The exhaust threads on one of my spare /5 heads are are M52x2. The thread dimensions can be calculated using a Machinery's handbook and there are some web pages with this dimensional information. I always wondered how to make a jig for the head to single point thread the exhaust threads in a lathe? I expect someone has figured out a way. I have seen dies for sale on Ebay from Germany for ~$100.00 + shipping.
Maybe an easier solution is to get a piece of 6061-T6 extruded rod, bore a hole ~39mm in diameter, single point thread the OD to make a threaded nipple, and then cut off the damaged threads on the head and TIG weld on the nipple? The welding raised portion of the new exhaust nipple ID could be line bored in a milling machine or hand filed smooth. Maybe one day I will try this with one of my spare heads, but my TIG welding has to improve first.
Stan_R80/7 screwed with this post 04-03-2013 at 04:16 PM
Reason: Edit: the exhaust ID is 39mm (for the < 1000 cc heads)