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MG TD TF 1500 - Cylinder Head Spark Plug Holes - cross threaded

Looking for advice on repairing (what I believe to be) cross threaded spark plug holes in my cylinder head.

Two of the spark plug holes appear to be cross threaded in the "new" AEF118 head on my '55 TF-1500. The head is installed on the engine. Plugs are Champion #123 RN5C.

The holes in the head still have threads, so I hope that I will not need to ream and install inserts.

Not looking forward to it, but I don't mind pulling off the head to repair the threads if necessary. But it would be nice to fix in place.

I found a number of spark plug thread chasers. All are inexpensive. Harbor Freight @ $3.99 to KD Tools at $10.49. Should I try one? Any suggestions on brands or usage?

Thanks for your help,

Lonnie
TF7211
LM Cook

Lnnie - I have used a thread chasers to clean up the spark plug threads. The biggest problem is going to be removal of whatever swarf is generated that falls down into the cylinder unless the head is off the engine. As for which chaser to get, I would opt for the KD Tools one. I won't use any precision tools from HF. Cheers - Dave
David DuBois

Hello Lonnie. In dealing with your situation. I believe it is important to qualify the purpose of thread chasers .As Dave pointed out he has used thread chasers to clean out threads and if I may add at the same time they will remove burrs and yield a better thread surface finish . To use thread chasers to rectify cross threads, you are inviting a host of problems which could lead to engine failure and significant damage......... Your suggested alternative of installing inserts is the recommended procedure to follow. John
J .R.C Cavey

Use the KD tool and be very careful to be perfectly straight. The cross threads can't go too far. I would not go to helicoils or other inserts except as a last resort. I speak from experience. As a kid I worked with the bosses nephew, a drug head he was minding for his sister. That dummy stripped so many oil drain plugs that we cut a set of wrenches down to lengths so short that he couldn't. He was either so stupid or brain dead that he just couldn't get the idea that although the plug had a 1" head, it was only into sheet metal and the long wrench would strip it.

He was just as bad with spark plugs. He'd get one cocked then apply a socket and ratchet until it wouldn't go any more. Of course it either leaked and made a noise or was discovered the next time it came in for a tune-up.

Anyway, one of the rest of us would end up fixing his blunders and yes, a good thread chaser, carefully used, would find the good threads and clean up the rest. A good chaser is tapered and will get beyond the fouled threads.

I remember being happy when he was too drugged to remember to come to work. He was so oblivious that we had to watch him so that he wouldn't pump gas with a cigarette in his mouth.

I just was flashed back!
JE Carroll

If you are going to use the thread chaser..you could place a vacuum hose near the boss to try to suction out any chips created by the thread chaser. Regards, Tom
tm peterson

Alwa˙s start plugs by hand. ....after that you can go for it, but not too tight!
ZBMan

You can chase the threads safely without yanking the head.

Get a small neodymium rare earth magnet and stick it on the end of the thread chaser. Virtually all the filings will cling to the chaser.

Threads provide the mechanical structure to hold the plug in, they are not responsible for the seal.

Make sure you have the correct plugs to seal- either the flat washer or the chamfered seat seals compression.

....and double check the length, 1/2" vs 3/4".
JRN JIM

If you do take the head off you might consider re-tapping the thread from inside the combustion chamber, the end of the thread is unlikely to be damaged so there is less chance of continuing with the cross threading.
Max Irvine

Max. Brilliant! Lateral thinking at its very best!
Peter TD 5801
P Hehir

Thanks for the information and tips. I'll remove the head in a couple of days to chase the threads in the spark plug holes. Don't want to take a chance of chips falling into the cylinders with the head on the engine. I'll try to start the chase on the piston-side of the head. I hope that I won't find that I need to enlarge the holes and install inserts.

Jim - WSM calls for 3/4" reach plugs.

While it's off, I'll clean and paint the engine and components. Also check the condition of freeze plugs etc.

Lonnie
TF7211
LM Cook

Hey Lonnie, how about this little tool, I've not tried it but it's made in the USA and cuts the thread from the bottom of the hole as you draw it out, bringing the swarth with it.

http://www.amazon.com/Powerbuilt-640811-Back-Tap-Thread-Repair/dp/B0014FGVVW
CJ Harvey

CJ -

I saw similar tools in my search. It's pretty neat, but I am not confident enough to work blind from the inside out.

My head was freshly installed, so it should be easy to remove and put on my bench for best viewing as I work on the threads.

I won't know until I am able to see it from all sides if the threads are crossed or if crud lodged in the threads during the machining process.

Thanks for the info,

Lonnie
TF7211
LM Cook

Just curious- were you trying to remove or change plugs, or installing for the first time?
George Butz

George - check the email in-box at your office.

Lonnie
TF7211
LM Cook

You make think this is wrong but I wouldn't worry at all about anything getting into the engine.

First, you're not drilling or cutting new threads, just cleaning a couple of crossed ones. Anything that happens to fall in will just either blow out if you crank the engine with the plugs out or pass harmlessly out the exhaust. It's not like introducing contaminates into the lower end where you worry about a scratched bearing or damaged oil pump. Hard carbon deposits are always breaking off in the combustion chamber and blowing out the exhaust. Even if a couple of tiny chips evade the vacuum or magnet that has been suggested they're too small to be of any consequence and won't scratch anything.

Of course if you're wanting an excuse to disassemble the engine then by all means, those are dangerous particles. :-)
JE Carroll

I,ve use helicoils a number of times and have the tool that breaks off the tang somewhere, under the bench. Recently, however, I was at a local engine rebuilders and he swears by inserts made by Wurth. I believe these are made in the States. How do they differ from helicoils?


Jan T
J Targosz

This thread was discussed between 18/05/2014 and 20/05/2014

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