Topic: What are my options?

[SurfinBird]

'78 CB750F SS

So, I fix one thing and more problems pop up after running fine for a year, and it all started by tightening the boots from the air box to the carbs. This time it resulted in me needing to pull the carbs to clean and hopefully fix an overflow problem. I have taken them apart and cleaned them, and replaced a failed gasket on #2. I'm putting them back together and find that 2 screw holes on the #2 and 1 on #4 are stripped. One of the ones on #2 threads, but won't tighten all the way it's just spins, the second on #2 isn't catching any thread at all. I knew #4 was tapped to a bigger size, but if i have to patch 2 I might as well do a 3rd. What can I do to keep all the screws the same size and not have to tap to a larger size? Also 2 of the holes (one on #2 and one on #4) don't go all the way through the carb body, they are blocked by the backing plate screws.

[Brantley]

Helicoil or Permacoil.

[SurfinBird]

Any idea what size helicoil I would use? I'm not familiar with using it and want to make sure to get the correct size.

[lucky]

Any idea what size helicoil I would use? I'm not familiar with using it and want to make sure to get the correct size.

Can you just drill the hole and install a screw
 same size- but longer with a nut on the back side?

[SurfinBird]

The backing bracket screws run under the hole so I cant drill through.

[Dimitri13]

On my 022A carbs I had one carb's threads strip out so I tapped it with a 5mm tap. The carb body was in the way preventing the tap tapping all the way through because of the taper. I tapped as much as I could (dug into the body just a hair to get a little more), then used my 5mm bolt and some cutting oil to tap the rest of the way. Worked just fine.

Just for some info, the stock bolt size is 4mm, and 5mm is the biggest you can go. Any larger and you'll drill into the float bowl body causing leaks, and there's not enough material on the carb body to accept any larger.

[SurfinBird]

That's good info, thanks. I'd rather not drill bigger holes, the body has sleeves that sit in the holes (like a guide), so i'd have to drill out the body as well. The less I have to drill the better. Hopefully the helicoil route will work.

[madmtnmotors]

Any idea what size helicoil I would use? I'm not familiar with using it and want to make sure to get the correct size.

I just ordered my M4x.7 thread repair kit from McMaster Carr along with ten extra thread inserts, should be here tomorrow. Damn ham-fisted PO's have stripped bowl screws on more than one set of carbs I'm trying to revive, AND a tach cable retention screw in one of the valve covers. I'll post before/during/after results.

The backing bracket screws run under the hole so I cant drill through.

I plan to remove the offending bracket screw (only one in my case) while the repair is made to insure full depth repair of the bowl screw. Should not affect the bracket screw when complete, just a small hole where some of the thread used to be. This is a fairly long and beefy screw, no worries.

[SurfinBird]

Photos would be great madmtnmotors, please post pics. My biggest worry is drilling nice and straight. I'm sourcing a kit now, do you need a tapping tool or does the kit have one included?

[cgswss]

I guess I'm to late, but I just taped them to an american Machine screw.  I, THINK I used a 8-32.  No drilling, just turn in the tap.  If you do one, its a great idea to do all so you don't try to put the wrong screw in one of the holes, and wipe out the threads for good.

[phil71]

7/32.. And , not to be too cavalier , but if your floats are dialed in, you can run 3 screws and save yourself a whole lotta hassle. I'd be concerned that helicoil will just bust off the whole tab off.  Not a lotta meat there for that.

[SurfinBird]

Got my Helicoil kit. The screws don't thread into the inserts (inserts are big by a a hair) , but, i'm sure that will change once the holes are drilled and tapped. madmtnmotors, have you done yours yet?

[SOHC Digger]

Got my Helicoil kit. The screws don't thread into the inserts (inserts are big by a a hair) , but, i'm sure that will change once the holes are drilled and tapped. madmtnmotors, have you done yours yet?

The helicoils will tighten up when you insert them.  If they say 4mm, then that's what they are.

[SurfinBird]

Thanks Digger, I figured as much. Have you tried helicoils on your carbs?

[SOHC Digger]

Thanks Digger, I figured as much. Have you tried helicoils on your carbs?

Not my carbs, but as a mechanic, I've helicoiled a lot of #$%* in a lot of different sizes.

