Could someone please tell me what this thread size should be, my Jumbo currently has an unsightly M12 x 1.5 hex head bolt and nut fitted which will only screw in about 10 mm before it meets resistance. It's been messed up by a previous owner as the thread in the head looks awful, I would just like to return the thread to what it was originally either by re-tapping or helicoiling and then fitting a suitable socket head bolt to block it off.  A couple of pics attached, Cheers.

Edited October 2, 20187 yr by fourex
naughty word used.

On 280 looks like M11 by 1.5.

Does it matter, the threads mushed. Most decompressor valves are the same thread as the spark plug, it will need helicoiling or a thread insert boss fitting. All of these can be got on that website that has the same name as a dam great river begins with Am, and not too dear either. The insert boss will look good but most people would opt for a helicoil, from an access point of view you should be able to do the repair in situ. I may be wrong but it looks like you have an insert in there already. If the hole is too big you will need a thread boss insert, these are totally different to a helicoil and I suggest you do a search on google before spending out on an unsuitable repair. If you are going to fit a decompressor valve a kit is available from the usual dealers at approx£40 with cable, lever etc. If just blanking off then it is important to completely fill the hole down to the roof of the combustion chamber. The current bodge up has reduced your compression ratio slightly.

Edited October 2, 20187 yr by section swept

Thanks, I've got a pretty good idea how to fix it once I know what it once was.

Try a spark plug, think standard plug threads are 14x1.25 or 12x1.25, so seeing a 12x1.5 is close but not right, good guess its 12x1.25

Edited October 2, 20187 yr by faussy

I asked myself the same question as you the other year, but decided to thrash de whole decompressor kit, took the head off and welded up the hole from both ends. Strange, but I have never missed the decompressor after that, as the Rotax engine is such an easy starter (when set up right). 

I like to have the decompressor it makes  starting much easier and does not stress the kick start mechanism so much.

I think about putting one to my TXR too.

The Rotax type decompression unit works very smooth much better then the Montesa type for example.

 

Looks like M12 x 1.75 to me.

 

Thanks, I ended up installing a 12 x 1.25mm helicoil and plugged it with a socket head bolt and alloy washer, I never really intended to fit a decompressor just wanted to return it to whatever was the original thread.  If the 1.25 thread pitch is not as per original than it doesn't really matter now.

Edited December 19, 20187 yr by fourex