Exhaust thread repair

[mgill]

I've got a head thats been cross threaded, and need it repaired.
Prefer someone in North America.
Any one know the size and thread pitch of the exhaust nut?

[EVguru]

I can't recall the thread size off the top of my head (pitch is 1.5mm I think), but I can tell you the thread is a special. The diameter and pitch are a standard Metric Fine combination, but the thread is a close tolerance.

I've got a pair of gas flowed heads on the shelf from MEZ Porting and I presume they also did the exhaust port repairs. The exhaust nuts are loose in the threads because they were cut to standard tolerance.

I've now done three exhaust port repairs. I have a jig to mount the head on the mill so the port can be bored out. I then press in a threaded insert (with the correct close tolerance thread) and weld it in place. It's not a quick job!

Do NOT machine the end of the port flat! It needs to be conical to give a good seal to the exhaust pipe.

If the threads just need cleaning up, then there is a little trick you can try. You use a tap of the necessary pitch and adapt it to the thread diameter with a wooden or plastic plug.

[mgill]

[quote="EVguru] press in a threaded insert with the correct close tolerance thread[/quote]

Where might I find an insert?
NLM?

[EVguru]

Where might I find an insert?

Surrounded by a huge pile of swarf on my Lathe!


Your best bet is probably to find someone with a home workshop in your neighborhood.

NLM offer the port repair for about £70 I think, but the shipping wouldn't be fun.

[mgill]

Thanks Paul.
Mat.

[norbert]

The size is metric:

350 ccm (125 und 250er V perhaps the same?): 42x1,5
500 an 501 : 45x1,5

For the 500 you can use the nuts from the MZ TS 150. At least they are the same size and normaly very much cheaper.

norbert