Here's a cut-and-paste from one of GSXR's earlier posts on the topic of prechambers:

New prechambers are not required. the stupid WorldPac note about needing new PC's is wrong and pisses me off because it misleads a lot of people. Your new head may work with the old prechambers, or may not. If not, you need to modify the PC's (mill them slightly) to seat correctly. Or, you can convert to the inclined/angular injection, which will fit without modifications, but costs $1000-$1500 new and is obviously not worth it. This may be a viable option if you buy a used #17 head that includes all the inclined injection parts. Some people are lucky and bolt in old PC's with no problems.

The prechamber issue affects some newer heads - possibly #17 or #20 as well, definitely some #22, since it happened to me. Metric Motors says they find it on maybe half the new 603 heads they buy when rebuilding 603's. Some need machining, others don't. Apparently the design spec changed on the newer heads for the angled injector prechambers or something. Only way to know is to insert the p/c with the head off, measure protrusion, then insert a shim of known thickness (say, 0.5mm) and measure again, the difference should be 0.5mm - if not, you need to machine & repeat the measurements until they come out correct. Or after the head is installed and you get the nice "puff puff puff" when cranking (like I did!), yank the p/c's, machine them, and reinstall!

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Chris

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