[lucky]

I guess I'm to late, but I just taped them to an american Machine screw.  I, THINK I used a 8-32.  No drilling, just turn in the tap.  If you do one, its a great idea to do all so you don't try to put the wrong screw in one of the holes, and wipe out the threads for good.

You do not know what size tap you were using?
How will you know what kind of screws to buy?

[Fritz]

I guess I'm to late, but I just taped them to an american Machine screw.  I, THINK I used a 8-32.  No drilling, just turn in the tap.  If you do one, its a great idea to do all so you don't try to put the wrong screw in one of the holes, and wipe out the threads for good.

You do not know what size tap you were using?
How will you know what kind of screws to buy?

Yes, Lucky: If you tell the truth you don't have to remember anything. (Mark Twain)
In this case: The less you cheat, the less you'll have to curse in the future.

[madmtnmotors]

Got my Helicoil kit. The screws don't thread into the inserts (inserts are big by a a hair) , but, i'm sure that will change once the holes are drilled and tapped. madmtnmotors, have you done yours yet?

Surfinbird,
    Got the Helicoil kit Thursday and did the repair that evening. The repair coils were 6mm in length which leaves one full coil extended out the back side. When I repair another carb body I will likely cut one turn off the top first (NOT the bottom, that's where the tang is that the insertion tool uses) but it
is not entirely necessary. I could order 4mm repair inserts, but either way the repair went well. I use red loctite for the insert and after 24 hrs (min) for that to set up I then run a tap through the repair (4mm x .7) to make sure all is well and to clear any excess red loctite (don't want THAT grabbin the screw!). Careful when drilling, there ain't much material and it drills through quick with a tendency to grab, so get a firm grip on the carb first!

    When tapping the carb body in preparation for the insert I would recommend lubing the tap and going slow. Forward a little, back a little, etc... then back it out slow when you're done. Then a shot of brake cleaner or carb cleaner (to clean out the cutting oil so the loctite will grab) followed by a shot of compressed air). My tap had two threads that got fouled with cut material stuck between them and removing the tap required more effort than it should and I was afraid the newly cut threads were being ruined. All is well and repair is complete, I may have been moving a little to quickly in my haste to get it fixed. I can post some picts if you like.

[Fritz]

My tap had two threads that got fouled with cut material stuck between them and removing the tap required more effort than it should and I was afraid the newly cut threads were being ruined.

Did you lube it? Ethanol or Ethanol mixed with motor oil is siad to work fine to tap alloy.

[cgswss]

I was typing inside at night,  Didn't seem important to go out and look as he already ordered the heli-coils.  It was a 8-32 and there was no drilling involved.  I normally would only use #6, #8, or #10 for something like this as you can get screws and cap screws anywhere in those sizes.

[madmtnmotors]

My tap had two threads that got fouled with cut material stuck between them and removing the tap required more effort than it should and I was afraid the newly cut threads were being ruined.

Did you lube it? Ethanol or Ethanol mixed with motor oil is siad to work fine to tap alloy.

Nope, in too big a hurry to be bothered, hence my advice to lube the tap (sound of forehead slap here...) 

[SurfinBird]

Thanks madmtnmotors, I've never tried this so any advice is appreciated. I was wondering about the length of the inserts, i'll take off a bit. Photos would be great.

[cgswss]

By the way, if you clean the threads with Acetone just before you use the loc-tite, it will cure faster and make a stronger set.  If you screw up, you will need to heat it to remove it.

[madmtnmotors]

Thanks madmtnmotors, I've never tried this so any advice is appreciated. I was wondering about the length of the inserts, i'll take off a bit. Photos would be great.

Measure the lenth of your repair insert first, they come in at least three lengths: 4mm, 6mm, and 10mm. If you've got 4mm then you're set, and if you've got 6mm then you can remove one coil, but these things are tiny so make sure you don't damage what's left. Removing a coil isn't absolutely necessary, even the repair I made that encroached on the screw boss for the mounting plate had no problems (there's more material there).

I' ll post some picts, might be an hour or so...

[madmtnmotors]

Here ya' go:

Completed repair:


Chasing the new threads to insure there are no surprises:


Between the last photo and this photo you can see one coil of thread protruding above the carb body (no worries):


And finally, the completed repair that encroaches on the screw boss for the carb plate (the holes overlap, but the new threads do not extend into this screw pocket):