Topic: i went and screwed up the easy bit... help required

[superchode]

so i finished off the '78 750 engine rebuild last week... set all the clearances, good to go.  just needed to wait for my hex bolt set to come in - clamp the cover down and toss the engine in the frame.  bolt set shows up, cover installed... and i think to myself that i may as well toss in the new spark plugs while the engine's still on the bench.

well i don't have a spark plug socket for the spark plug that this thing takes - so i go at it with a long 18mm socket... fine for 3 of them - but i've gone and cross-threaded the 4rth.  i'm really bummed about this... i wanted to be done and i rushed into hosing myself for however long it takes to fix this.

i was thinking i'll just go buy a tap for whatever thread this plug is and use it to re-align the threads. does anyone know what the thread is on this plug? is there any advice on how i should go about repairing this?  i can't make myself tear the top end down again... a black abyss appears in my thoughts just thinking about having to do that.

advice/assistance would be greatly appreciated.

[Noel]

Simplest fix: Carefully get the plug started correctly, torque it to minimum spec, and leave it.

Somewhat more complicated fix: Load up the tap with grease to prevent chips from falling in, clean up the threads, and hope the plug holds afterwards.

Correct fix: Head comes back off, plug hole gets drilled, tapped, and helicoiled.

IMO, #2 is the worst plan. I'd try #1 and hope for the best, realizing that #3 is in my future, either sooner or later.

Threading is 12mm -- I think. It is on the 500 and I believe it's the same throughout the range.

[superchode]

well the plug won't seat all the way... and i imagine that will have a noticable negative effect on combustion - don't see how i can leave it torqued halfway in.

[Noel]

Old Honda aluminum is famously bad. I'd be very concerned that running a tap through there would leave the threads hopelessly loose, which is why I believe #2 is a poor idea.

Honestly, with the engine out of the frame, I would really consider yanking the head and doing it right. Yeah, it's a lot of work, but it'll be a lot more work if a lazy fix fails a couple of months down the road and you have to take the engine out again.

[Noel]

Bear in mind, though, that this is all just my opinion. You may get lucky and one of the truly knowledgeable folks will come along to explain to us that cleaning up with a tap will work just fine.

[TwoTired]

Can't tell from here how bad it's screwed up.

Cross threading pushes or bends the metal away from its desired position.  A standard tap will cut away the deformed metal.  You will have to deal with metal chip control, and of course the metal removed makes the installation significantly weaker.

A possibly better solution is the push the metal back into position without removing it.  This is called thread rolling rather than thread cutting.

I have been able the roll threads back into proper position on occasion.  I used an old spark plug, first cleaning and chasing the threads on it with the correct die.  The steel threads on it are tougher than the aluminum head.
Indexing the thread beginning is the tough or luck part.  Plus you must be certain that the plug is started perpendicular to the spark plug seating surface.
Indexing is automatic if you thread from the opposite end (from the combustion chamber side).
But often, there is enough material left at beginning of thread to start the plug in the correct remaining groove instead of the one you cross threaded.  Oil the plug threads well, start and force the plug into the threads at the correct 90 degrees to the plug seating surface.  Go one quarter turn at a time.  Back off to get oil into the thread forefront and repeat.
This method is not without risk.  If the plug does not index correctly it can seize and removal will take out enough head material to require a timesert.
http://www.timesert.com/html/mtrcsert.html

However, when it does work, once you get past the buggared thread portion, it screws in easily, and slightly over tightening the spark plug realigns all the threads without cutting.
Clean all the oil off the threads before putting the real spark plugs in.  The heat can turn the oil to glue and when the spark plug come out next time so do the aluminum threads.  Anti seize is your best bet for steel plug installation into aluminum material. 

You didn't say which position you cross threaded.  Number two or number three positions need a tapered socket to correctly align the plug 90 degrees to the plug gasket sealing surface.

best of luck!

[Jonesy]

I know it's late now, but a trick I learned long ago was to take a rubber plug boot an push it over the plug and use it to start the sparkplug into the threads. Since it's flexible it will help line up the plug properly and if it cross-threads, the rubber boot will just spin on the plug, before the plug is forced in and does damage. Once the plug is in, just pluck the boot off and tighten down the plug with a wrench.

[superchode]

it's the #4 plug thread that's cross-threaded.  i did spend a bunch of time trying to gently find the proper alignment and patch up the threads - without luck.  pretty much resigned myself to pulling it apart, now - i don't want to think about bits of metal sitting on top of my piston.

it's just a matter of how i repair the threads now - have to see how it looks when it comes apart.

[Bodi]

You should be able to start a plug or tap from the combustion chamber side "the right way". I've used a plug to clean up carbonized threads this way since I didn't have a tap (12X1.25mm or something?). I filed a groove in the (old used) plug threads to act like a tap's cutters. It worked quite well. Removing the plug washer lets the threads go almost all the way through the hole.
If you've just crossthreaded the outer part and the plug still engages some virgin threads near the bottom it should be OK. If a new plug spins before you can turn it the 3/4 turn to compress the new washer then you will need to helicoil it.

[jsaab2748]

Not sure if you'd want to go this route at this point, but they do make a tap especially for spark plug
holes. Some even have a 3/8 square hole in the "drive" end making it possible to get in fairly deep spots using an extension, NAPA has them, but not sure what sizes they carry.

[superchode]

well... i disassembled the top end to repair my handiwork.  first tried threading a sparkplug in from behind to fix the thread - but it wouldn't go in far enough from the combustion chamber side.

luckily my local hardware store had the right size tap (even though they didn't have any bolts w/ the correct thread pitch) and i ran that in from the combustionchamber side... all is well.  there's plenty of unmolested threads near the chamber and the plug tightens and seals well... so i'm happy.

very glad i took it apart, though.. there were shavings on the piston that i certainly wouldn't have been able to clean out with the engine fully assembled.