Spark plug thread was crossed by prev owner

[keithr906]

1974 R90/6. I inherited this problem. Anyone with experience with helicoil or timesert to fix the spark plug thread with the head on the bike? How about Back Tap?

[strichzwei]

Pull the head, have it welded and re-tapped or get a replacement head, you'll sleep better !

[weh8127]

What he said!

[srankin]

Same here, St.

[schrader7032]

About 1/4 of the way down this page, Snowbum mentions welding/tapping the new hole but also says he has installed Helicoils for this repair.

https://bmwmotorcycletech.info/sparkplugs.htm

[keithr906]

After research and investigation, I will pursue the repair using a Helicoil. Much respected BMW airhead expert, Snowbum, writes in detail about his preference for this choice stating that the result may be expected to be better than original in strength. In fact, to support that in practice, my father has Helicoiled the other cylinder many thousands of miles ago with lasting positive results. There are sometime several appropriate answers to any question. This is one of them. I know that if for any reason my repair is not successful, I will have more options. If you are not familiar with Snowbum, I recommend that it might be time for you to do so. I also appreciate and thank you for your valued responses. I am informed. Keith

[srankin]

I guess a helicoil would work. Most people don't ride their bikes enough to find out one way or another. Despite Snowbum's advice, I would not do it. To me a helicoil is a temporary fix. But, that is me and my opinion.

Good luck. St.

[sherman980]

Keith,
Before you go down the "sky is falling" rabbit hole, can you give us a little more information? How far down the spark plug hole are the threads damaged? Typically when a spark plug gets started crooked, it will damage the first few threads and then jam leaving the the remainder of the threads intact, that is, unless someone forced things well beyond what a "reasonable" person would do. Your head uses long reach plugs so there is likely plenty of undamaged thread depth left.

Assuming the damage is only in the top part of the hole, a spark plug thread chasing tap of the correct size would be my first step. Use lots of grease on the tap, particularly in the "grooves" in the tap meant to catch the shavings. I position the engine so the intake valve is fully closed and the exhaust valve open but the piston still near the top of the bore, and then carefully try and "catch" the original thread with the chase tap. If it works, go part way in, back out, clean then regrease the tap, and go again until you've "fixed" the damaged area. NO need to run it all the way down the hole if the damage is only near the top. I "wind" a piece of paper towel tightly around a flat blade screw driver so it is smaller than the diameter of the hole, insert and then turn the screw driver in the opposite direction of how I wound it to "expand" the paper towel and clean the threads of any shavings. Repeat. Once clean to the eye, use compressed air through the spark plug hole to blow any nasties left out the exhaust port. I'll sometimes squirt some brake cleaner down the hole followed by compressed air to clean things a bit more. If this works, the repair (insert and start plugs carefully with dab of high temp anti-seize paste) will likely be good longer than the rest of the head will last. If it doesn't work, move on to a helicoil - no harm done... I've repaired many heads of various makes and sizes this way with no ill effects from a running perspective.

Hope that helps.

[sherman980]

Keith,
One last thought. Welding up and remachining a spark plug hole is not something that should be left to the "average" welding and/or machine shop that is not well versed in this type of repair. LOTS OF BAD THINGS CAN HAPPEN when trying to weld closed a long reach spark plug hole in an aluminum head, and, even if that is successful, without proper fixturing, remachining the new hole. This would be my LAST option for repair. (If things were that bad, I'd be tempted to just find a good used head to replace the damaged one. They're readily available on eBay and elsewhere, probably for less than it would cost to repair the one you have if it's damaged that badly.) I've used helicoil repairs for this issue for decades and have put thousands of miles on bikes with this repair over decades, so I wouldn't worry if that is the route you ultimately go.

[Randolph]

I've had some bad experiences trying to line up a tap with what I thought were the original threads. I'd yank the head off and start the (M14 x 1.25) tap from the inside of the head.That's much more likely to catch the original threads and the cleanup is easier.

-Randy