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Discussion Starter #1
Hey All, I'm a new rider and new to the forum so I apologize in advance for any lack of know-how navigating around here. A buddy of mine recently decided to just give up his 76 Honda CB750 that had been sitting in his garage and I took on the gift(project)! I'm not an experienced mechanic by any means, but I'm a bit handy and have turned a wrench or two. I cleaned the carbs and replaced the battery on the bike and she fired right up. I decided to change the spark plugs next. The three plugs went in smooth, but the fourth I decided to give a turn too many and stripped it!!! The plug now backs out fine and goes in smooth until the last bit...almost snugs up but then rotates through. I've been searching around and apparently I need an M12 x 1.25 spark plug repair kit which is the equivalent of a unicorn. Can anyone shed some light on the matter or guide me in a direction that won't cost the mortgage??? Thanks!!

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MONIonline

6m ago*-*#38
 

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Can you use a heli-coil?
 

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Discussion Starter #3
Thats the route i was looking to go, im just a bit uneducated on the details in using them. The m12 "spark plug kit" is like non existent. I found one on ebay. Ive been told by a few people that the kit to be specifically a "spark plug kit"????
 

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Is the plug or the cylinder stripped?
Did you try another plug in that head?
 

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Discussion Starter #5
I haven't. ....would be the simplest step. I think i just dismissed the thought of that when i weighed it against my luck. Ill try tomorrow. Thanks for responding so quick. Whats the chances the plug strips before the head? And if it is the head...whats the next step?
 

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Let's take one step at a time. One would like to believe that the plug would be weaker metal than the head.

There are people on here with a lot more experience than I have who can help you if it gets to that stage
 

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The head is likely stripped since aluminum is softer than the steel plug.

An auto parts store will stock a helicoil kit. Bring in one of the correct spark plugs to match up the size to make sure you get the right one.

The head should probably be removed to drill the old threads out to make sure the drill bit goes in straight and to keep the shavings out of the engine. Just take your time and follow the directions. It's pretty easy to install new threads.
 

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Discussion Starter #8
Copy that. I appreciate it man. A buddy of mine mentioned to me that the head is aluminum and not so for the plug. Ill definitely try to see if a different plug will work tomorrow.
 

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head jobs...

I just thought about a little thing from my childhood. As helicoils were designed for steel heads back in the day,m I don't think you will want to try it.

aluminum is a completely different monster, they sell a stuff like jbweld for aluminum, if you can get a tap the right size, and get the heads off, you could probably use it, I would find a good bike mechanic in your area, take it to their shop, and ask them what to do. I would say, the best repair is replacement. Find a head, and swap them out. Stripped heads, and plug size are a couple things that can really change the performance of your machine, take it in to the shop, have them do a complete porting and spice it up nice for oversized plugs on all the holes.. just an idea.
 

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Discussion Starter #11
Dods, I meant to mention that this particular spark plug size doesnt seem to be very common. At least not as far as thread repair systems go. The auto stores carry an m10 and then an m14 as standard. For some reason this m12 is like from a different realm. Ive been searching for the last two weeks online and have only found two kits and theyre about $160.
 

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M12 is a bit rare, though if you look around you'll find one. It will be more expensive (most likely) than the 10 or 14. I found one at my local auto parts store which had been on the shelf for a loooong time and they gave me a decent price for it.

The sad fact is that not many vehicles out there use the m12, so it costs more - which is a bummer.

Edit: Heck, if you live locally bring the head over and we'll heli-coil it. I probably have a spare.
 

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Discussion Starter #14
Cmonstart,
If i can track one down, is it feasible for me to do the job with the head intact? Where ya located?
 

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Southern NH.

Do you mean with the head on the engine? It's not the smartest way to go for sure. Really the best thing is to pull the head, which means pulling the engine in the 750. Much safer. It will also give you a chance to look things over really well.

You can heli-coil a plug hole with the head on in an emergency. Grease up the tap really well, turn it in a bit, pull it out, wipe the grease off, add new grease, repeat. The grease holds the metal filings the tap produces. Not the smartest way to go - any metal gets into the cylinders and it could really mess things up fast!!! I've done it once or twice because I HAD to - not the smartest way to go.
 

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Discussion Starter #17
Had a fellow suggest turning the kick over to reduce the amount of depth beneath the ports and just blowing it out after the threading????
 

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That may sound like a good idea, but it snot. You will not be able to get all the crap out of the combustion chamber and little shards of metal aren't going to do much for the cylinder walls or valve seats should they get slammed shut on a piece.

If you're going to pull the engine, which you should do, you may want to consider more extensive work. Not what you want ot hear probably, but if you've gone to the trouble of pulling the engine and removing the head, you can at least inspect the bores. Multiple measurements at top, middle, and bottom can be taken just by rotating the crankshaft. Don't use the bits on the right side of the motor where the points are though. If you see pitting (from sitting) or if the bores are out of spec the cylinder block needs to come off.

This may sound like a lot more work, but all the grunting you'll be doing to get the engine out puts you in place to do this.

Take note of O-ring locations and consult a manual about removing the head. The nuts should be tackled in a systematic approach to avoid warping the head while removing and installing it. Clymer makes a decent manual for the SOHC 750s that will give you all the detailed info you need.

At this point you may also want to consider replacing the cylinder head. eBay appears to have a lot of them available. If there is a salvage yard for bikes near you that offers the option to have a look at it prior to purchase and avoid shipping (heavy/bulky item). Have a look at all the mating surfaces before you buy. Screwdriver gouges indicate someone pried the head off in a rather ham fisted fashion. I looked at one on eBay that was loaded with them and I'd give it a pass. You'll want the mating surface checked for warping too.

What you currently have is a bike in need of engine work. Your lack of experience makes this the perfect opportunity to gain some and get to know a decent bike while you're at it. You may be wanting to get this thing on the road ASAP, but as a word of caution, this isn't going to be a simple project.

You say your friend has had it and just decided to give it up. That suggests it has been sitting for a long time. If so it needs maintenance. Even if it hasn't been sitting a long time, do you know when the last brake fluid change occurred or the last fork oil change took place? The last swing arm service?

Brake calipers get corrosion around the outer most aspect of the cylinder bore. They're apt to stick at some point and cause all manner of problems. A good going through the front and rear suspension will work wonders for the bike's handling provided you accept that this is 70s tech, not a modern sport bike Same for the brakes. You want them working at their best though.

Your initial investment (free?) leaves you some room to drop some cash - you will need to do so, trust me. Doing it right the first time is cheaper than doing it right the second time.
 

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Helicoils

I used heli coils everywhere on my Norton race bike. We opened the engine often. The heli coils were mostly used in aluminium casings. Without them the motor would have come apart. Wonderful invention.

Unkle Crusty*
 

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Heli-coil repairs have saved a LOT of aluminum engines. No reason not to use them. Think about it... the spark plug is steel going into aluminum.
 
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