctite.
Then you mill off the excess up top to your groove, tap the plug threads, then flatten the top where your plug seals and chamfer the threads.
Then clean up the end in the chamber, usually clean up the end of thread to prevent hotspots.

Thanks. A mill and lathe? I've got the sohc(what forum we in lol?)

I realize it could be saved but I'd be looking at TWO sizes up now at this point since I was drilling for one size up for my thread insert. And as simple as the steps look on paper I don't know if I trust myself at this point to not mess up. I even nicked the valve drilling down in there. Not thinking of that or about the basic angle I was drilling in not so sure I won't mess things up again. Lol, like I can't trust myself to take a few breaths and think something simple through atm. I've been very stressed. Back hurts, energy is low and eyesight isn't good.

IDK, maybe onepieceatatime will come up with something good and ready for me. Or I'll look into these ebay heads. They look pretty good. And some have a 30 day return policy

Thanks for the replies

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Author Topic: Need to fix a spark plug hole thread. Need correct specs (Read 10828 times)