In situ spark plug thread repair

[koolio]

I just rebuilt a CB500T engine that I bought in bits and I did not notice that the RHS spark plug has actually been cross threaded.

It's going to be a real headache to pull it out of the frame, break the cam chain and take the head off to helicoil it.

Can I get away with drilling out the old threads and helicoiling it with both valves closed? Or do I really just have to take the head off.

[Pete.]

It's been done many times over. You can weigh up the odds yourself, but personally I would just do it and use a small piece of hose to hoover out as much debris as I could before cranking. There's a small chance that some swarf might get caught in a valve seat but that's it.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[c_dug]

Is it worth running a correct size tap through first to see if you can clean up what's there, or is it a complete gonner?
____________________
I am a bellend, I am a man of constant sorrow, I am a gummy bear, I am a rock.

[Jim painter]

If you have to drill and tap/helicoil put some grease on the drillbit and the tap. It helps the swarf stick to the bits rather than falling in side. Don't drill all in one go so you can keep cleàning the swarf off.

Jim

[koolio]

Ok guys thanks will try it.. I am terrified about the valve seats however.

[c_dug]

With the piston being at tdc to keep the valves closed you want to be sure not to drill into the top of it too.
____________________
I am a bellend, I am a man of constant sorrow, I am a gummy bear, I am a rock.

[Ariel Badger]

I would do it at BDC with a small pipe gaffer taped to the vac. This way you can get into the corners.
____________________
Bikers make great organ donors, get 115 on your licence today.

[Teflon-Mike]

A Thread 'Saver' might be worth a try. Unlike a tap, it has no cutting edge, so re-forms the metal back into the thread, without making swarf.. One I have is about the size of a spark plug and fits in plug spanner... so if you can get a sparker in the hole, you can get a thread saver in there. May be worth a shot, if its only a couple of start threads crossed.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[bladerunner]

When I've done it in situ...I used a compressor to blow any debris out the exhaust valve...didn't need to drill it first as the tap was designed to cut from std plug size to insert size. It's worked fine the few times I've been asked to do it!
____________________
Current bikes...cbr929, KDX200's, Rd125lc mk2, RGV250's

[Vincent]

Sounds like you need one of these kits but, if it's the old 500T, it'll have the bigger plugs:-

www.ebay.co.uk/itm/BGS-Werkzeug-Spark-Plug-Thread-Repair-Kit-M10-x-1-00-/271452791015
____________________
Space Is Deep

[Pete.]

[c_dug]

Why not? (genuine question)

Tef did stipulate if it was only the first couple of threads were crossed.
____________________
I am a bellend, I am a man of constant sorrow, I am a gummy bear, I am a rock.