Cross threaded spark plug hole threads on 72 SF 750 - solutions?

Paul LeClair

Paul LeClair

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Location
Calgary, Alberta, Canada
Hi all

still some very late season riding going on here as winter closes in. A friend turned up yesterday on his 72 SF 750 limping towards home with electrical troubles and a cross threaded spark plug.

The bike stopped on the highway west of Calgary with electrical issues, and in his efforts to get it going again he removed and replaced spark plugs at the side of the road with primitive tools, and accidentally cross threaded one of the plugs putting it back in, left cylinder.

as I live on the far west edge of Calgary, he made it as far as my place with the bike. I sorted his electrical issues, but do not have a tap large enough to even attempt to tackle a cross threaded spark plug hole with a tap smeared in grease to catch any material coming off. Not that keen to attempt to re tap the hole with head in place anyway.

so, what are the best known longest lasting repairs of a cross threaded spark plug hole in a 72 SP 750 head? I have already warned him the head is likely going to have to come off to do the repair properly, but if there is a way to do this with head left on he would be more enthused...... Anyone?

Paul LeClair
 
Paul, don't ya just hate that dont know how many times in the past have been confronted with cross threaded plugs in heads.
Depends on how much is cross threaded, if he has wound it all the way in and buggered the whole thread then is harder to correct.

You need to get a purpose built spark plug tap as they have not only a chaser on the lead in but also a small amount of material designed to help keep the thing square. Or you can get a tool which you insert in all the way then it cleans the thread up when removing it by cleaning in reverse.
Like you said heaps of grease on the tap. if not crossed all the way down once you clean up the area of concern with one of the above. here is the link to the neat AST tool


If buggered or non salvagable then you can get pupose built time sert kits ( no not helicoils or their clones as they are a waste of the raw materials that they are made off IMHO ) http://www.timesert.com/ which have a nice self centering tap so again if push comes to shove you could repair in situ,. just use heaps of grease on the tap and then heaps of compressed air when finished.
At least on a bike you have the option of removing the head which is not so hard but when you are working on something which is like 12 hours for a head R&R sometimes an in situ repair is the best option.

Good luck that's what I say
 
Paul,
When you get the thread repaired, tell your friend to use a bit of hose when fitting spark plugs. If the plug cross-threads the hose will spin on the plug with no damage to the thread.
 
PLC-Consider using the Helicoil sav-a-thread kit. The insert is thin walled and solid construction, not the junk coil type insert. Main concern is correct insert length. Insert stakes in for secure assembly. Use red Loctite 271 for insert. I've used the product with success many times-it works as designed.
Bob-
 
The best bet is to remove exhaust and inlet manifold. Rig up something so you can put compressed air into the inlet and a vacuum extracting out the exhaust. Then tap the 14mm out to a 18mm thread and put in an insert with Locktite 263 or whatever is high strength. Did this an old Roundcase Duc and all was fine

http://www.ebay.co.uk/itm/SPARK-PLUG-THREAD-REPAIR-INSERT-M14x1-25mm-/141561036505?hash=item20f5b1fed9:g:B5wAAOxyF0pTg0Ss
 
Paul,
Depending on how bad the threads are chewed up, you may get away with one of these.
Helped me in the past on 2 heads with cross threaded plug holes.
I sourced mine in Germany, but HF or Whitney etc. should have them as well.

HTH
Gerald
 

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One remark that I haven't read:
Usually the crossthread is only in the aluminium upper part of the spark plug hole. The thread in the cast iron section is the strongest, so if you can reclaim the upper section then chances are good that the thread will last, in my case 38 years and 220.000 km..  :)

Marnix
 
Gerald said:
Paul,
Depending on how bad the threads are chewed up, you may get away with one of these.
Helped me in the past on 2 heads with cross threaded plug holes.
I sourced mine in Germany, but HF or Whitney etc. should have them as well.

HTH
Gerald
Click to expand...

Umm read the first line of my second paragraph as its exactly what I was referring to. 

Interestingly the second tool which I suggested would start in the cast iron scull bit and wind back tother way.

 
take an old (long reach, as standard) plug, remove the earth tang, cut a cross with a hacksaw from top to bottom of the thread, close up the lead in part of the thread, in a vice or use self grippers, to make the thread "over tapered",  coat with grease and attempt to thread the thing in squarely, use your fingers and eyes to start it, when you are sure it has picked up square, fit a socket, extension, and a tee bar, then turn it in a half turn and back out a quarter, repeat until bottomed, then run in and out a few times, then do the same with a good plug.
Nothing to lose, if it wont hold up when you finally install your working plug, the head comes off, not hard at all on an SF

if you can pressurise the cylinder through the inlet or exhaust ports then so much the better, (rubber cone?) the updraft through the plug hole will be a good thing, achieves nothing unless a good flow of air is coming out all the time
CLEM
 
Brett said:
Umm read the first line of my second paragraph as its exactly what I was referring to. 
Click to expand...

I did! just added my 2 cents with a tool that is most likely obtainable in Canada without delay.
Clems DIY solution is another option.
 
thanks very much, all.

owners initial preference is not to remove the head if that can be avoided.

I really liked the look of the "back tap" thread repair tool suggested by Brett. North American equivalent appears to be this

https://www.amazon.ca/Powerbuilt-640811-Back-Tap-Thread-Repair/dp/B0014FGVVW/ref=sr_1_1?s=automotive&ie=UTF8&qid=1477971928&sr=1-1&keywords=14+mm+back+tap

I have asked the owner of the bike to order one of these tools and we will give it a go first before considering more drastic options.

Paul LeClair
 
Gents,
I can't say I have done this but heard of getting the engine coming up onto compression ( valves closed / closing) and then pump the head with grease.  Do the tapping, cutting, helicoil etc.
Finish the compression Stoke, with plug out and the grease should push the scarf out.  I'd wash the bore and use vacuum after but might be worth the risk.
Good luck
 
Gerald said:
Paul,
Depending on how bad the threads are chewed up, you may get away with one of these.
Helped me in the past on 2 heads with cross threaded plug holes.
I sourced mine in Germany, but HF or Whitney etc. should have them as well.

HTH
Gerald
Click to expand...
Got mine in Halfords 😊.
 
i have used this tool successfully in the past, and find its the best of the "re-threaders"  - but use care (lots of grease)  as if you cross thread it when trying to rethread your bad sparkplug hole  then your truly f'd

Paul LeClair said:
thanks very much, all.

owners initial preference is not to remove the head if that can be avoided.

I really liked the look of the "back tap" thread repair tool suggested by Brett. North American equivalent appears to be this

https://www.amazon.ca/Powerbuilt-640811-Back-Tap-Thread-Repair/dp/B0014FGVVW/ref=sr_1_1?s=automotive&ie=UTF8&qid=1477971928&sr=1-1&keywords=14+mm+back+tap

I have asked the owner of the bike to order one of these tools and we will give it a go first before considering more drastic options.

Paul LeClair
Click to expand...
 
Perma-Plug M14 X 1.25 Spark Plug Thread Repair Kit or Perma-Coil! - Worked great on my friends Harley - Sure a similar Perma-Plug kit suitable for the 750 Plug will work on a Breganzie.
 
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