SOHC.co.uk Forums > CB350/400

Spark plug threads

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oldchuffer:
I've got two problems with spark plug holes on my restoration:

1. One plug will screw to the bottom but continues to turn so i guess the threads are shot.

2. One plug will screw in about 3 turns and then gets so stiff that i guess its cross threaded.

Whats the best plan of action for both of the above?  The head is off the bike.

Many thanks.

K2-K6:
Ordinarily if the threads are definitely gone and past any reasonable tolerance (the spinning one) then an insert which is screwed and bonded in will give a competent long lasting fix.

The one that's crossed, potentially,  can you put a plug socket on it and then put a long extension bar on that ? As you turn it outwards from it's position can you see the extension end move in enlarged circle (indicating off centre) if so, you can wind it out until it's at thread end (it clicks as threads finish) then see if you can pick up on the original thread start point by aligning the extension bar concentric to the hole. It can then pickup the correct thread line and go in straight if you can.

heli_madken:
For the cross threaded I was going to suggest a thread chaser but 12mm ones seem to be hard to come by now - https://www.ebay.co.uk/itm/164760112423?mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338781099&toolid=20006&customid=V2CztEsOAAAAOh7WwmrIZc80laQGAAAAAA&_trkparms=ispr%3D1&amdata=enc%3A1MBULEJRJRvCLVqGG0nx-Iw47

The beauty of them is that they work in reverse using the good threads further into the hole and chasing them back outwards. A little grease on the chaser stops any bits falling into the cylinder but there generally isnt that much swarf

Bryanj:
Some months ago i did 4 inserts on a 350/400 for a member and had to buy 4 kits to do it plus a correct drill. Helicoils can unscrew after fitting and be impossible to redo. The kits were about £10 each from memory

McCabe-Thiele (Ted):
Personally I would take the cylinder head to a good engineering firm & ask them to sort it out - might be the tight thread is a case of carbon deposits caused by a loose plug and it can be cleaned out - the one that just turns sounds as though it needs an insert. Worst case is it might need two inserts. Ask them to check the remaining two cylinders. Last option look for a replacement cylinder head Nurse Julie has them up for sale sometimes.

Not sure if you can find one where you live or in North Wales but maybe worth asking Trigger if he can sort it sending the head by courier service.

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