Help with a thread mistake

Hello All

This is a thread to help out with a thread

I am making the Wyvern model IC engine and am nearly finished. I ordered a spark plug, which arrived today. When i tried to screw it into the cylinder head it didn't fit - I then realised I had used an M10 x 1.25 tap while the thread on the spark plug is M10 x 1.

Can anyone advise on the best way forward? I don't really want to remake the cylinder head, as it took ages. What about filling the hole with JB Weld and remachining it or can I do something with the plug?

Any help would be really appreciated.

Andrew

FWIW, Plug the hole with a blank, Loctited in, redrill & tap with correct thread... or drill out next size up & use thread insert to bring back to correct size...

George.

Well the easiest way would be to buy a helicoil kit and re -do the thread the correct size. JB weld won't be up to the job ( or jb perhaps) . A lot of motorcycle heads get helicoiled  when the threads gall or strip. A helicoil is a 'stronger job' anyway. However as we are on the Model engineers Forum I have no doubt that suggestions will abound around making a threaded Titanium bush, or some other more complicated suggestion followed by 11 pages of posts about the appropriate tapping size, or the correct grade of material for the job. Helicoil kits are available cheaply online. Inserts can be shortened if you are careful or different lengths are available. Once the plug is in no one but you will know it is there.....

Edited By stevetee on 23/02/2017 12:26:21

Thanks guys. Good suggestions. I am kicking myself at the moment - which is quite painful.

That's the name of the insert ... I couldn't remember at the time...

​George.

http://www.ebay.co.uk/itm/15-Piece-Helicoil-Thread-Repair-Kit-M10-x-1-0-x-13-5mm-Free-Delivery-/222359502193?hash=item33c5a8bd71:g:sy8AAOSwU-pXqDEI

is that the one?

Andy

When I made my Wyvern I manged to need to redo the holes for both inlet and outlet valves. I bored these out and plugged them with ally before re-working, the plugs being retained by JB weld in one case and an aluminum solder/braze in the other. Both have been satisfactory in use. Documented at post #146 here **LINK**

HTH,

Rod

Thanks John - very kind offer. I might take you up on that. Thanks Rod, I saw your posts.

I always fit helicoils in aluminium instead of just tapping straight into the ali. I used to work for company that did sub-contract work for Rolls Royce and this was always specified on the drawings we received.

If you do go the Heli Coil way, I suggest that you use a bronze insert rather than stainless, if you use the latter there is a chance of over heating the spark plug (there is reference somewhere in my paper work of an air accident report of an engine failure in NZ caused by a wrong insert in an earo engine).

I'd drill out and make a threaded aluminium  plug, and Loctite it in place then re-drill and tap 10 mm x1 mm(I'v got a tap you can borrow, but it might be a bit far away).

Ian S C

Edited By Ian S C on 24/02/2017 10:43:48

Standard repair in VW alloy heads plug holes. Many years ago, reclaimed cylinder heads fitting helicoils for rocker shaft clamp bolts after an engine recon firm said it would need a new head at £35. (Well I did say MANY years ago).

Tap and 10 inserts were £2.50. Had to grind flats on tap down to M6 size and make a "Long series" tubular tap wrench due to offending holes being in lower part of head casting. Anyone conversant with V dub beetle motor will understand problem.

Local Tractor manufacturing plant fitted Helicoils as standard in CI engine blocks during machining as it was cheaper than having to strip engines down and do it in the event of a whoopsie when assembling new tractors, long gone now.

Regards Ian.

Edited By Circlip on 24/02/2017 11:02:09

The only problem that I see with using helicoils on model engines is that most of the time you simply don't have enough extra metal or enough room

For example the Wyvern in question only has a 1/2" dia boss to be tapped for teh plug, drilling and tapping that out for a helicoil won't leave much in teh way of wall thickness. Things will also come quiet close to the cvalve seating, I have done models where the M10x1 thread is less than 1mm away from the valve seats.

You are right Jason, there is not much room. I think the hole diameter for the helicoil is 10.3 mm, so the wall thickness will be less than 1mm. The valve seatings will also be tight. Its a shame because I had got the valve seatings pretty spot on and had been pleased. I have ordered the helicoil kit now - i might try boring the hole rather drilling it out.

Could you make a plug and tap it to take a 1/4 -32 rimfire plug

Vince

I have gone with the helicoil option. I carefully bored out the hole rather than use the drill, that way I could get right to the valve seats without damaging them. I also ground down the helicoil tap at the end to get as much threading as possible. I had to cut down the coils as they are too long for the hole.

It went OK, the cheapo helicoil kit from eBay uses a bar with a split end to wind the coils in and is a bit clumsy, the first one got cross threaded and i had to pull it out. The second go was better, but I misjudged the length of the coil - its too short which has left only 3 threads for the plug to screw into. The plug does seem secure when tightened up. I am hoping this will be OK - not sure it will be possible to remove the helicoil from the hole and fit a new one.

Thanks all for your ideas and advice. Andy

If you need to remove the helicoil, grind up a piece of silver steel to a shallow point and pick the end of the helicoil out then pull it out .